The interpretation of spirometric measurements in epidemiologic surveys

Spirometry is a fundamental tool in pulmonary epidemiology. Published standards provide specific information on instrumentation, techniques of testing, calculation of results, and physiologic mechanisms. They do not provide guidelines on how to interpret the test results once obtained, e.g., on such questions as which predicted values to use, what value is abnormal, how many people in the general population are likely to be abnormal? It is with these questions that this paper is concerned. Standard normal values in wide use are not ideal but should be utilized for all subgroups of the population investigated, as well as for the comparison population. Conventional definitions of abnormality lack statistical rationale and result in many false-positive and some false-negative results. This is especially true for fixed ratios such as forced expiratory volume (-1 sec)/forced vital capacity <0.70. A statistical definition of abnormality would place 95% of the normal population above the lower limit (lower 95% confidence limit). Such a definition may be more applicable to overt than to early disease. The spirometric values of well-defined diseased groups must be characterized before it is possible to state that abnormality (a value below the lower limit of normal) means disease. The distribution of values (including the mean and standard deviation) should be used to assess epidemiologic risk factors. Prevalence rates separate abnormal from normal subjects. Such data may be used to identify those requiring preventive intervention, medical care, or compensation. Pathogenic mechanisms can be more easily isolated when abnormal subjects are separated from the majority of normal. Frequencies of spirometric abnormality in standard or normal populations serve as a reference to compare rates of similar abnormality in a population under investigation.