Tag Archives: Zero Offset

Some DLCO errors the 2017 standards will probably fix

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Last week I ran across a couple errors in some DLCO tests that I don’t remember seeing before, or at least not as distinctly as they appeared this time. If I hadn’t been looking carefully I could have missed them but both sets of errors will be a lot more evident when the 2017 ERS/ATS DLCO standards are implemented.

The first error has to do with gas analyzer offsets. What alerted me was a set of irreproducible DLCO results.

Test 1: Test 2: Test 3: Test 4:
DLCO (ml/min/mmHg): 24.53 17.21 12.91 6.74
Inspired Volume: 1.99 2.06 2.32 2.26
VA (L): 3.83 3.52 3.63 2.60
Exhaled CH4: 43.27 49.19 54.80 74.14
Exhaled CO: 16.09 23.15 31.39 49.46

When I first looked at the graphs for each test, there wasn’t anything particularly evident until I pulled up the graph for the fourth DLCO test:

This graph showed that the baseline CH4 and CO readings were significantly elevated, but this hadn’t been evident in the previous tests.

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DLCO overestimated from an apparent zero offset error

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I’ve had some concerns for a while now about how the CO and CH4 concentrations are being calculated from the DLCO analyzer calibration zero offsets and gains on our test systems. For this reason I’ve been looking carefully at all of the raw data from our DLCO tests and today I came across an oddball test result. There are several reason why this is probably not the best example for this particular problem that I could come up with but it illustrates an important point and it’s in front of me so I’ll go with it.

In order to use the output from a gas analyzer you need to know the zero offset and the gain of the signal. Presumably the analyzer remains stable enough between the time it was calibrated and the time it is used for the zero offset and gain to be meaningful. When looking at the calibration data I’ve noticed that some of our test systems show relatively large changes in zero offset from day to day. These changes are still within the operating limits of the analyzer so no red flags have gone up over this. The test systems and analyzers are turned off over night so in order to see if the analyzers go through these kind of changes during a normal day I once did a series of calibrations each separated by five or ten minutes on one of the more suspect testing systems. What I saw was that although there were small changes from calibration to calibration, I didn’t see anywhere near the changes I’ve seen from day to day which at least implied that the analyzer remained reasonably stable during a given day.

Today a patient’s report came across my desk and as usual I took a look at the raw test results. What I saw was that two out of three of the DLCO tests had been performed with the correct inspired volume but that the one with a much lower inspired volume had a much larger VA and DLCO when compared to the other results. This got me scratching my head since the patient has severe COPD and that usually means that a lower inspired volume leads to a lower DLCO and VA. When I noticed the analyzer signals during the breath-hold period that’s when I could see right away why the results had been overestimated.

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DLCO Dilemma

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For the last several months I’ve noticed what appears to be a greater than normal number of patient test results where the VA volume from the DLCO test was greater than the TLC. This is not impossible of course, but it usually tends to be more on the rare side and when I’ve seen this in the past and inspected the results closely there were usually either problems with the lung volume test or the difference was only a few percent and within the error bar for both tests. We’ve been seeing VA’s that were larger than TLC more frequently lately but when I look at the results closely most of the time I have been unable to see anything wrong with either the lung volume or the DLCO test. At the same time, we have a number of patients that are frequent fliers and have seen what looks to be bigger differences in DLCO from visit to visit than usual as well as a number of patients that have had larger DLCO results than we would have expected.

The problem is that these apparent problems are really just suspicions with very little real evidence. I’ve been paying very close attention to lung volumes since our hardware and software upgrade last summer so my paranoia level is on the high side and I may well be overreacting. Late last week however, I found myself on the horns of a dilemma. The test results for a patient with a helium dilution TLC that was 68% of predicted but at the same time with a VA that was 93% of predicted and a DLCO that was 129% of predicted came across my desk.

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