Acceptable and Repeatable Spirometry Curves: ATS 2019 Criteria

A spirometry test can produce a technically valid-looking curve and still be worthless for medical surveillance if the effort behind it doesn't meet acceptability and repeatability standards. The 2019 ATS/ERS technical standard replaced the older 2005 criteria with tighter, more specific rules for grading test quality, and most modern spirometers now score sessions automatically using this framework. Understanding what the software is actually checking — and what an A, B, C, D, or F grade means for a compliance file — matters for anyone reviewing occupational spirometry results.

What makes a spirometry effort acceptable?

An acceptable effort under ATS/ERS 2019 has to clear several hurdles in sequence: a good start of test (minimal hesitation, sharp initial rise), no cough during the first second, no glottis closure or early termination, no evidence of a Valsalva maneuver, no leak around the mouthpiece, and no obstruction of the mouthpiece by the tongue or dentures. The exhalation also has to run long enough — either reaching a volume plateau (less than 0.025 L change over one second) or lasting at least 6 seconds for adults (3 seconds for children under 6). A curve that fails any single criterion isn't averaged in or partially counted; it's excluded from the session's usable efforts entirely, which is why technicians coach patients through multiple blows rather than accepting the first attempt.

What is back-extrapolation volume and why does it matter?

Back-extrapolation volume (BEV) is the technical measure of how clean the start of the test was. The spirometer draws a line through the steepest part of the volume-time curve and extrapolates it back to zero time, then measures the volume that would have been exhaled before that point — essentially quantifying hesitation or a slow ramp-up before the real effort begins. ATS/ERS 2019 sets the acceptable limit at the lesser of 150 mL or 5% of FVC. A high BEV means the patient started too slowly or hesitated, and the resulting FEV1 and PEF values will read artificially low even though FVC may still look reasonable — which is exactly the kind of error that produces a false "reduced" flag on an otherwise normal worker.

What does repeatability between efforts require?

Acceptability grades a single blow; repeatability compares the best two acceptable efforts against each other. Under the 2019 standard, the two largest FVC values must be within 0.150 L of each other, and the two largest FEV1 values must be within 0.150 L of each other — for patients with an FVC under 1.0 L, that tolerance tightens to 0.100 L. A session needs at least two acceptable, repeatable efforts to grade above an F, and three is the practical minimum most protocols require before accepting results into a medical surveillance record. If a patient can't produce two efforts within tolerance after multiple attempts, the session should be flagged for review rather than reported as a clean baseline.

Grade	Acceptable efforts	Repeatability (FVC and FEV1)
A	3 or more	Within 0.150 L (0.100 L if FVC < 1.0 L)
B	2	Within 0.150 L (0.100 L if FVC < 1.0 L)
C	2	Within 0.200 L
D	2	Within 0.250 L
E	1	Not applicable — only one acceptable effort
F	0	No acceptable efforts obtained

Does a low grade mean the test has to be repeated?

Not automatically, but it should trigger a closer look. A grade of C or D can still be clinically useful for trending an individual worker over years, since the software flags the imprecision rather than hiding it. Where grading matters most in an occupational setting is in surveillance programs under standards like OSHA's respirable crystalline silica rule, where spirometry results feed medical evaluations and fitness-for-duty decisions — a D-grade session used as a program baseline can produce years of miscalibrated year-over-year comparisons. The practical rule most occupational health providers follow: A and B grades go straight into the record, C and D get a supervisor or physician review before acceptance, and E or F sessions get rescheduled.

How Foster handles spirometry quality control

Foster Special Instruments trains and certifies technicians on ATS/ERS acceptability and repeatability criteria as part of every on-site spirometry program we run, and our equipment grades each session in real time so a low-quality effort gets flagged before the patient leaves the testing station rather than discovered weeks later during a records review. We also calibrate the spirometers themselves against a 3-liter syringe on a documented schedule, because a grading algorithm is only as good as the volume measurement feeding it. That combination — trained technicians plus verified instrument accuracy — is what keeps a multi-year surveillance dataset comparable instead of noisy.

Frequently asked questions

What changed between the 2005 and 2019 ATS/ERS spirometry standards?

The 2019 update tightened several thresholds, refined the end-of-test criteria to favor volume plateau detection over a fixed time cutoff alone, and restructured the grading scale from A-through-F with more explicit repeatability tiers. It also clarified back-extrapolation volume limits and gave more specific guidance on pediatric testing.

How many blows should a patient be asked to perform?

Most protocols start with three attempts and continue up to eight if acceptability or repeatability criteria aren't met, though testing typically stops if the patient is unable to produce two acceptable curves after a reasonable number of tries or shows signs of fatigue or dizziness.

Can a patient with a real lung condition still get a poor repeatability grade?

Yes — grading measures test quality, not lung function. A patient with genuine airway obstruction can still produce highly repeatable curves, while a healthy patient with poor technique can produce inconsistent ones. Repeatability grade and the underlying spirometry values (FEV1, FVC, FEV1/FVC ratio) are separate pieces of information that both need review.

Does a single F-graded session invalidate a whole surveillance record?

No — it just means that particular test date shouldn't be used for trending or clearance decisions. The patient should be rescheduled for a valid test, and the F-graded session noted rather than silently dropped from the file.

Who is qualified to grade spirometry results?

Modern spirometers grade acceptability and repeatability automatically per the ATS/ERS algorithm, but a NIOSH-approved spirometry course-trained technician should review flagged sessions, and a physician or other licensed health care professional should interpret results and any that fall below normal reference ranges. See our note on NIOSH spirometry certification requirements for technician credentialing.

Need a spirometry program with trained technicians and calibrated equipment behind every session? Request a quote or learn more about our on-site occupational health testing and respirator fit testing services.