What are the ATS guidelines for interpreting spirometry results?

ATS Guidelines for Reading Spirometry

Spirometry interpretation requires systematic evaluation of test quality first, followed by assessment of acceptability and repeatability criteria, with the best values selected from at least three acceptable maneuvers that meet specific technical standards. 1

Quality Grading System

The ATS/ERS quality grading system (A through F) should be applied to assess confidence in results 2:

• Grade A: ≥3 acceptable tests with repeatability within 0.150 L (or 0.100 L if FVC <1.0 L)
• Grade B: ≥2 acceptable tests with repeatability within 0.150 L
• Grade C: ≥2 acceptable tests with repeatability within 0.200 L
• Grade D: ≥2 acceptable tests with repeatability within 0.250 L
• Grade E: Only one acceptable test achieved
• Grade F: No acceptable tests

Tests with grades A, B, or C are generally usable; grade D is suspect; grade E may only show values "within normal range" 2. Even lower-grade results can provide diagnostic information when carefully considered alongside clinical context, particularly to rule out disease.

Within-Maneuver Acceptability Criteria

Each individual spirogram must be "acceptable" by meeting ALL seven conditions 1:

1. Good start of test: Extrapolated volume (EV) must be <5% of FVC or <0.150 L, whichever is greater
2. No cough during first second that affects FEV₁ measurement
3. Satisfactory end of test: Either:
   • Duration >6 seconds for adults (>3 seconds for children <10 years), OR
   • Plateau in volume-time curve (volume change <0.025 L for ≥1 second), OR
   • Subject cannot/should not continue
4. No Valsalva maneuver (glottis closure) causing airflow cessation
5. No leak at the mouth
6. No obstructed mouthpiece (tongue, teeth, biting)
7. No extra breath taken during maneuver

Note: A "usable" curve only needs to meet conditions 1 and 2, while an "acceptable" curve requires all seven 1.

Between-Maneuver Repeatability Criteria

After obtaining acceptable spirograms, apply these tests 1:

• Minimum requirement: Three acceptable FVC maneuvers
• Repeatability achieved when:
  • Difference between largest and next largest FVC ≤0.150 L
  • Difference between largest and next largest FEV₁ ≤0.150 L
  • For FVC <1.0 L, both values must be ≤0.100 L

If criteria not met: Continue testing up to (but usually no more than) eight maneuvers 1. Large variability often indicates incomplete inhalations; brief rest periods may be needed between maneuvers.

Test Result Selection

Select the largest FVC and largest FEV₁ from all usable curves, even if they don't come from the same curve 1. This is critical—do not assume best values must come from a single maneuver.

Maneuvers with unacceptable start or cough (unusable curves) must be discarded before applying repeatability criteria. However, maneuvers with early termination or Valsalva may still be used for selecting largest FVC and FEV₁ 1.

Common Pitfalls to Avoid

Early termination: While maneuvers not meeting end-of-test criteria shouldn't count toward the three acceptable maneuvers, don't discard all results—FEV₁ may still be useful depending on exhalation length 1.

Prolonged exhalation: For obstructed patients, exhalation >6 seconds is often needed, but times >15 seconds rarely change clinical decisions and risk syncope, fatigue, and discomfort 1.

Repeatability vs. exclusion: Repeatability criteria determine when additional maneuvers are needed—they should NOT be used to exclude results from reports or subjects from studies. Label results that don't meet repeatability criteria but don't discard them 1.

Temperature correction: All values must be reported at BTPS (body temperature, ambient pressure, saturated with water vapor). Spirometer temperature should be measured for each maneuver, not assumed constant 1.

Equipment Validation

Spirometers should be validated using the 24 standard ATS waveforms with accuracy within ±3% of reading or ±0.050 L for FVC and FEV₁ 1.

Interpretation Framework

Once quality is established, interpret using z-scores and lower limit of normal (LLN) rather than fixed cutoffs like 80% predicted or FEV₁/FVC <0.70, which introduce age and sex bias 2, 3. The 2022 ERS/ATS technical standard recommends GLI reference equations with z-score cutoffs of -1.645 (LLN), -2.5, and -4 for severity grading 3, 4.