PRINCIPLES OF TOXICOLOGY - Biology East Borneo

518 EPIDEMIOLOGIC ISSUES IN OCCUPATIONAL AND ENVIRONMENTAL HEALTHTABLE 21.4 Incidence Data: Rate Ratio and Rate DifferenceDisease+ –+ A BExposure– C DIncidence of disease in exposed population = A/PY (person-year)Incidence of disease in unexposed population = C/PYRate ratio = A/PY divided by C/PYRate difference = (A/PY – C/Py)rate that is not due to the exposure. Therefore, standardization for age is necessary prior to makingcomparisons of the cancer rates between the two different populations.A variation on the SRR used extensively in Occupational Epidemiology is the standardizedmortality ratio (SMR) with regard to mortality or standardized morbidity ratio with regard to morbidity.First, the person years of exposure in the study group are multiplied times the rate of death froma particular disease in the reference or standard population (i.e., number of cases/number ofpersons in the standard population/year) to generate an expected number of cases of death fromthe disease. The SMR is the number of cases of death in the study population divided by thenumber of cases expected from the standard population (SMR = observed/expected × 100). In theexample described above, the incidence rate of lung cancer in worker populations without asbestosexposure is 1/1000 workers/year while the rate of lung cancer in an asbestos-exposed populationis 4/1000 workers/year with 1000 person-years. The expected number of lung cancer cases in theexposed worker group, using the unexposed worker population as the reference or standardpopulation, is 1 (i.e., 1000 person-years × 1/1000 per year). Therefore, the SMR for lung cancerwith asbestos exposure would be 4/1 × 100 or 400. A SMR greater than or less than 100 indicatesincreased or decreased risk of disease. SMRs and SRRs can be tested for statistical significanceusing the χ 2 test and a confidence interval.In occupational epidemiologic studies, the proportional mortality ratio (PMR) is frequently usedto measure the risk or association between exposure and disease. The PMR is used in settings whenonly the population and the cause of death (or disease) are known, not the number of person-years ofexposure. In a PMR study, the expected number of deaths due to a particular disease is computed usingthe proportion of these deaths in a standard population (such as the general population), rather thanthe death rate as in the SMR study. The PMR study results approximate those of a SMR study whenthe particular cause of death is relatively uncommon. A PMR greater than or less than 100 indicatesan increased or decreased risk of disease or death.In case–control studies, the total number and proportions of persons with and without a disease aredefined by the study design, rather than the true proportions in the total population. As such, usuallytrue rates of disease cannot be derived in case–control studies. Therefore, the rate ratio is not anappropriate measure of risk; the odds ratio is used as an approximation of the rate ratio for the risk ofdisease associated with exposure (Table 21.5). An odds ratio greater than or less than 1 indicates anTABLE 21.5 Case–Control Data: Odds RatioDisease+ –+ A BExposure– C DRate ratio = odds ratio = AD/BC