Spirometry and Flow-Volume Testing
What makes a maneuver acceptable
Spirometry depends on effort, technique, and coaching; acceptability criteria are the backbone of most questions.
An acceptable forced vital capacity maneuver has no artifact (no cough in the first second, no glottic closure, no leak around the mouthpiece, no variable effort, no early termination), a good start of forced exhalation (back-extrapolated volume ≤0.150 L or ≤5% of FVC), and a satisfactory end of test - a plateau (volume change <0.025 L over 1 second), sufficient exhalation duration, or a patient who genuinely cannot continue.
Obtain at least three acceptable maneuvers and, ideally, three that meet repeatability: the two largest FVC values within 0.150 L of each other and the two largest FEV1 values within 0.150 L (older standards used 0.200 L). Report the largest FVC and the largest FEV1 even when they come from different curves, and take the FEV1/FVC ratio from the trial with the largest sum of FVC plus FEV1.
Interpretation: obstruction when FEV1/FVC falls below the lower limit of normal - a fixed 0.70 threshold overdiagnoses older adults. Severity is graded from FEV1 percent predicted. A bronchodilator response requires an increase of at least 12% and at least 200 mL in FEV1 or FVC.
Flow-volume loops localize obstruction: variable extrathoracic - inspiratory limb flattening; fixed - both limbs; variable intrathoracic - expiratory limb flattening. FEF25-75 is effort-dependent and highly variable; never diagnose small-airways disease from it alone.
Trap: a cough after the first second may invalidate the FVC but not necessarily the FEV1.
Worked example: FEV1 2.00 L before and 2.30 L after albuterol = +15% and +300 mL - both thresholds met, positive response.