Came across an excellent post discussing forced vital capacity end-of-test (EOT) criteria. It is well worth the read and can be found here.
To give you an idea of what he has to say, here is how it begins ...
I was reviewing spirometry reports and noticed several patients in a row with an early termination of exhalation that was not reflected in the reported expiratory time. This is far from the first time I’ve noticed this but seeing several examples more or less at the same time got me curious about how the software was determining expiratory time.
The ATS/ERS standards are somewhat mute on how the total expiratory time of a spirometry effort is determined. The closest the standard comes to addressing this is the end-of-test (EOT) criteria:
“The volume–time curve shows no change in volume (<0.025 L) for >=1 s[econd]”
But this is meant primarily as a quality indicator to show the patient has really achieved an adequate exhalation and not as a definition of expiratory time. In fact total expiratory time is not defined in the ATS/ERS standards at all and this leaves it up to individual equipment manufacturers to determine when a spirometry effort is over. This is a problem because the ATS/ERS standards also say that patients should:
“exhale for >=3 s in children aged <10 yrs and for >=6 s in subjects aged >10 yrs.”
So despite not being defined, the length of a spirometry effort is a critical quality indicator.