Retrospective cohort studies

Observational studies Prospective cohort study Prospective case-control study Retrospective cohort study Retrospective case control study... 

The problems often encountered in retrospective cohort studies include poor exposure data and incomplete follow-up of all individuals. The accuracy of health outcome data may also be low. 

Retrospective Study—A type of cohort study based on a group of persons known to have been exposed at some time in the past. Data are collected from routinely recorded events, up to the time the study is undertaken. Retrospective studies are limited to causal factors that can be ascertained from existing records and/or examining survivors of the cohort. 

Several retrospective cohort studies of workers exposed to unquantified levels of trichloroethylene have been conducted. All of these studies have limitations that restrict their usefulness for evaluating the carcinogenicity of trichloroethylene. None has shown clear, unequivocal, evidence that trichloroethylene exposure is linked to increased cancer risk. 

In contrast, three European studies have found slight but statistically significant increases in cancer in workers exposed to trichloroethylene. A survey of Finnish workers exposed to primarily trichloroethylene found an association of limited statistical significance between exposure and incidence of stomach, liver, prostate, and lymphohematopoietic cancers (Antilla et al. 1995). However, the study did not reliably separate the effects of individual solvents, so attributing these cancers to trichloroethylene exposure alone was not possible. A significant association between workplace exposure to trichloroethylene and kidney cancer was found in a retrospective cohort study of German cardboard factory workers (Henschler et al. 1995). The... 

Gerhardsson L, Lundstrom NG, Nordberg G, et al. 1986b. Mortality and lead exposure A retrospective cohort study of Swedish smelter workers. Br J Ind Med 43 707-712. 

Lima N, Guerrant RL, Kaiser DL, Germanson T, Farr BM A retrospective cohort study of nosocomial diarrhea as a risk factor for nosocomial infection. J Infect Dis 1990 161 948-952. 

Nienhuis H, Goldacre M, Seagroatt V, et al. Incidence of disease after vasectomy a record linkage retrospective cohort study. BMJ 1992 304 743-6. 

Another large retrospective cohort study followed 3916 smelter workers and reported an overall standardized mortality ratio of 372." Lung cancer mortality was related to intensity of exposure but not to duration. Histologic types of lung carcinomas were similar to those seen in smokers. 

A retrospective cohort study of 6635 male workers employed for more than 15 years in seven Chinese factories found that the SMRs for lung and liver cancer among those highly exposed to CTPV were 4.3 and 2.25, respectively."... 

A cohort study of 5668 NG-exposed workers found an increased standardized mortality ratio for deaths from ischemic heart disease. The increase was more pronounced for those with 10 or more years of exposure and was statistically significant for the 40- to 49-year age group, whereas a deficit of cardiovascular mortality had been anticipated because of preplacement and annual medical examinations designed to exclude persons with cardiovascular abnormalities. These results were confirmed in a retrospective cohort mortality study that found a significant excess of ischemic heart disease mortality among workers actively exposed to NG and under the age of 45. ° (Note this study failed to detect a chronic cardiovascular effect as excess risk was only associated with workers actively exposed to NG.)... 

Epidemiological studies have different strengths and weaknesses associated with their design (Table 4) shows some of the strengths and weaknesses of the two main methods of prospective cohort studies and retrospective case-control studies. 

Cragle DL, Hollis DR, Newport TH, et al. 1984. A retrospective cohort study among workers occupationally exposed to metallic nickel powder at the Oak Ridge Gaseous Diffusion Plant. In Sunderman FW Jr, Aitio A, Berlin A, eds. Nickel in the human environment. lARC scientific publication no. 53. Lyon, France International Agency for Research on Cancer 57-64. 

A 2004 report by the Institute of Medicine s Immunization Safety Review Committee concluded that available evidence favored rejection of a causal relation between thimerosal-containing vaccines and autism. In like manner, a recent retrospective cohort study conducted by the CDC did not support a causal association between early prenatal or postnatal exposure to mercury from thimerosal-containing vaccines and neuropsychological functioning later in childhood. 

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