When DLCO tests are performed my lab’s standard policy to average two or more results that meet the criteria for quality and reproducibility. It is not unusual for us to perform three DLCO tests and have all of them meet quality criteria but to have one test result that is higher than the other two. Unlike spirometry tests, bigger isn’t necessarily better for DLCO, so in a circumstance like this we will average the two closest results rather than choose the highest result. Even though the higher test results can come from a DLCO test with good quality, I think that reproducibility trumps this and that choosing by reproducibility gives us results that are more clinically reliable.

When I review spirometry results and either lung volumes or a DLCO test has also been performed, I will always check the Slow Vital Capacity(SVC) from the lung volumes and the Inspiratory Volume (Vinsp) from the DLCO test to see if they are larger than the reported Forced Vital Capacity. If either of them is I will manually re-calculate the FEV1/VC ratio to see if it indicates the presence of airway obstruction. This is in line with the ATS-ERS recommendations to use the largest Vital Capacity, regardless of the source, for the FEV1/VC ratio.

I have been reviewing the raw test data for all DLCO tests (as well as all the lung volume tests and regular spot checks on spirometry) performed in my lab for at least the last year. Since our software and hardware upgrade a year and a half ago we’ve found a number of problems that have significant effects on the DLCO test results. Depending on the problem they are capable of causing the results to be over- or under-estimated. All of the technicians performing the tests are now well aware of these problems and there haven’t been any problematic DLCO tests selected for a while. Nevertheless, I always check the raw data just to be sure.

Today, I ran across a report that looked quite straightforward. A set of spirometry and DLCO tests had been performed on a frequent-flier patient with pulmonary fibrosis. The patient has restrictive lung disease and lung volumes measured about a year ago were 64% of predicted. Even though the patient’s FVC has decreased since then there is no clinical reason to repeat the lung volume measurements. The results looked like this: 

When I looked at the raw DLCO data, what I saw was: 

Both tests met all ATS-ERS quality criteria (including Vinsp > 90% of the FVC) and they were eminently reproducible, but the second test had a significantly larger Inspired Volume. This fact alone is not a reason to reject the first test. Because the VA was essentially the same for both tests this says to me that the DLCO gas mixture was probably equally well distributed through the patient’s lungs for both tests.

However, when I took the Inspired Volume from the second test and recalculated the FEV1/VC ratio it came out as 65.9 which is 91% of predicted. We use an FEV1/VC ratio of less than 95% of predicted as our criteria for airway obstruction. (I’ve discussed this in the past and we do this partly because we think that it is more accurate than using the LLN and partly because of continuity). Once I saw the higher Vinsp, I de-selected the first DLCO test so that the reported Vinsp was no longer averaged between the two tests and that it was instead reported from just the second test.

Normally I would hesitate to do something like this, but in this case the DLCO and VA results were essentially identical between the two tests so excluding the first test made no significant difference whatsoever in the reported DLCO results. More importantly, the higher Vinsp showed that the patient had an obstructive component to their lung disease as well as a restrictive one.

My lab’s software always reports the highest SVC regardless of which test it comes from but it averages the Inspired Volume. Reporting the highest SVC is in line with the ATS-ERS criteria but the ATS-ERS does not speak to how Vinsp should be reported. Since I believe that averaging DLCO results is the proper thing to do I haven’t previously questioned whether or not Vinsp should be averaged along with DLCO and VA but now I think that the highest Vinsp should be reported instead.

I’ve often suspected that many patients with pulmonary fibrosis also have some airway obstruction as well. I base this on the fact that despite having a significantly reduced vital capacity many of these patients can take a lot longer than 6 seconds to completely exhale their FVC (assuming their dyspnea lets them exhale that long) and this happens even when their FEV1/FVC ratio is normal or elevated. I wonder if other criteria like FEV3/FVC ratio might highlight some of these patients or whether performing SVC maneuvers along with FVC maneuvers to try to get a higher VC is a better approach.

Anybody that reviews and interprets PFT reports needs to be aware how critical numbers like SVC and Vinsp end up on the report.  This is a somewhat unusual case where the results from a DLCO test were used to show airway obstruction. Although I knew that Vinsp was averaged, in this case the care I’ve been taking to review the raw data from DLCO and lung volume tests has paid off because if I hadn’t looked at the raw data for the DLCO test this patient’s airway obstruction would have gone un-reported. 

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