International Journal of Epidemiology, Vol 28, 106-112, Copyright © 1999 by International Epidemiological Association
D Sethi, J Wheeler, LC Rodrigues, S Fox and P Roderick
BACKGROUND: One of the aims of the Study of Infectious Intestinal Disease
(IID) in England is to estimate the incidence of IID presenting to general
practice. This sub-study aims to estimate and correct the degree of
under-ascertainment in the national study. METHODS: Cases of presumed IID
which presented to general practice in the national study had been
ascertained by their GP. In 26 general practices, cases with computerized
diagnoses suggestive of IID were identified retrospectively. Cases which
fulfilled the case definition of IID and should have been ascertained to
the coordinating centre but were not, represented the under-ascertainment.
Logistic regression modelling was used to identify independent factors
which influenced under- ascertainment. RESULTS: The records of 2021
patients were examined, 1514 were eligible and should have been ascertained
but only 974 (64%) were. There was variation in ascertainment between the
practices (30% to 93%). Patient-related factors independently associated
with ascertainment were: i) vomiting only as opposed to diarrhoea with and
without vomiting (OR 0.37) and ii) consultation in the surgery as opposed
to at home (OR 2.18). Practice-related factors independently associated
with ascertainment were: i) participation in the enumeration study
component (OR 1.78), ii) a larger number of partners (OR 0.3 for 7-8
partners); iii) rural location (OR 2.27) and iv) previous research
experience (OR 1.92). Predicted ascertainment percentages were calculated
according to practice characteristics. CONCLUSION: Under- ascertainment of
IID was substantial (36%) and non-random and had to be corrected. Practice
characteristics influencing variation in ascertainment were identified and
a multivariate model developed to identify adjustment factors which could
be applied to individual practices. Researchers need to be aware of factors
which influence ascertainment in acute epidemiological studies based in
general practice.
ARTICLES
Investigation of under-ascertainment in epidemiological studies based in general practice
MRC Epidemiology and Medical Care Unit, Wolfson Institute of Preventive Medicine, St Bartholomew's and the Royal London Hospital School of Medicine and Dentistry, UK.
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