8.3 Evaluating Spirometry Acceptability & Repeatability

Validity is Domain II.C—not optional “nice to have”

NBRC II.C Evaluate Validity of Results task II.C.2 (spirometry, outline a/b) asks whether you can judge acceptability of individual efforts and repeatability across efforts. RPFT-level items often show a grid of trial values or describe a curve defect and ask: accept, reject, repeat, or report with caution. This section operationalizes ATS/ERS technical quality concepts used after you have performed maneuvers in 8.2.

Definitions to keep separate:

• Acceptability — Is this single effort free enough from defects to use its FEV1 and/or FVC?
• Repeatability — Among acceptable efforts, do the best values agree closely enough?
• Usability — Even if not fully ideal, can part of the data (often FEV1) still be reported with a quality statement?

Acceptability checklist (ATS/ERS quality concepts)

Use a mental (or written) checklist on every forced maneuver. Software grades help but do not replace technologist judgment when sensors mis-classify cough or leak.

Master acceptability table

Start-of-test detail: back-extrapolation volume (BEV)

A “good start” is quantified by back-extrapolation. If the patient hesitates, the volume–time curve rises slowly and the extrapolated zero-time error grows. When BEV exceeds the greater of 5% of FVC or 100 mL, the effort fails start-of-test acceptability for standards-based grading. Coaching fix (8.2): explosive onset after full inspiration.

Exam trap: High PEF with huge hesitation is uncommon; more often PEF is low and BEV is high—both point to poor start, not “instant severe restriction.”

Cough rules (high-yield)

• Cough in the first second: do not use that effort’s FEV1.
• Cough after the first second may still allow FEV1 use if the first second is clean, but FVC and the later curve can be compromised—judge FVC separately.
• Repeated coughing sessions may yield usable FEV1 with limited FVC—report with notes.

Early termination vs obstruction

Severe obstruction produces long emptying times; that is not the same as early termination. Early termination is a premature stop while significant volume remains (no plateau, short FET, patient stops cooperating). Do not label every low FVC as “acceptable obstruction pattern” without inspecting EOT quality.

Repeatability criteria

After you have acceptable efforts, check whether the best values cluster.

FEV1 and FVC agreement (ages ≥6 years—exam workhorse rule)

For patients 6 years and older, ATS/ERS technical standards commonly require that the two largest FEV1 values from acceptable efforts agree within 0.150 L (150 mL), and the two largest FVC values agree within 0.150 L. If they do not, perform more efforts (within fatigue/safety limits) to improve repeatability.

Younger children

Preschool and young pediatric cutoffs are tighter in relative or absolute terms depending on the standard version and age band (often 0.100 L or 10% type rules appear in pediatric pathways). For RPFT items, read the age in the stem: do not blindly apply 150 mL to a toddler if the question highlights pediatric criteria.

Which values are selected for reporting

• Report the largest FEV1 and largest FVC from acceptable efforts—even if they come from different maneuvers (classic rule).
• PEF is often taken as the largest PEF from acceptable starts; labs follow ATS/ERS and manufacturer reporting rules.
• Do not average all trials as the primary reported FEV1/FVC when standards call for maximum acceptable values.

Minimum number of acceptable efforts

Exam trap: “Must always have exactly three curves that are identical.” Reality: three acceptable efforts with FEV1/FVC within repeatability limits—not identical tracings.

Acceptability vs repeatability decision table

Grading and reporting when criteria are not fully met

Real patients—especially children, neuromuscular disease, acute illness, and elderly—do not always produce A+ sessions. Domain II.C expects mature judgment:

Do report (with transparency)

• Best usable FEV1 even if FVC failed EOT, when clinical need exists and defects are documented.
• Number of acceptable efforts, which criteria failed, and whether repeatability was met.
• “Session ended for dizziness after six attempts; two acceptable FEV1 within 140 mL; FVC not repeatable due to early stops.”

Do not

• Discard all numeric results solely because the session was imperfect if usable efforts exist and the lab/reporting standard allows graded reporting.
• Select a middle “pretty” curve that is lower than a higher acceptable FEV1.
• Edit raw points to force BEV or repeatability compliance.
• Report post-bronchodilator change using pre efforts that were not acceptable (see 8.4).

Quality grades (conceptual)

Many systems assign letter grades or star ratings based on number of acceptable efforts and repeatability. Know the concept: higher grades need multiple acceptable, repeatable efforts; lower grades flag caution for interpreters. Your narrative comment still matters when the grade is intermediate.

Waveform pattern recognition for validity

Special populations and validity

• Neuromuscular / supine testing: Acceptability rules still apply; patients may meet FEV1 usability before full FVC. Positional comparison is only meaningful if both positions have transparent quality.
• Severe obstruction: Long FET is expected; do not fail FVC solely because the patient blew longer than 6 seconds.
• Pediatrics: Prioritize free effort and start quality; use age-appropriate repeatability; document effort cooperation.

RPFT validity vignettes

Vignette A: Trials FEV1 = 2.50, 2.48, 2.20 L (all acceptable starts). Two largest differ by 0.02 L → repeatable. Report 2.50 L (largest).

Vignette B: Trials FEV1 = 3.10, 2.80, 2.78 L, all acceptable. Two largest differ by 0.30 L → not repeatable—obtain more efforts before finalizing.

Vignette C: Best-looking loop has cough at 0.5 s; another effort has clean first second but early stop. Use clean-first-second effort for FEV1; do not prefer the “pretty” coughed curve.

Vignette D: Only two acceptable efforts possible after eight tries; FEV1 within 100 mL. Report with grade/comment that only two acceptable efforts were obtained—do not fabricate a third curve.

Link to Domain III (preview)

Validity decisions feed reference comparisons and clinical calls in Domain III. A repeatable but poorly acceptable session misleads percent predicted as badly as a math error. Master II.C.2 so later interpretation questions start from trustworthy numbers.

Numbers and rules to memorize

• BEV ≤ max(5% FVC, 0.100 L) for good start
• No cough in first 1 second for FEV1 acceptability
• Repeatability (≈≥6 y): FEV1 and FVC within 0.150 L
• Goal ≥3 acceptable efforts; practical max ~8 attempts
• Report largest acceptable FEV1 and FVC (may be different trials)
• Incomplete sessions → grade + comment, don’t invent data

Link forward

Bronchodilator response studies (8.4) require valid pre and post sets under the same acceptability/repeatability logic, plus correct drug timing and significance thresholds from current ATS/ERS interpretive strategy documents.