Cohort study

When the data as a whole are reviewed for studies on humans exposed to ethylene oxide, no conclusion can be made that there is an increase in mortahty associated with those exposed to ethylene oxide. Two Swedish studies (247,248) indicated an increase in leukemia for workers exposed to multiple chemicals including ethylene oxide however, in a recent larger Swedish study (249) of workers exposed to only ethylene oxide, there was no association of any type of cancer increase for these workers. In a recent study sponsored by NIOSH, there was no significant increase in mortahty observed for cancer when all types are combined or for certain individual types of cancer, even for those people who worked the longest and were observed the longest. However, a statistically significant increase in mortahty from certain types of lymphoma was observed for male workers. This is contrary to the results observed for female workers. In addition, four other cohort studies of ethylene oxide-exposed workers have been pubhshed (250—253), but no unequivocal increase in the risk of cancer was observed. 

The benefit of a prospective cohort study is the possibility for accurate exposure assessment. However, these are not common, because many occupational diseases (including cancers which are being intensely investigated currently) require long exposure times to develop. It is not practical or ethical to wait for decades before one obtains the result. 

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. 

The two basic types of analytical studies are the cohort and the case-control study. Each has strengths and weaknesses as well as different resource and time requirements. The cohort study involves the study of indi iduals classified by e.xposure characteristics, e.g., a group of welders. The study then follows the development of disease in the welders group as well as in an unexposed comparison population. The measure that assesses the magnitude of... 

Cohort Study—A type of epidemiological study of a specific group or groups of people who have had a common insult (e.g., exposure to an agent suspected of causing disease or a common disease) and are followed forward from exposure to outcome. At least one exposed group is compared to one unexposed group. 

Prospective Study—A type of cohort study in which the pertinent observations are made on events occurring after the start of the study. A group is followed over time. 

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. 

Data from a number of clinical cohort studies and clinical trials iUusfrates that HlV-1 is predominantly classified as R5 in tteatment-naive patients (Brumme et al. 2005 Moyle et al. 2005). In tteatment-experienced patients (with low nadir CD4- -count), there is an increase in CXCR4-using virus, which is almost entirely due to an increase in D/M virus (Moyle et al. 2005 Melby et al. 2006a). Pure X4 virus remains rare and indeed, even in treatment-experienced patients, 47-62% of patients continue to have only R5 virus. 

In two published cohort studies, pegylated IFN-a was effective in approximately one-third of HBeAg-positive patients in whom standard IFN-a or lamivudine had failed (Flink et al. 2006a Leemans et al. 2006). 

In HBeAg-negative patients, 12 weeks of lamivudine monotherapy followed by 36 months of pegylated lFN-a2b therapy (including 3 months of concomitant administration) has been compared with 48 weeks of lamivudine monotherapy (Vassiliadis et al. 2007). At the end of follow-up, the rate of ALT normalization was significantly higher with the sequential therapy than with lamivudine alone. No difference in the proportion of patients with undetectable HBV DNA was observed. Similar results were obtained in a Chinese cohort study (Shi et al. 2006). 

McArthur JC (2004) HIV dementia an evolving disease. J Neuroimmunol 157(l-2) 3-10 McArthur JC, Hoover DR, BaceUar H, MUler EN, Cohen BA, Becker JT, Graham NM, McArthur JH, Seines OA, Jacobson LP et al (1993) Dementia in AIDS patients incidence and risk factors. Multicenter AIDS Cohort Study. Neurology 43(ll) 2245-2252 McManus CM, liu JS, Hahn MT, Hua LL, Brosnan CE, Berman JW, Lee SC (2000) Differential induction of chemokines in human microgUa by type I and II interferons. GUa 29(3) 273-280 McQuibban GA, Butler GS, Gong JH, BendaU L, Power C, Clark-Lewis I, OveraU CM (2001) Matrix metaUoproteinase activity inactivates the CXC chemokine stromal ceU-derived factor-1. J Biol Chem 276(47) 43503 3508... 

Sacktor N, Lyles RH, Skolasky R, Kleeberger C, Seines OA, Miller EN, Becker JT, Cohen B, McArthur JC (2001) HIV-associated neurologic disease incidence changes Multicenter AIDS Cohort Study, 1990-1998. Neurology 56(2) 257-260... 

McDermott DH, Zimmerman PA, Guignard F, Kleeberger CA, Lehman SF, Murphy PM (1998) CCR5 promoter polymorphism and HIV-1 disease progression. Multicenter AIDS Cohort Study (MACS). Lancet 352 866-870... 

Bacellar H, Munoz A et al (1994) Temporal trends in the incidence of HIV-l-related neurologic diseases Multicenter AIDS Cohort Study, 1985-1992. Neurology 44(10) 1892-1900 Banki K, Hutter E et al (1998) Molecular ordering in HIV-induced apoptosis. Oxidative stress, activation of caspases, and cell survival are regulated by transaldolase. . J Biol Chem 273(19) 11944-11953... 

McArthur JC, Hoover DR, Bacellar H, Miller EN, Cohen BA, Becker JT, Graham NM, McArthur JH, Seines OA, Jacobson LP et al (1993) Dementia in AIDS patients incidence and risk factors. Multicenter AIDS Cohort Study. Neurology 43(ll) 2245-2252... 

Parra A, Kreiter KT, Williams S, Sciacca R, Mack WJ, Naidech AM, Commichau CS, Fitzsimmons BF, Janjua N, Mayer SA, Connolly Jr. ES, Effect of prior statin use on functional outcome and delayed vasospasm after acute aneurysmal subarachnoid hemorrhage a matched controlled cohort study. Neurosurgery 2005 56 476 84 [discussion 476 84]. 

Craig J, Chua R, Russell C, Wootton R, Chant D, Patterson V. A cohort study of early neurological consultation by telemedicine on the care of neurological inpatients. J Neurol Neurosurg Psychiatry 2004 75 1031-1035. 

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... 

Axelson O, Andersson K, Hogstedt C, et al. 1978. A cohort study of trichloroethylene exposure and cancer mortality. J Occup Med 20 194-196. 

Axelson O, Selden A, Andersson K, et al. 1994. Updated and expanded Swedish cohort study on trichloroethylene and cancer risk. J Occup Med 36 556-562. 

Shindell S, Ulrich S. 1985. A cohort study of employees of a manufacturing plant using trichloroethylene. 

Tola S, Vilhunen R, JarvinenE, et al. 1980. A cohort study on workers exposed to trichloroethylene. J Occup Med 22 737-740. 

GOLDBOHM R A, HERTOG M G L, BRANTS HAM, VAN POPPEL G and VAN DEN BRANDT P A (1996) Consumption of black tea and cancer risk a prospective cohort study , JNatl Cancer Inst, 88, 93-100. 

KNEKT p, JARIVNEN R, REUNANEN A, MAATELA J (1996) Flavonoid intake and coronary mortality in Finland a cohort study, British Medical Journal, 312, 478-81. 

Data concerning gastric cancer are scarce. The prospective Netherlands Cohort Study found no correlation between lutein dietary intake and gastric cancer risk, whereas findings from the Physicians Health Study and the ATBC study reported no effect of P-carotene on gastric cancer incidence. Two case-control studies and three intervention trials (ATBC, CARET, and the Physicians Health Study ) showed no association of P-carotene, lycopene, lutein, zeaxanthin, and P-cryptoxanthin. 

Dauchet, L. et al., Fruit and vegetable consumption and risk of stroke a meta-analysis of cohort studies, Neurology, 65, 1193, 2005. 

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