Case-Control Studies

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In a case-control study a number of patients with the disease in question (here glomerulonephritis) are asked whether they have been exposed to the risk factor (here toxic chemicals) more often than a similar number of control individuals without the disease and of the same age and sex. 

To-day, nineteen case-control studies of hydrocarbon exposure have been performed (Zimmerman et al 1975, Lagrue et al 1977, Ravnskov 1978, Ravnskov et al 1979, Finn et al 1980, van der Laan 1980, Franchini et al 1982, Ravnskov et al 1983, Bell et al 1985, Harrison et al 1986, Harrington et al 1989, Steenland et al 1990, Sesso et al 1990, Yaqoob et al 1992, Porro et al 1992, Stengel et al 1995, Nuyts et al 1995, Asal et al 1996, Huber et al 2000). In the large majority of these studies patients with glomerulonephritis had been exposed more often to hydrocarbons (mainly fuels, glues, paints, solvents and motor exhausts) than had the control individuals (Ravnskov 2000a). In a few studies the exposure difference was trivial, but two of them included only patients with early stages of the disease (van der Laan 1980, Harrington et al 1989), and two studies included one group of patients with normal or mildly reduced renal function and little exposure, and one group of patients with chronic renal failure and much exposure (Stengel et al 1995, Asal et al 1996). In four studies, that included patients with end-stage renal failure only, 60-80 per cent of the patients had been exposed (Zimmerman et al 1975, Finn et al 1980, Steenland et al 1990, Yaqoob et al 1992).

Thus, taking all studies together, a strong, inverse association was found between degree of exposure and renal function, and a direct association between degree of exposure and stage of the disease indicating that the most important effect of the exposure is a worsening of renal function (Ravnskov 2000a).

Patients with glomerulonephritis and normal or only slightly depressed renal function may not necessarily have been exposed more than most of us who live in a western society, whereas patients with glomerulonephritis and more advanced renal failure have been exposed more often, in particular patients with end-stage renal failure. 

However, even if two studies that included acute glomerulonephritis did not find an increased exposure to hydrocarbons we cannot exclude that such exposure also may start glomerulonephritis because some individuals may be more sensitive to hydrocarbon exposure than others. If so, a short, but intensive exposure may suffice to start the disease, but as most people experience such exposure once or perhaps several times in their life, the total exposure of patients with early glomerulonephritis may not necessarily exceed the exposure of most people. To know if a short exposure may start the disease it is necessary to study the timely association between the exposure and the onset of the disease. One of the case-control studies had addressed this question. As mentioned in the section named The postinfectious occurrence the acute onset of the disease was closely time-associated with exposure to organic solvents in ten of the fifteen patients.

Next section: The Cohort Studies