Spirometry Test Components for Asthma
The spirometry test for asthma comprises forced expiratory maneuvers that measure FEV1 (forced expiratory volume in 1 second), FVC (forced vital capacity), and the FEV1/FVC ratio, displayed as both flow-volume and volume-time curves to assess airway obstruction and reversibility 1.
Core Measurements
The essential spirometric parameters include:
- FEV1: The maximal volume of air exhaled in the first second from full inspiration, expressed in liters at body temperature and pressure saturated with water vapor (BTPS) 1
- FVC: The total volume of air forcefully exhaled from maximal inspiration 1
- FEV1/FVC ratio: The primary indicator of airway obstruction
- FEF25-75%: Forced expiratory flow between 25-75% of FVC, which reflects small airway function 1
Display Requirements
Both flow-volume and volume-time curves must be displayed for optimal quality control 1. The flow-volume display provides detail for the initial portion of the maneuver and helps assess effort magnitude, while the volume-time graph shows detail for the latter part of the maneuver and allows evaluation of test completion 1.
Technical Specifications
The spirometer must:
- Accumulate volume for at least 15 seconds
- Measure volumes up to 8 liters with accuracy of ±3% or ±0.050 L (whichever is greater)
- Display at least 0.25 seconds (preferably 1 second) before exhalation starts to calculate back-extrapolated volume
- Show the last 2 seconds of the maneuver to confirm satisfactory test completion 1
Reference Values and Interpretation
Results are compared against reference equations (GLI-2012 equations are recommended) and expressed as z-scores, with the lower limit of normal set at the 5th percentile (z-score of -1.645) 2. The report should include predicted values, measured values, percent predicted, and z-scores for each parameter 2.
Quality Control Elements
Multiple maneuvers (typically 3-8) are performed to ensure repeatability 1. The technician must visually inspect each maneuver for quality before proceeding, and the report should include technician comments on test quality and any relevant issues that may aid interpretation 2.