A spirometry quality grading system. Or is it?

A set of guidelines for grading spirometry quality was included with the recently published ATS recommendations for a standardized pulmonary function report. These guideline are similar to others published previously so they weren’t a great surprise but as much as I may respect the authors of the standard my first thought was “when was the last time any of these people performed routine spirometry?” The authors acknowledge that the source for these guidelines is epidemiological and if I was conducting a research study that required spirometry these guidelines would be useful towards knowing which results to keep and which to toss but for routine clinical spirometry, they’re pretty useless.

I put these thoughts aside because I had other projects I was working on but I was reminded of them when I recently performed spirometry on an individual who wasn’t able to perform a single effort without a major errors. The person in question was an otherwise intelligent and mature individual but found themselves getting more frustrated and angry with each effort because they couldn’t manage to perform the test right. I did my best to explain and demonstrate what they were supposed to do each time but after the third try they refused to do any more. About the only thing that was reportable was the FEV1 from a single effort.

This may be a somewhat extreme case but it’s something that those of us who perform PFTs are faced with every day. There are many individuals that have no problems performing spirometry but sometimes we’re fortunate to get even a single test effort that meets all of the ATS/ERS criteria. The presence or absence of test quality usually isn’t apparent in the final report however, and for this reason I do understand the value in some kind of quality grading system. But that also implies that the grading system serves the purpose for which it is intended.

In order to quantify this I reviewed the spirometry performed by 200 patients in my lab in order to determine how many acceptable and reproducible results there were. To be honest, as bad as I thought the quality problem was, when I looked at the numbers it was worse than I imagined.

The spirometry quality grading system is:

Grade: Criteria:
A ≥3 acceptable tests with repeatability within 0.150 L (for age 2–6, 0.100 L ), or 10% of highest value, whichever is greater
B ≥2 acceptable tests with repeatability within 0.150 L (for age 2–6, 0.100 L ), or 10% of highest value, whichever is greater
C ≥2 acceptable tests with repeatability within 0.200 L (for age 2–6, 0.150 L ), or 10% of highest value, whichever is greater
D ≥2 acceptable tests with repeatability within 0.250 L (for age 2–6, 0.200 L ), or 10% of highest value, whichever is greater
E 1 acceptable test
F No acceptable tests

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Calculating VA the mass balance way

One of the more significant changes that appeared in the 2017 ERS/ATS DLCO standards was the requirement that rapid-response gas analyzer (RGA) systems calculate VA using a mass balance approach. This is actually more straightforward than it sounds but it does raise several issues that weren’t fully addressed in the 2017 standards.

Up until this time VA has been calculated from the inspired volume and by the amount of dilution of the tracer gas in the exhaled alveolar sample. Specifically:


Which is described by:


VI = inspired volume

Vd = Anatomical and Machine deadspace

Fitrace = Inspired tracer gas concentration

FAtrace = Exhaled tracer gas concentration

The basic concept behind the mass balance approach to measuring VA is relatively simple and is described in the 2017 standard as:

…the tracer gas left in the lung at end exhalation is equal to all of the tracer gas inhaled minus the tracer gas exhaled.”

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ERS/ATS 2017 DLCO standards

The new ERS/ATS standards for DLCO testing were published in the January issue of the European Respiratory Journal. The article was published as open access and can be downloaded from the ERJ website.

The biggest difference between the new standards and those from 2005 is that they are now primarily oriented towards Rapid-response Gas Analyzers (RGA). The authors explicitly state that the new standards do not make older systems that use discrete alveolar sampling and slower gas analyzers obsolete, but many of the new suggestions and requirements for labs and manufacturers require systems with a RGA.

The differences between the 2017 and 2005 standards that I’ve been able to find include:

♦ Flow accuracy was not specified in the 2005 standard but is now required to be ± 2% over a range of ± 10 L/sec.

♦ Volume accuracy is now required to be ± 2.5% (± 75 ml) instead of ± 3.5%. Notably the 2005 standard included a ± 0.5% error in the calibrating syringe. The accuracy of the 3-liter syringe is now stated separately. In the 2005 standard volume accuracy was over an 8-liter range. No volume range is specified in the 2017 standard.

♦ RGA response time (analyzer rise time) had not previously been specified but is now required to be ≤150 milliseconds. Sample transit time was discussed but no specific recommendations were made. Sample transport issues such as Taylor dispersion, gas viscosity and turbulence at gas fittings was also discussed and although it was suggested that manufacturers attempt to minimize these effects no specific recommendations were made.

♦ Analyzer linearity for both RGA and discrete sample systems has been relaxed to ± 1.0% in the 2017 standards from ± 0.5% in the 2005 standards.

♦ CO analyzer accuracy for both RGA and discrete sample systems is now specified as ≤10 ppm (which is ±0.3% of 0.3% CO). It was previously specified as ± 0.0015% (which is ± 0.5% of 0.3% CO).

♦ Interference from CO2 and water vapor for both RGA and discrete sample systems is now specified as ≤10 ppm error in CO (when CO2 and water vapor are ≤5%). Interference was recognized as a problem in the 2005 standard but error limits were not specified.

♦ Digital sampling rate was not discussed or specified in the 2005 standards. It is now specified as a minimum of ≥100 hz with a resolution of 14 bits. A 1000 hz sampling rate is recommended.
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