Quick calculation of longitudinal FEV1 decline

Interpretation of results

The above Figure shows the observed FEV1 values in relation to the relative limit of longitudinal decline (LLD) (blue line) and two cross-sectional limits: the lower limit of normal (LLN) (burgundy) which represents the lower 5th percentile in healthy non-smokers, and the 0.1th percentile (yellow) which approximates 60% predicted to indicate a moderate airflow impairment (1).

If the last observation is below the LLD limit (blue line), either the decline in FEV1 may be excessive or the FEV1 variability may be higher than expected. The LLD takes into account the expected FEV1 within-person variation and the duration of follow-up. LLD is useful for early (1-8 years of follow-up) identification of those at risk of developing chronic excessive decline in FEV1, those who have experienced a large drop in FEV1 within a short period of time as has been observed in response to some occupational exposures, or those whose FEV1 variability is higher than expected. However, the aptness of the longitudinal limit depends on the spirometry quality. The longitudinal limit based on within-person variation of 4% (a default) is appropriate if the quality is very good (i.e., testing follows all ATS/ERS recommendations). Alternatively, for individuals or programs with lesser spirometry quality, the value of 6% should be used and an effort be made to improve spirometry quality (2). The American College of Occupational and Environmental Medicine (ACOEM) recommends a longitudinal limit based on an annual decline of 15%, which is comparable to LLD using within-person variation of 6%. For spirometry monitoring programs with computerized spirometry data, SPIROLA software can be used to determine and monitor the program’s average within-person variation and spirometer generated quality grades to maintain these at an acceptable level.

Beginning with 8 years, the regression line and slope provide better indicators than LLD on whether the observed decline is excessive and, in conjunction with the cross-sectional limits, indicate the likelihood of developing airflow limitation. The precision of the regression slope (i.e., average rate of decline) increases with the number of FEV1 measurements and duration of follow-up. Excessive decline established over 5 or more years is an important predictor of increased morbidity and mortality regardless of the level of lung function (4). The normal rate of FEV1 decline is about 30 ml/y. In individuals without obstruction or restriction at baseline, the rates of FEV1 decline of 60 to <90 ml/y and of ≥90 ml/y were associated with 1.6 (95%CI 1.1-2.6) and 2.2 (95% CI 1.3-3.5) times higher risk of death in comparison to those with the rate of decline <30 ml/y (5).

Both the limit of longitudinal decline and the regression slope, serve as a guide and should be considered together with other information (e.g., spirometry quality, respiratory symptoms, exposure to respiratory hazards such as tobacco smoking or occupational exposures, radiological findings). Before making a final decision, the quality of the baseline and final spirometry tests should be evaluated, and if needed spirometry should be repeated.

Note: SPIROLA Quick Calculation does not keep data. To keep longitudinal spirometry data use SPIROLA and the 'Create Empty Database' function. Alternatively, read spirometry data directly into SPIROLA database.