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Forced vital capacity, abbreviated FVC, is the total gas volume expelled in a forced expiration following maximal inspiration, under the stated maneuver and acceptance criteria. It is a difference between lung volumes, not the total lung capacity: residual lung volume remains after expiration. The 2019 technical statement defines the maneuver and separately evaluates whether its start and end support the reported quantities.^[ref]

An original compartment illustration starts with 6 L of lung gas and ends with 2 L. The expelled volume is 4 L. Starting with 8 L and ending with 4 L gives the same FVC, despite different absolute volumes. Thus absolute-volume ambiguity in spirometry applies even to a perfectly measured, fully completed expiration.

Duration also affects what is collected. For a constructed exponential emptying curve,At $V_{\infty }=4$ L, $\tau =2$ s and $T=6$ s, about 3.80 L has been expelled and about 0.20 L of this model's eventual expelled volume remains. This is an explanation of finite-time capture, not a universal breathing time requirement or a patient model.

Early termination bias in forced vital capacity can also change a ratio. Suppose a complete hypothetical maneuver has FEV1 of 2.4 L and FVC of 4.0 L, giving a FEV1 to FVC ratio of 0.60. If a recording ends later than the first second but captures only 3.0 L, treating that incomplete total as FVC would yield 0.80 with the same numerator. An apparently larger ratio can therefore result from losing late expired volume, rather than from an improvement in early emptying.

The technical statement distinguishes FEV1 and FVC acceptability because errors can affect the two quantities differently.^[ref] Reported volume, repeatability and maneuver completeness are distinct pieces of evidence. Agreement between repeated incomplete traces does not independently establish that their common endpoint is valid.

Spirometry measures the volume of air moved into or out of the respiratory system as a function of time. A spirometer records displaced gas directly or derives volume by integrating volumetric flow rate. The resulting trace describes gas movement at the mouth; it does not independently determine alveolar pressure, pleural pressure or lung compliance.^[ref]

Two important outputs of a forced expiration following maximal inspiration are forced vital capacity, the total volume expelled in that maneuver, and forced expiratory volume in one second, the volume expelled during its first second. Their ratio compares early emptying with the eventual expelled volume.^[ref] Equal final volumes can accompany substantially different emptying rates.

An original mathematical example sets expelled volumewhere the numerical time $t$ and time constant $\tau$ are in seconds. Both $\tau =1$ and $\tau =2$ give an asymptotic expelled volume of 4 L. Their first-second volumes are instead $4(1-e^{-1})\approx 2.53$ L and $4(1-e^{-1/2})\approx 1.57$ L. The corresponding ratios to the limiting volume are approximately 0.632 and 0.393. These smooth toy curves explain the measurements; they are not measured traces or diagnostic reference limits.

Spirometry measures volume differences. The gas left after maximal expiration is residual lung volume, which has not crossed the instrument. Consequently a volume trace alone does not determine total lung capacity: different absolute starting volumes can produce the same expelled-volume trace. This is the absolute-volume ambiguity in spirometry.

Timing, effort, leaks and early termination can alter what a trace represents. Distinguish an instrumentally repeatable signal from a maneuver that answers the intended physiological question. The primary technical statement treats equipment quality assurance and maneuver acceptability separately.^[ref]