Glossary

A reference for the clinical and statistical terms used throughout pft. Every definition is keyed to the canonical source where the term is formally defined.

Measure abbreviations

FEV1 — Forced Expiratory Volume in 1 second. The volume of air expired during the first second of a forced expiratory maneuver, in litres.

FVC — Forced Vital Capacity. The total volume of air expired during a forced exhalation from full inspiration, in litres.

FEV1/FVC — Ratio of FEV1 to FVC. Dimensionless; typical adult values are 0.70 – 0.85.

FEF25-75 — Forced Expiratory Flow between 25% and 75% of FVC. Litres per second.

FEF75 — Forced Expiratory Flow at 75% of expired FVC.

FRC — Functional Residual Capacity. The volume in the lungs at the end of normal tidal expiration.

TLC — Total Lung Capacity. The maximum volume of air in the lungs after full inspiration.

RV — Residual Volume. The volume remaining in the lungs after maximum exhalation.

RV/TLC — The ratio of residual volume to total lung capacity.

ERV — Expiratory Reserve Volume.

IC — Inspiratory Capacity.

VC — Vital Capacity (slow VC, distinct from FVC).

TLCO / DLCO — Transfer factor / diffusing capacity for carbon monoxide. “TLCO” is the SI-units name; “DLCO” is the traditional-units name; they describe the same physiological measure.

KCO — Carbon-monoxide transfer coefficient, equal to TLCO/VA (or DLCO/VA). Expressed in SI or traditional units accordingly.

VA — Alveolar volume, used in the single-breath DLCO maneuver.

Statistical terms

M (median / predicted value) — The age-, sex-, and demographic- adjusted central value for a measure. In the GLI LMS framework, the expected value for a healthy individual matching the inputs.

S (coefficient of variation) — A scale parameter from the LMS framework that captures the spread of healthy values at each age/sex combination.

L (skewness / Box-Cox transform) — A shape parameter from the LMS framework that adjusts for non-normality of the underlying distribution.

LMS method — A statistical framework (Cole TJ. Stat Med. 1988;7(3):305-12) that models reference distributions via three age-varying parameters L, M, S. Used by the GLI equations for FEV1, FVC, FEV1/FVC, TLCO, lung volumes, and others.

LLN (lower limit of normal) — The 5th percentile of the reference distribution: the value below which a healthy individual falls only 5% of the time. Equivalent to a z-score of −1.645.

ULN (upper limit of normal) — The 95th percentile, equivalent to z-score +1.645.

z-score — A measure of how far an observed value is from the predicted median, expressed in standard deviation units. In the LMS framework: $$z = \frac{(measured / M)^L - 1}{L \cdot S}.$$ Implemented by the <measure>_zscore outputs of pft_spirometry(), pft_volumes(), and pft_diffusion().

Percent predicted — (measured / M) × 100. A traditional expression of departure from predicted; superseded by z-scores in the Stanojevic 2022 standard but still widely used in clinical practice.

Patterns and clinical entities

Normal — All measured values at or above their LLN.

Obstructed — FEV1/FVC below LLN with TLC at or above LLN (or unknown).

Restricted — TLC below LLN with FEV1/FVC at or above LLN.

Mixed — Both FEV1/FVC and TLC below their LLNs.

Non-specific pattern — Low FVC with normal FEV1/FVC and normal TLC. By definition not restrictive (TLC is normal) and not obstructive (FEV1/FVC is normal). The label is descriptive only; Stanojevic 2022 Table 5 enumerates clinical contexts.

PRISm — Preserved Ratio Impaired Spirometry. Low FEV1 with FEV1/FVC at or above LLN. A spirometry-only screening label (no TLC required).

Dysanapsis — A normal-variant pattern with normal FEV1, high FVC, and low FEV1/FVC. Listed in Stanojevic 2022 Table 5 but not emitted as a separate label by pft_classify() (folded into “Obstructed” when FEV1/FVC is below LLN).

Tests and gradings

BDR (bronchodilator response) — Per Stanojevic 2022: a change of more than 10% of the predicted value in FEV1 or FVC between pre- and post-bronchodilator measurements. Implemented by pft_bdr(). Replaces the 2005 standard (≥12% AND ≥200 mL from baseline).

Severity grading — Per Stanojevic 2022, a uniform three-level system applied to any z-score:

Grade	z-score
Normal	z ≥ −1.645
Mild	−2.5 ≤ z < −1.645
Moderate	−4 ≤ z < −2.5
Severe	z < −4

Implemented by pft_severity().

GOLD COPD severity — Per the Global Initiative for Chronic Obstructive Lung Disease, in patients with confirmed airflow obstruction:

Grade	FEV1 % predicted
GOLD 1	≥ 80%
GOLD 2	50 – < 80%
GOLD 3	30 – < 50%
GOLD 4	< 30%

Implemented by pft_gold().

CCS (conditional change score) — A z-score-style index of whether the change between two measurements computed as (z2 − r * z1) / sqrt(1 − r^2) exceeds the within-subject variability expected by regression-to-the-mean alone. |CCS| > 1.96 is the Stanojevic 2022 two-sided 95% normal-limits threshold (Box 2). Implemented by pft_change().

Spirometry quality grade (A–F) — Per Graham et al. ATS/ERS 2019, a grade based on the number of acceptable maneuvers from a session and the difference between the two best values:

Grade	Acceptable	Best-two diff (adult)	Best-two diff (child ≤ 6)
A	≥ 3	≤ 0.150 L	≤ 0.100 L
B	2	≤ 0.150 L	≤ 0.100 L
C	≥ 2	≤ 0.200 L	≤ 0.150 L
D	≥ 2	≤ 0.250 L	≤ 0.200 L
E	≥ 2	> 0.250 L, or 1 maneuver	> 0.200 L, or 1 maneuver
F	0	n/a	n/a

The child thresholds (column “Best-two diff (child ≤ 6)”) are additionally floored at 10% of the highest measured value per Graham 2019 Table 10’s footnote.

Implemented by pft_quality().

Notation: the 4-character pattern combination

pft_classify() emits an ats_pattern_combination column with a four-character string. Each character is A (below LLN) or N (at or above LLN), in the fixed order:

1. FEV1
2. FVC
3. FEV1/FVC
4. TLC

So "NNAN" means normal FEV1, normal FVC, abnormally low FEV1/FVC, normal TLC. This is by definition pure airway obstruction.

Race / ancestry categories (GLI 2012 only)

GLI 2012 distinguishes five ancestral groups for spirometry:

• Caucasian — European-ancestry populations.
• AfrAm — African American.
• NEAsia — North-East Asian (Han Chinese, Japanese, Korean).
• SEAsia — South-East Asian.
• Other/mixed — A multi-ethnic composite category constructed by the GLI Task Force from populations not captured by the four above.

GLI Global 2022 is race-neutral; the race column is ignored when calling pft_spirometry(year = 2022).

Common validation errors

If your cohort is unexpectedly all-NA or you see a warning about unrecognised inputs, check the following before anything else:

Sex must be canonical "M" / "F". Common dataset values like "male", "Male", "MALE", "m", "f", "Female", "woman", "boy", "girl" are auto-normalised with a warning. Anything else (e.g. "Unknown", "X", "NB") is set to NA. Prior to this behaviour any value other than "M" was silently treated as female; make sure your data isn’t relying on that.

Race must be one of the five GLI 2012 categories. Common variants like "caucasian" (lowercase), " Caucasian" (whitespace), "white", "black", "African American", "european" are auto-normalised with a warning. Anything else ("Asian" ambiguous between NEAsia/SEAsia is mapped to NEAsia; other strings like "Hispanic", "Latino", "Native American" are not in the GLI 2012 framework) is set to NA.

year = 2012 without a race column errors rather than silently producing all-NA output. Either supply a race column or call pft_spirometry(data, year = 2022) for the race-neutral equations.