DLCO overestimated from an apparent zero offset error

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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Tidal Expiratory Flow Limitation and NEP

Expiratory flow limitation exists when increasing expiratory effort fails to increase the expiratory flow rate. This can been seen when a patient exhales along the same flow-volume curve during quiet, tidal breathing as they do during a forced expiratory effort. Tidal expiratory flow limitation (EFLt) is a key concept in understanding dyspnea, hyperinflation and orthopnea in patients with COPD and in assessing the functional severity of COPD.


The notion that EFLt could be detected through flow-volume loops was proposed in the 1960’s but subsequent research has shown this approach to be unreliable for a variety of moderately esoteric reasons which include the differences in static lung recoil and airway resistance that occur following a deep inhalation, differences in time-dependent lung emptying and the viscoelastic forces in the lung. When tidal flow limitation as seen on flow-volume loops is compared to other techniques it is apparent that flow-volume loops overestimate the presence of EFLt by approximately one-third.

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Odd results from a leak during a DLCO test

One of our testing systems has been leaking during the breath-holding period of DLCO tests. This has been an on-again, off-again problem caused by a leaking balloon valve. The odd thing about this is that the results from tests that show a patient leak are often higher than the results where they don’t.

This is not the pattern I would have expected. My initial thought would be that in the DLCO tests with a leak, the patient is spending more time at a lung volume below TLC which in turn means a lower surface area. A lower surface area should lead to a lower DLCO but this doesn’t appear to be happening.

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Should PFT Lab staff be credentialed?

The issue of PFT staff licensure or certification has recently been a topic of discussion on the LinkedIn Pulmonary Function Studies group and the Diagnostics section of the AARConnect discussion board. The overwhelming majority of those posting comments have been in favor of some form of licensure or certification. CPFT (Certified Pulmonary Function Technician) and RPFT (Registered Pulmonary Function Technician) certification is discussed most often but not surprisingly on the AARConnect board most posters feel that RRT (Registered Respiratory Therapist) certification should be required as well. Continue reading


FEV6 (the volume of air forcefully exhaled after 6 seconds) has been proposed as a replacement or surrogate for FVC in spirometry. Given that using FEV6 would simplify and speed up the spirometry test this is a seductive notion.

The use of an expired volume with a fixed expiratory time as a replacement for FVC was proposed at least 25 years ago, although at that time FEV7 was proposed as being slightly more accurate than FEV6. The first reference values for FEV6 however, were not available until the results from NHANESIII study were made available in 1999 and most studies of FEV6 post-date that.

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