Diffusing Capacity (DLCO)

What is it? 

This test is used to estimate the transfer of oxygen from the alveoli in your lungs to your bloodstream. The diffusing capacity (DL) of oxygen is technically very difficult to measure, and the test actually measures the diffusing capacity of carbon monoxide (DLCO) which provides a valid estimate of the oxygen diffusion. The diffusing capacity test is safe and has no lasting effects.

What should you expect?

This test requires complicated equipment and special testing gases. It will always be performed in a pulmonary function lab.

For this test:

  • You will be asked to sit upright in a chair.
  • You may be asked to loosen your bra or your belt if these could restrict your breathing.
  • If you are using supplemental oxygen then you will be asked to take your nasal cannula off.
  • You will have your nose clipped so that you will breathe only through your mouth.
  • You will be asked to breathe through a flanged rubber mouthpiece. It is important that you keep your lips snug on the mouthpiece in order to get a tight seal so that air does not leak.

When the diffusing capacity test starts:

  • You will be told to breathe quietly for several breaths and then to blow out as much air as you can.
  • When you have emptied your lungs you will be asked to take a quick deep breath in, as deep as you can, and then to hold it for 10 seconds. The inhaled gas contains a very low concentration of carbon monoxide.
  • At the end of the breath-holding period you will be asked to blow the air out quickly. The staff person giving you the test should be coaching you the entire time you are performing the test.

You will probably need to perform the diffusing capacity test at least twice. More attempts may be necessary and this will be based on test quality and reproducibility but there should not be more than four attempts. The staff person who is performing your test should tell you each time whether you did the test correctly, or if not, which part of the test you need to improve.

You may become very short of breath during the breath-holding period. This is normal. Despite this you need to hold your breath for at least 10 seconds and not come off the mouthpiece until you are told that you can. If you become too tired, short of breath, or uncomfortable please take time to recover between tests. If you are using supplemental oxygen you can use it between tests if this will help you recover. You can drink water if your throat is uncomfortable or dry. Kleenex should be available if you start coughing.

The nose clip and the mouthpiece should both be new and clean at the start of your testing session. The staff person performing your test should be wearing gloves or at a minimum should have performed hand hygiene before your testing session. Kleenex should be available if you start coughing.

What is a normal measurement?

The diffusing capacity test results are compared to normal values for someone that is your height, age, gender and ethnicity. These normal values will come from one of several different population studies and there are two different ways of making this comparison:

Percent predicted:

A DLCO result that is at least 80% of the predicted value is considered to be within normal limits.

Lower Limit of Normal (LLN):

The lower limit of normal is based on a statistical analysis of the study population. A DLCO result above the LLN is considered to be within normal limits.

When DLCO results are below normal, the severity of any decrease is assessed as follows:


What affects test quality?

  • Your test results will probably be underestimated if you do not empty your lung as much as you can and then take as deep a breath as you can.
  • Your test results will probably be underestimated if you do not hold your breath for the full ten seconds or if you leak air during the test.
  • Your test results will probably be underestimated if you breathe in too slowly at the start of the test or if you breathe out too slowly at the end of the test.
  • Because hemoglobin is the molecule in your red blood cells that carries oxygen in the bloodstream the test results will be underestimated if your hemoglobin levels are low and overestimated if they are high. The diffusing capacity results can be corrected for your hemoglobin level but the blood test for hemoglobin needs to have been done recently. There are no clear standards for how recent a hemoglobin measurement needs to be in order to correct DLCO test results but anything more than a month old should not be used for this purpose.
  • Because the diffusing capacity test uses small amounts of carbon monoxide any extra carbon monoxide in your blood will reduce the test results. Smoking cigarettes puts extra carbon monoxide in your blood and for this reason you should not smoke for 24 hours before the diffusing capacity test. It is possible to correct for blood carbon monoxide levels but you will need to have an arterial blood sample taken and measured in a laboratory instrument called a co-oximeter. This blood test should be done no more than an hour from the time the diffusing capacity test is performed.
  • Your predicted DLCO is directly related to your height so your test results cannot be assessed correctly if your height has not been measured accurately. Your height should be measured regularly and it should be measured with your shoes off with you standing straight while looking directly ahead.

Note: For a technical and in-depth discussion of the normal values for the DLCO test see Whats normal about DLCO?

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PFT Patient by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

37 thoughts on “Diffusing Capacity (DLCO)”

  1. what does it mean if a patient could not complete this test? A baseline could not be reached and 85% could not be maintained. The patient lost color and felt lightheaded.

    1. The patient was most probably performing a Valsalva maneuver, that is increasing the pressure inside their lung and trying to blow out while the valve was closed. This extra pressure decreases blood flow through the lung and is likely what caused the lightheadedness. Because it can decrease blood flow a Valsalva maneuver can decrease the measured DLCO. A Mueller manuever is the opposite of a Valsalva manuver, i.e. the patient tries to keep inhaling, which causes low pressure inside the lung and increases blood flow into the lung. A Mueller maneuver can increase DLCO by increasing the lung’s blood volume. Both Valsalva and Mueller maneuvers are contraindicated because they can affect the measured DLCO. An individual performing a DLCO is supposed to relax after taking the full inhalation of the gas mixture and should neither try to continue to inhale or to try to exhale.

  2. My dlco was only 77%. I recently suffered from a pulmonary embolism and am still taking blood thinners for it. Shortness of breath is still a symptom for me throughout my recovery. I am wondering if my number being a bit low could be from the blood clot or even the blood thinner. My fev1 was 102%

    1. Theresa –

      The job of the lungs is to get air and blood together so they can exchange gases (O2 in and CO2 out). A pulmonary emboli affects the blood flow through the lung and when this happens the part of the lung that’s affected can continue to be ventilated but with no blood flow, there is no gas exchange. DLCO generally decreases in step with the severity of the pulmonary emboli and as an example, the DLCO results from an individual tested in my lab with a severe pulmonary emboli that completely blocked the blood flow to one of their lungs was half of what it should have been. Assuming you have no other reasons to have a low DLCO (smoking history, for one) the decrease in your DLCO is most probably due to the emboli. Blood thinners are usually given to prevent you from having any more pulmonary emboli (they’re not given in every case but I’m a technologist, not a physician, and can’t give you the reasons why) but should not affect your gas exchange (DLCO).

      FEV1 however, is a measure of how fast you can blow your air out which has to do with the condition of your airways, not your pulmonary circulation. Pulmonary emboli usually do not affect the lung’s airways and this means you can have a normal FEV1 and still have a pulmonary emboli (and a reduced DLCO).

      Best wishes on your continued recovery – Richard

      1. Thank you for this explanation, it is very helpful. I have a collection of autoimmune stuff happening and my dlco dropped over 6mos. I find it hard to take breaths, some days i feel all I can do is lay/sit and breathe. My pulm isnt concerned so far….

  3. Bob B.
    I had 4 yearly LFTs performed in the same lab. The DLCO norms used were 16.4, 16.2, 16.3, and16.3. The following year I had 2 tests performed at different labs. The DLCO norms used were 24.1 and 26.9. The actual results over these 6 tests were 8.4, 9.9, 9.3, 9.4, 8.6, and 9.1. As will be obvious, expressed as a percentage of norm, my DLCO performance dropped from Moderate to Severe, yet the actual results were pretty stable. What am I to believe from this wide difference? Why would the norms used vary so greatly?

    1. Bob –

      There are well over 20 different DLCO reference equations from around the world. About a half dozen or so of these are in common use in the USA and Europe. The studies these equations come from were performed at different times, in different places, with different types of equipment, with different equipment settings and, of course, different study populations. Each pulmonary function lab is supposed to choose the reference equations (for all the tests they perform) that “match their local population” but other than that piece of advice there are no particular guidelines on how you are supposed to do this, so as you may guess it is implemented with varying degrees of success. The differences between the different equations usually aren’t as big as what you describe (not doubting you, BTW). If I knew your height and age I could probably pinpoint which reference equation each lab used and would be able to tell you more but unless you are both short and elderly (from your name I’m assuming you are male) the DLCO norms from the first set of tests looks to be more than a bit on the low side (at least compared to the average).

      FWIW, the European Respiratory Society (ERS) and American Thoracic Society (ATS) just released new DLCO testing standards. They still do not specifically recommend one particular DLCO reference equation but they published a short list of DLCO studies that were performed according to the 2005 DLCO standards (and most of the reference equations in common use are not on it). The new standards address many testing issues and I hope that all PFT labs will be reviewing their equipment, procedures, software and reference equations and hopefully bringing as many of them up to par as they can.

      Regards, Richard

      1. Thank you Richard and if I may I would like to explore this a little further. I am male and the following are my age and height for each of the tests conducted:
        2012: Age 73 Height 67inches
        2013: Age 74 Height 67inches
        2014: Age 75 Height 67inches
        2015: Age 76 Height 67inches

        2016: 1st Test (April) Age 77 Height 67inches
        2016 2nd Test (Sept.)Age 77 Height 67 inches

        All of the first 4 tests were conducted in the same Hospital lab. The first test in April 2016 was conducted in a Lab opened by my Respirologist (which is located about 200 yards from the same hospital). The second test in September 2016 was conducted at a second hospital 60 miles east and closer to my home. All Labs are in Canada.

        1. Bob –

          The normal values from 2012-2015 do not match any reference equations that I have. The closest is that from Burrows (Burrows B, Kasik JE, Niden AH, Barclay WR. Clinical usefulness of the single-breath diffusing capacity test. Am Rev Respir Dis 1961; 84: 789-806) which always produces the very lowest DLCO normal values for any age or height but even it is a point or so higher. Another possibility is that the normals from 2012-2015 are actually the lower limit of normal (LLN) rather than the mean value. The most of the commonly used reference equations are not published with an LLN but it can be derived from the standard deviation of the study’s population. How this is done however, can be idiosyncratic so I’d only be guessing to say which reference equation was actually used. However, if it was an LLN then your results should not have been compared as a percent of predicted which is how the DLCO is scored as mild, moderate or severe.

          The first of your 2016 tests used Miller’s reference equation (Miller A, Thornton JC, Warshaw R, Anderson H, Teirstein AS, Selikoff IJ. Single breath diffusing capacity in a representative of Michigan, a large industrial state. Am Rev Resp Dis 1983; 127: 270-277). The second 2016 test is less clear but this may to do with how accurately inches are converted to centimeters and whether or not age is expressed as a whole number or includes a fractional year but the closest is Crapo (Crapo RO, Morris AH. Standardized single-breath normal values for carbon monoxide diffusing capacity. Am Rev Resp Dis 1981; 123: 185-189).

          Miller’s study is well regarded and Crapo’s is usually considered to be on the high side. When all of the equations I have on hand are averaged, the normal values for somebody your (current) age and height would be 24.92. FYI, the normal values for Gutierrez’s study on Canadians is 24.44 (Gutierrez C, Ghezzo RH, Abboud RT, Cosio MG, Dill JR, Martin RR, McCarthy DS, Moorse JLC, Zamel N. Reference values of pulmonary function tests for Canadian Caucasians. Can Respir J 2004; 11(6): 414-424).

          Basically, the reference values from your 2016 tests appear to be a lot closer to “reality” than those from 2012-2015.

          – Richard

  4. Richard:
    I’ve been thinking some more about your response and I’d like to ask one more question. In your comments you say “…..if it was an LLN then your results should not have been compared as a percent of predicted which is how the DLCO is scored as mild, moderate or severe…..”. How then should the results have been compared and presented?

    1. Bob –

      Percent of predicted should be made using the mean value (i.e. what it usually presented as the normal value). I’ve never seen a PFT report that reported a percent of the LLN.

      – Richard

  5. I’m a 77yr old White Australian male, Scots/Irish ancestors, 177cm and 80kg,generally in good health apart from well managed Atrial Fibulation,Xarelto 20mg/day,, and Basal Ideopathic Pulmonary Fibrosis.Flat walking causes no symptom,s however hills, long staircases and exertion cause a degree of shortness of breath which is not distressing and is measured on a scale of 2/10.
    I have been on a Trial of Perfenidone,3x3x267mg daily, since diagnosis15 months ago. A recent MRI showed no appreciable changes.All Spirometry has been stable during the trial period,both FEVand FVC readings of 4l+ at 110% with the exception of DLCO which has declined from 55 to 48% predicted.Oximeter readings are consistent at 96/98 and 6min walk test is 600metres+.Please let me know of anything that I can do to halt decline/improve my DLCO. Kind regards

    1. Geoff –

      The fact that you can walk 600 meters and that your oxygen saturation stays normal says that you are doing well. There is always a certain amount of visit-to-visit variability in DLCO and for my lab it is only considered significant if it changes +/-10% and 2.0 ml/min/mmHg. Even then I look very carefully at test quality and there are number of ways that results can be underestimated despite the best efforts of you and the technologist performing your tests.

      Were your DLCO results were corrected for your hemoglobin levels? Hemoglobin needs to be measured near the time you have your DLCO measured and different labs have different policies about this. My lab cannot order a hemoglobin test (or perform it) and we have to rely on any blood work that was done recently (within the last month). If we don’t have a hemoglobin measurement we can’t correct your DLCO so sometimes what looks like a change from one visit to another is just the fact that we had a hemoglobin measurement on one visit and didn’t on the other. The pulmonary physicians (and oncologists and CT surgeons) we work with know this and adjust their thinking about the results accordingly.

      DLCO is a measure of the functional surface area of the lung and it’s not like a muscle that you can build up with exercise. As far as I know Perfenidone is the only medication that has shown any success in treating pulmonary fibrosis but I’m a technologist, not a physician, and I don’t keep up with the treatment side of things (the testing side is complicated enough). I will say that the things that our mothers told us when growing up; eat right, exercise and get plenty of sleep have been shown over and over to be the best things we can do for ourselves.

      I would recommend that you join an IPF support group (just google it). You need to talk to people who are dealing with the same problems that you are. You’re not alone and they may well have ideas about how to help yourself.

      Best wishes, Richard

  6. Recently while testing some of my patients are getting an invalid effort error even tho they are performing the maneuver well. Could this be effected if they have a severe obstruction even tho they are giving maximum effort?

    1. Nick –

      That’s not a terribly informative error message. There are several things your system’s software may be looking at. Inspired volume needs to be >=85% of the highest VC, and it’s usually the FVC that it’ll be compared to and less commonly the SVC as well. Breath-holding time (BHT) should be >8 and <12 seconds. If your system is using the Jones-Meade protocol, BHT starts at 1/3 the inspiratory time (inhalation to 90% of max inspired volume) and ends half-way through the sampling period. At the end of breath-holding the patient has 4 seconds to exhale their washout volume and alveolar sample volume (nominally 1 second for washout and 3 seconds for the alveolar sample).

      Since you say this message is associated with severe airway obstruction then it's the expiratory side that's most likely to be the problem and that's either a prolonged BHT or a prolonged alveolar sample time. If your system software lets you adjust the DLCO test settings you might decrease the BHT (usually defaults to 10 seconds) to 8 or 9 seconds. One suggestion would be to tell the patient at the end of the breath-holding period to "let their air out" rather than telling them to "blast it out". They might well have higher expiratory flow rates with a relaxed effort than with a forced effort (negative effort dependence).

      Other than that it depends on what type of system you have (continuous sampling vs alveolar sample bag) as to whether this is really a problem. The downside of alveolar sample bag systems is that you have to preset the washout and sample volume (computer may be doing this for you but it's still a guess). For these systems I would be careful about decreasing the washout volume, but you can probably get by with a small alveolar sample volume. If you have a continuous sampling system then you can adjust washout and alveolar volume afterwards. BHT is not really as important as the standards make it sound to be. I've found that patients (with or without airway obstruction) have similar DLCO's when BHT is anywhere from 9 to 14 seconds.

      Regards, Richard

  7. My inspired volume at my last DLCO test just could not get over 85% despite more than 5 attempts. What would cause this when in the past I have been able to without difficulty. My FVC remained the same from previous tests. Is this difficulty a sign of lung function decline? The technician said she could not report the results since they were invalid. How soon should I go back to repeat?

    1. Jane –

      Two possibilities come to mind. First, the technician wasn’t cuing the test system or instructing you properly. You need to exhale as much as you can first before you take a deep breath in. If the technician didn’t tell you or have you do this then your inspired volume will not match your FVC; or if they were triggering the test system too soon then you wouldn’t have enough time to completely exhale before you were supposed to inhale.

      Second, there could be an inspiratory leak in the DLCO sub-system such that your inspiration wasn’t measured accurately. Most test systems have valves of some kind and it’s possible that one didn’t close (or open) properly, or that there was a leak in the tubing or other parts.

      My experience is that a lot of people have a problem with the exhaling-completely-before-inhaling part of the maneuver and unfortunately this means that more often than we’d like we have to try to make sense of suboptimal results. The way this is done is to look at the VA (Alveolar Volume). The VA is a quick lung volume measurement of the total capacity of the lung (TLC). If the VA is roughly the same as the TLC (measured by a different technique such as helium dilution, nitrogen washout or plethysmography) then we’ll accept a DLCO with a suboptimal inspired volume and that is because in this instance the VA is saying that the DLCO test gas mixture was well-distributed through the lung.

      Regards, Richard

  8. Hi, I am a 34 year old male, about 220 lb, about 5/9″.

    I have been dealing with something (I don’t know what exactly, but the only diagnosis thus far is RADS) for the last 9 months or so.

    All of my spirometry numbers are normal (my 25-75% flows tend to be a little lower, but have ranged between ~70-95% in various PFTs over the months).

    I have had multiple DLCO measurements over the past 9 months and, prior to today, they ranged from about 117-140%. Today I had PFTs and the DLCO came out to 99%. In June, my DLCO was 120%.

    The technician told me (before I even took the test) that anything over 100% for DLCO is essentially a testing error. Yet I have seen that normal results are between 80-120% online.

    My question really boils down to what could cause such variability in DLCO results? Should I be concerned with a drop to 99% given that my DLCO has been consistently above 115%?

    I have read that pulmonary vascular disease and early emphysema or ILD should be considered when there is an isolated drop in DLCO, but my DLCO is still “normal” (right smack in the middle of the normal range). Yet the drop makes me nervous given my ongoing symptoms of shortness of breath and chest pain.

    I am not looking for a diagnosis here (obviously), but I don’t understand if such a variation could be normal or maybe the test was done incorrectly or if I should be concerned. I do have a follow up with a pulmonologist and plan to discuss this at that appointment, but in the meantime I am scratching my head.

    Thanks for considering.

    1. Scott –

      The technician doing your tests is mistaken since results above 100% of predicted are relatively common. Partly it depends on which reference equations are being used since some tend to be on the high side and some tend to be on the low side of any given population (see Whats normal about DLCO). Partly it’s because human DLCO results follow a bell-shaped curve and by definition half of everybody is “above normal” (and that’s because it’s the mean value of the population is considered to be “normal”).

      More importantly (and probably most pertinent to your question), DLCO tends to be elevated in people with asthma and values from 120% to 140% of predicted are not uncommon. The reasons for this remain unclear and the possibilities for this range from increased vascularization of the airways to an overall elevated volume of blood in the pulmonary capillaries to a more even distribution of blood flow between the apex and bases of the lung. I’ve never seen a research study that tried to correlate DLCO with changes in clinical asthma status but my experience is that many patients with an elevated DLCO often have changes from one visit to another so the changes you describe aren’t completely unusual. I have to temper that however, with the fact that DLCO is not a routine measurement in patients with asthma so those patients with asthma who have frequent DLCO measurements often have some coexisting problem that is being monitored. One final point is that DLCO can also be over- and under-estimated due to testing and equipment issues.

      I can’t tell you not to be concerned that your DLCO results have dropped to “normal” when they were always elevated, but I’d have to say that emphysema and ILD would seem to be unlikely given the fact that your other test results are normal.

      Regards, Richard

  9. I have scleroderma, Sjogren’s, probably vasculitis, and a few other autoimmune diseases. My rheumatologist put on a report that I had a significantly low DLCO. That was from a pulmonary function test over a year and a half ago.

    Since the winter I have had a real problem with shortness of breath. My family doctor prescribed a puffer but it did nothing to help me breathe. I had a recent pulmonary function test but can’t get the results until October 31st.

    Recently, I had to go to the ER because of the shortness of breath and pain around the bottom of my rib cage. Although the ER is the same hospital where I had all the testing done, they would not give me the results of the PFT nor of an MRI of my T-spine (The neurologist is trying to determine if I have peripheral neuropathy or a Sjogren’s related issue but I don’t see her again until the middle of December.)

    My pulse ox was anywhere between 84% and 99%. Do the pulse ox monitors placed on your finger measure DLCO?

    I was informed by the ER doctor that the recent CT scan of my lungs showed either an infection or inflammation. This did not show on a chest x-ray done in the ER. A CT scan done 3 months prior showed a small granulomas mass and some small nodules in my lungs.

    I realize that my medical conditions are too complex for you to provide a diagnosis so I am not looking for that, but I am very concerned about what is going on since my research indicates that 80% of scleroderma patients have lung involvement and it is now the number one cause of death and disability.

    No one will address my concerns and from everything I have read, it is important to get early treatment if there is lung involvement to help to slow progression.

    I have two questions that I am hoping you can answer.
    1. Am I over reacting or do I need to kick up a stink with my primary care physician to be referred to a specialist?
    2. If I purchase a pulse ox finger monitor will it help me to determine if I need to go to the ER again? I’m not really sure what it measures. I can breathe in enough air ok but I feel like I am suffocating. The ER made me feel like I was wasting their time because my pulse ox was considered normal even though I had to stop talking to take a couple of breaths. It wasn’t until the doctor in charge looked at my CT scan that anything was done. They gave me a prescription for antibiotics and sent me home. Now I don’t know what to do since I can’t seem to get any of my specialists to return my call and my primary care physician does not know anything about my conditions.

    Americans think that Canadians have a great health system. In some senses we do but if you are really sick with something rare you can’t see the people you need to. The neurologist appointment I just had was a referral from 2015 and I don’t go back for results and referral to another specialist until December!

    1. Marie –

      A pulse oximeter measures how much oxygen is in your bloodstream. A DLCO test measures how good your lungs are at getting oxygen into your blood stream. They’re related, but are measuring completely different things.

      Oxygen levels don’t usually vary between 84% and 99% unless the ER staff has done something like putting you on supplemental oxygen. My experience is that when the oxygen level measured by a pulse oximeter varies a lot in a short time it usually means that the finger they placed the probe onto has poor circulation. The reason they may be dismissing some of your concerns is that they are likely looking at only the high readings and ignoring the low ones. Pulse oximeters are relatively inexpensive (I’ve seen prices as low as $25 on Amazon and Ebay) so it probably wouldn’t hurt to get one. It would either alleviate some of your concerns or give you evidence to give your physicians (and I would try using it on your thumb since that digit has better blood flow than the fingers do).

      I don’t know how to navigate the Canadian health system so I am unable to give you any specific advice but I think you have more than sufficient reason to push for a referral to a pulmonary specialist particularly since past testing showed a low DLCO. It would help if you had your PFT (and MRI) results and I don’t understand why you aren’t allowed to see them. I’m not going to say the US health system is better since I am all too aware of its many shortcomings but at least here there is no question about being able to access your test results.

      Best wishes, Richard

      1. Thank you Richard. I will push for the referral. There is a way to force the doctors to allow you to see your records and I could get copies but they charge $30 per page. Due to the fact that both my husband and I are disabled and I lost my job due to health reasons 2 years ago, I just can’t afford that.

        I will have my husband come with me to my primary care physician and take the report where my urologist states that I should be referred to Internal Medicine to put all the pieces together. That report has been ignored so far. I have it because I had my rheumatologist mail back my application for disability.

        1. I do have another question before I buy a monitor. If I am having shortness of breath (this heat wave and humidity is making things miserable) would the pulse ox monitor show that I have less oxygen in my blood? What I’m trying to determine is if DLCO corresponds to lack of oxygen in the blood or not.

          I am having difficulty figuring out why I feel I can’t breathe and yet my pulse ox appears to be in normal range.

          1. Marie –

            Shortness of breath can have many causes and a low oxygen level is just one of them. A pulse oximeter would at least be able to tell you whether you have low oxygen levels or not. Importantly, your oxygen level can be completely normal when you are sitting quietly but could drop when you exercise so you should measure your oxygen levels both ways.

            Hot and humid air actually has a slightly lower oxygen concentration than cool, dry air but the difference is small. What bothers people and makes them feel short of breath is that breathing is one of the ways that people lose body heat and when the air is hot and humid, the cooling effect doesn’t work and this will make you uncomfortable.

            Best wishes, Richard

    1. Linda –

      When you say your oxygen is normal what you probably mean is that your oxygen saturation measured by a pulse oximeter is normal. DLCO is a measure of how good your lungs do at getting oxygen into your blood stream. You can have a low DLCO and if you aren’t exercising (i.e. your body’s oxygen demand is low) then your oxygen saturation can remain normal. Whenever you do exercise however, and your body’s oxygen demands are high, then your oxygen saturation will decrease and that’s when you’ll need supplemental oxygen. People who get short of breath easily tend to exercise less and may not notice their limitations. I’m not going to say this applies to you, but this is what I’ve seen in the past.

      Regards, Richard

      1. thank you for the reply. my walking test was normal, fev1 normal, overnight oxycimetry was normal but dlco was at 46 percent so I don’t understand???

        1. Linda –

          That does seem a bit contradictory. With that low of a DLCO your oxygen saturation while walking should have decreased. The only reasons I can think it didn’t would be that you were walking very slowly or that there is an error of some kind in your DLCO test and that it isn’t really that low.

          Regards, Richard

  10. Hi, I’m 54 male 5′.9″. in the past 6 month have some SOB when climbing stairs. Normally healthy. Heart tests (stress and catheterization) all fine. Chest x-ray fine and no sounds in lungs. Did PFT and the hospital said results are normal. The Dr sent for CT since DLCO/VA was slightly lower at 73. The DLCO is 81. All other parameters seems fine. what can cause DLCO/VA to be lower while DLCO to be fine? could this be a test error?

    1. Carel –

      The percent predicted DLCO/VA (KCO!) is the intersection of two different predicted values, DLCO and TLC (predicted VA = predicted TLC – predicted anatomical dead space), that come from different population studies. If your VA was above normal (according to your predicted TLC) but your DLCO was normal, then the DLCO/VA ratio will be reduced. In your case since the percent predicted of your DLCO (81%) is low normal, your VA doesn’t have to be elevated all that much for the percent predicted DLCO/VA ratio to be reduced.

      VA is a single-breath lung volume measurement and for this reason it should never be greater than TLC. Test errors almost always lead to VA being lower than it “really” is. When VA is larger than TLC it usually means there’s a problem with the TLC measurement, not the VA measurement.

      DLCO/VA (KCO!) is primarily useful when diagnosing a reduced DLCO in the presence of restrictive lung disease and isn’t of much use when lung volumes are normal (for a more technical and detailed discussion of this see “Using DL/VA (no, no, no, it’s really KCO!) to assess PFT results“). A personal observation (as far as I’ve been able to tell this hasn’t been formally studied) is that individuals with larger than normal lung volumes (i.e. elevated FVC and TLC) tend to have a normal DLCO, which also means their DLCO/VA is reduced.

      Finally, as I mentioned, the percent predicted DLCO and TLC almost always come from different population studies. Study populations differ so depending on which reference equations the PFT lab you were seen in is using the predicted DLCO/VA may or may not be a good fit for you.

      Regards, Richard

  11. Thank you Richard.
    During the test the technician did repeat few times I have large volume but on the results I got the following:
    FVC – 5.04L, 110%
    FEV1-4.07L, 111%
    FEV1/FVC – 81%
    TLC – 6.55L, 99%
    VC – 5.04L, 110%
    FRC PL- 3.36L, 100%
    ERV – 1.6L, 106%
    RV – 1.51L, 75%
    RV/TLC 23%
    DLCO -19.1, 80%
    DLCO Adj -19.1, 81%
    DLCO/VA – 2.98, 73%
    VA – 6.42L
    IVC – 4.84L

    The Dr was concerned of the lower DLCO/VA and asked for the CT to rule out Fibrosis. CT came clean. my symptoms which still were not fully clarified their cause is SOB when climbing stairs or mild exertion dizziness for few seconds when moving from sitting to standing. The Drs claim it might be that I’m simply out of shape… I used to be in high shape cycling race style long distances but stopped in the past 5 years.

    1. Carel –

      I had some trouble reconciling some of the numbers so working backwards I was able to determine that the assumed dead space (VA = TLC – dead space) is 0.77 L which seems high to me (anatomical deadspace is approx 0.25 L, machine deadspace is usually around 0.15 L), but would mean that your predicted VA is 111% of predicted so the percent predicted of your DLCO/VA makes a bit more sense.

      However, I also tried to determine which DLCO reference equation was being used and the only one that matches is Burrows B, Kasik JE, Niden AH, Barclay WR. Clinical usefulness of the single-breath diffusing capacity test. Am Rev Respir Dis 1961; 84: 789-806, which is a fairly old study and tends to produce the lowest predicted DLCO values when compared to all other DLCO reference equations. Using the GLI DLCO reference equations (just published last year) your predicted DLCO would be 27.73 and your DLCO would be 69% of predicted (LLN is 21.04). I’m sorry to say that this indicates that you have a more significant problem with gas exchange than your report suggests and also is somewhat of an explanation for your shortness of breath while climbing stairs. Your spirometry and lung volumes are very normal, so that rules out a lot of things, as does the clear CT and normal catheterization. I am not a physician however, so you should follow up with a pulmonary specialist. I would also suggest repeat DLCO testing at a different PFT lab.

      Regards, Richard

  12. Hi –

    I was diagnosed with cryptococcosis well over a year ago, and it has left me with significant lung scarring. Recently had my first pulmonary stress test and (frustratingly, since I feel like I can’t breathe all the time) everything seemed normal according to the technician despite my obvious struggling. Today I got the actual numbers and I was hoping to get your insight into two of the numbers, since I think they’re the only ones that show anything abnormal. My DLCOCOR (which I assume means corrected DLCO) is 43.36 ml/min/mmHG (128%) and my DLVA is 158%. Is this what the technician might have meant when she said I had “high diffusion” during the test? I’ve tried to do some reading on these values but I would have expected low numbers here considering the extent of my lung damage.

    Not looking for a medical opinion, just trying to learn so I can advocate for myself better. TIA for any light you can shed.

    1. JR –

      Having an elevated DLCO is usually a good thing, particularly with your history of cryptococcosis. Feeling short of breath can occur when you have too much carbon dioxide or too little oxygen in your bloodstream, but you can also feel short of breath when you can’t breath as deeply as you feel you should be able to. The wall of your thorax has stretch receptors that give feedback to your brain about how deeply you are breathing. The scarring in your lung may be preventing you from taking a deep breath but it’s also possible the cryptococcosis has affected the stretch receptors. I’ve known people who, because of injury or surgery, have had the nerves leading to the stretch receptors damaged and despite having normal lung function and normal ABG values, they always felt like they couldn’t breath properly.

      Best wishes, Richard

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