Overview
Begin with a Valid Measurement and an Appropriate Reference
A pulmonary function test is only as useful as the effort, equipment, and reference equation behind it.
A pulmonary function test is only as useful as the effort, equipment, and reference equation behind it. Before attaching a disease label to a number, confirm that the manoeuvres were technically acceptable, reproducible, and assigned a quality grade. A hesitant start, cough in the first second, early termination, leak, or inadequate inhalation can lower FEV1 or FVC. That is a performance problem until proven otherwise—not evidence of obstructive or restrictive disease. Spirometry measures: - FEV1: the volume forcibly exhaled during the first second after a full inspiration. - FVC: the total volume exhaled during that forced manoeuvre. - FEV1/FVC ratio: the proportion of the vital capacity expelled in the first second; this is the central airflow measure. The current interpretive framework compares the patient with an appropriate reference population using a z-score, which expresses how far the result sits from the predicted mean. A z-score at or above −1.645 is within the normal range; a value below −1.645 is below the lower limit of normal (LLN), or below the fifth percentile. Per cent predicted can look intuitive but is less...