5.2 ATS/ERS Spirometry Acceptability, Repeatability, and Usability Criteria

5.2 ATS/ERS Spirometry Acceptability, Repeatability, and Usability Criteria

To ensure that diagnostic spirometry yields reliable, clinically actionable data, the American Thoracic Society (ATS) and the European Respiratory Society (ERS) updated international standardized criteria for spirometry technical quality. The ATS/ERS 2019 standards dictate rigorous benchmarks for evaluating individual maneuver acceptability, between-maneuver repeatability, session usability, and result selection.

For the CPFT candidate, mastering these technical criteria is vital. Domain II of the CPFT examination places major emphasis on recognizing non-acceptable maneuver artifacts, applying mathematical quality control thresholds, and determining when a testing session satisfies diagnostic criteria.

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Individual Maneuver Acceptability Criteria

Acceptability criteria apply to every single FVC effort performed by the patient. A maneuver must satisfy quality standards across three distinct phases: the Start of Test, the Course of Test, and the End of Test (EOE).

 Volume (L)
   ^
4.0|             /-------------------- Plateau (<0.025 L for 1 sec)
   |            /  
3.0|           /   <- Continuous, unhesitating exhalation
2.0|          /    
1.0|   /|    /     
0.0|--/-+---/------------------------> Time (sec)
      ^     ^ 
     EV    t0 (Extrapolated Time Zero)
     (<0.100 L or 5% FVC)

1. Start of Test: Back-Extrapolation Volume ($EV$ / $V_{be}$)

To ensure that exhalation begins explosively from full TLC without hesitation, the start of test is evaluated using the back-extrapolation method:

• Mathematical Concept: A tangent line is drawn through the steepest slope of the volume-time curve (representing Peak Expiratory Flow Rate). The point where this line intersects the zero-volume baseline is designated as extrapolated time zero ($t_0$).
• Back-Extrapolation Volume ($EV$): The volume of air exhaled before $t_0$ is the back-extrapolation volume ($EV$ or $V_{be}$).
• ATS/ERS Standard: $EV$ must be $< 0.100\text{ L}$ ($100\text{ mL}$) or $< 5%\text{ of FVC}$, whichever value is greater.
• Clinical Significance: An $EV \ge 0.100\text{ L}$ (or $> 5%\text{ FVC}$) indicates false hesitation or a slow initial blow. Excessive $EV$ artificially depresses the $FEV1$ and $PEFR$, invalidating the maneuver.

2. Course of Test: Artifact-Free Execution

The maneuver must be free from technical and patient-induced artifacts throughout exhalation:

• No Coughing: No cough during the first second of exhalation. A cough in the first second distorts $FEV1$ measurement. (Coughs occurring after 1 second do not affect $FEV1$, though they may affect total $FVC$).
• No Glottic Closure or Premature Termination: Sudden cessation of flow caused by vocal cord closure or breath-holding invalidates the maneuver.
• No Mouthpiece Obstruction: The mouthpiece must not be occluded by the patient's tongue, teeth, or biting down.
• No Perioral Leak: Lips must maintain a tight seal around the mouthpiece throughout inspiration and exhalation.

3. End of Forced Expiration (EOFE) Criteria

The 2019 update renamed "end of test" to end of forced expiration (EOFE) and, importantly, retired the fixed 6-second minimum forced expiratory time. A maneuver satisfies EOFE when any one of the following is met, listed in order of preference:

• Expiratory volume plateau: volume change $< 0.025\text{ L}$ ($25\text{ mL}$) over a period of $\ge 1.0$ second.
• Forced expiratory time (FET) of $\ge 15$ seconds has been reached.
• Repeatable FVC: the FVC obtained is within the repeatability tolerance of, or greater than, the largest previously observed FVC (applied when the patient cannot or should not continue).

Retired number alert. "At least 6 seconds in adults, 3 seconds in children under 10" is the 2005 criterion. Many secondary review books still print it. Under ATS/ERS 2019 a 4-second blow that reaches a genuine 1-second plateau is acceptable, and a 7-second blow that never plateaus is not. Learn the 2019 rule as the standard and recognize the 2005 phrasing when a legacy item uses it.

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Between-Maneuver Repeatability Standards

Once individual maneuvers are confirmed to be acceptable, the technologist evaluates repeatability across maneuvers to verify consistent maximal patient effort. A minimum of 3 acceptable maneuvers must be performed.

Repeatability Thresholds

• Adults and Children > 6 Years:
  • The difference between the highest $FVC$ and the second highest $FVC$ must be $\le 0.150\text{ L}$ ($150\text{ mL}$).
  • The difference between the highest $FEV1$ and the second highest $FEV1$ must be $\le 0.150\text{ L}$ ($150\text{ mL}$).
• Children $\le 6$ Years:
  • The difference between the two largest $FVC$ values must be $\le 0.100\text{ L}$ ($100\text{ mL}$) or $10%$ of the highest value, whichever is greater.
  • The same limit applies to the two largest $FEV_1$ values.
• Very small vital capacities: when $FVC \le 1.000\text{ L}$, the adult tolerance also tightens to $0.100\text{ L}$.

The Rule of 8 Trials (Maximum Execution Limit)

If repeatability is not achieved after 3 acceptable efforts, testing continues until repeatability criteria are met. However, the technologist must adhere strictly to the Maximum 8-Trial Limit:

• Limit: No more than 8 FVC trials should be performed in a single testing session.
• Rationale: Executing more than 8 forced maneuvers leads to severe respiratory muscle fatigue, patient exhaustion, dynamic air trapping, and potentially exercise-induced bronchospasm. Exhaustion causes progressive drops in $FEV1$ and $FVC$, making repeatability impossible.
• Protocol when Repeatability Fails: If repeatability is not achieved after 8 trials, stop testing. Report the highest values obtained from acceptable maneuvers, document that repeatability standards were not met, and assign the appropriate ATS/ERS Usability Grade.

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Result Reporting and Selection Rules

A critical rule on the CPFT exam is how diagnostic values are selected for final reporting:

1. Reported FVC: The single highest FVC value obtained from any acceptable maneuver (even if it comes from a different trial than the highest $FEV1$).
2. Reported FEV1: The single highest FEV1 value obtained from any acceptable maneuver (even if it comes from a different trial than the highest $FVC$).
3. Reported FEF25-75% and Flow-Volume Curves: Selected from the single acceptable maneuver that produces the largest numerical sum of $FVC + FEV1$.

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ATS/ERS Usability and Quality Grading Matrix

The ATS/ERS 2019 standards established a standardized letter grading system (Grades A through F) to quantify the technical quality and degree of confidence in spirometry results:

For children aged $\le 6$ years the same structure applies with tighter limits: $0.100\text{ L}$ for grades A and B, $0.150\text{ L}$ for C, $0.200\text{ L}$ for D, and $> 0.200\text{ L}$ for E (or 10% of the highest value, whichever is greater).

Three points candidates routinely miss:

1. Grade B requires exactly two acceptable maneuvers. Three or more acceptable maneuvers within $0.150\text{ L}$ is Grade A, not Grade B.
2. $FEV_1$ and $FVC$ are graded separately. A session can be Grade A for $FEV_1$ and Grade C for $FVC$.
3. Grading applies to the whole set of pre-bronchodilator maneuvers, and again separately to the whole set of post-bronchodilator maneuvers — never to a single effort.

Usable versus Acceptable

The 2019 statement introduced usable as a category below acceptable. A curve is usable if it satisfies only the start-of-test requirements: an acceptable back-extrapolated volume and no cough in the first second. Usable-but-not-acceptable values may still be reported (Grade U) when nothing better exists, but the grade flags them as low confidence.