Using spirometry results in occupational medicine and research: Common errors and good practice in statistical analysis and reporting

Spirometry appears to be a simple and inexpensive method to measure disorders of the respiratory tract. In reality however, a simple spirometry test requires knowledge and skill to correctly conduct and evaluate the test and its results. This review addresses common misunderstandings in using, evaluating and reporting spirometry results in Occupational Health practice, clinical medicine and research. Results of spirometry need to be evaluated in relation to reference values. The factory medical officer has to decide first whether the test was technically correctly executed and is acceptable for medical interpretation. The next step is to compare results of the individual to published reference values. A 10% reduction of reference values for North Indians and Pakistanis and a 12 to 13% reduction for South Indians is recommended when Caucasian reference tables are used. In occupational health practice the worker's spirometry performance over time needs to be considered. Common errors in reporting summarized results, for instance from groups of workers, are the incorrect use of tests of significance and incorrect presentation of aggregated spirometry results. The loss of respiratory function is recommended as an indicator of difference between two groups. That way, early changes in function can seen without waiting for a drop of function below the usually used 80%-of-predicted limit. This procedure increases the sensitivity of medical surveillance. In research the more precise Lower Limit of Normal should be calculated and used. Correct reference equations, good patient coaching, decision on the technical quality (acceptability) of each spirometry test and critical re-evaluation of the machine's readout are essential parts of a correct spirometry test. A good understanding how results are calculated is crucial for further statistical evaluation