The effect of errors in Inspiratory Volume on DLCO.

Yesterday while reviewing reports I ran across an interesting error in the Inspiratory Volume (VI) from a DLCO test. I’ve probably seen this before but this time I realized what effect it could have on DLCO. Specifically, what I saw was that at the start of the DLCO test the subject had not finished exhaling and although the technician had started the test, the subject continued to exhale.

What makes this interesting is that the software used the subject’s volume at the start of the test as the initial volume. This means that the software measured the VI from the initial volume to the end of inspiration, not from the point at which the subject stopped exhaling to the end of inspiration. This also means that the VI was underestimated by 0.20 L and this affects both VA and the calculated DLCO.

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When an Inspiratory Volume really isn’t inspiring

I always like it when a patient does something during a test that makes me have to think about the basics of the test and what effect an error will have on the results. I was reviewing a report that had come across my desk and the technician performing the test had put “poor DLCO test reproducibility, fair quality in selected test” in the notes so of course I had to pull up the raw test data and take a look for myself.

The patient had performed three DLCO tests, two of which were completely unusable and one that was sort of okay but not really. Interestingly, the test system software thought it met the criteria for acceptability.


The ATS/ERS statement on DLCO testing says that the inspired volume needs to be at least 85% of the patient’s largest known vital capacity. Even though the patient’s inspired volume during most of the test was well below this threshold they made a further inspiratory effort just before exhaling and exceeded the threshold when they did. For this reason the software thought the effort was acceptable. This points out limitations in our testing system software, its hardware, and in the ATS/ERS statement as well.

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