FEV1FVCRatio

The FEV1/FVC ratio, also called the forced expiratory ratio (FER), is the proportion of the forced vital capacity (FVC) that is expelled in the first second of a maximal forced expiratory maneuver. It is most commonly derived from spirometry, where FEV1 is the volume exhaled in the first second and FVC is the total volume exhaled during a forced breath. The ratio is typically expressed as a unitless fraction or as a percentage.

Interpretation of the ratio depends on the clinical context and the overall spirometric pattern. In obstructive airway diseases such as asthma and chronic obstructive pulmonary disease (COPD), the FEV1 tends to be disproportionately reduced relative to FVC, leading to a lowered FEV1/FVC ratio. In restrictive diseases, both FEV1 and FVC are reduced but the ratio may be normal or even increased. Therefore the ratio alone does not establish a diagnosis; it must be interpreted alongside absolute values, other spirometric measures, and clinical information.

Diagnostic thresholds vary. A fixed cutoff of 0.70 (70%) is commonly used in some guidelines to indicate obstruction for COPD, but many clinicians prefer using the lower limit of normal (LLN) based on age, sex, height, and ethnicity to reduce misclassification in older adults. Post-bronchodilator testing can help distinguish reversible from irreversible obstruction; an improvement in FEV1 or FVC after bronchodilation may indicate asthma or reversible airway disease.

Limitations include dependence on effort and technique, as well as age-related changes. Proper quality control and reference standards are essential for accurate interpretation.