The
Cohort Studies
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In a cohort study a number of healthy individuals
(the cohort) are
followed during a period of time. Either all individuals are exposed and the
frequency of the disease is compared with the frequency in the general
population, or the cohort could represent a random sample of the population. In
the latter case the suspected disease factor is asked for among those who have acquired the
disease at follow-up and those who have not. However, the rarity of glomerulonephritis
demands that a conventional cohort study should include at least 22000 exposed individuals to be
able to demonstrate a statistically significant increase of the disease (Churchill
et al 1983).
Instead
of recording new cases among healthy individuals, renal function and degree of
exposure can be recorded in patients with established disease.
In
one such study fifty patients with
glomerulonephritis were followed for 7-8 years. At follow-up nine of 26 patients, who were heavily exposed to
hydrocarbons,
had end-stage renal failure, but this was not seen in any of 24 moderately
or rarely exposed patients (Bell
et al 1985).
In a more
recent study 68 patients with glomerulonephritis were followed for five years. At follow-up 21
of 29 patients with progressive renal failure had been exposed to hydrocarbons,
whereas exposure was noted only in five of 39 patients with stable renal
function. Furthermore, it was noted that when the exposure was eliminated in
these five patients, their renal function improved (Yaqoob et al
1993b).
Together
with the case-control studies these two
studies show that the most important effect of hydrocarbon
exposure is a worsening of renal function. The logical conclusion is of course
that all patients with glomerulonephritis and renal failure should be questioned
in depth about possible toxic exposure and if such exposure is present, attempts
should be performed to eliminate the exposure.
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