Investigation of under-ascertainment in epidemiological studies based in general practice

Background One of the aims of the Study of Infectious Intestinal Disease (I ID) in England isto estimate the incidence of IID presenting to general pra ctice. This sub-study aims to estimate and correct the degree of under-asce rtainment in the national study. Methods Cases of presumed IID which presented to general practice in the na tional study had been ascertained by their GP. In 26 general practices, cas es with computerized diagnoses suggestive of IID were identified retrospect ively. Cases which fulfilled the case definition of IID and should have bee n ascertained to the coordinating centre but were not, represented the unde r-ascertainment. Logistic regression modelling was used to identify indepen dent 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 i n ascertainment between the practices (30% to 93%). Patient-related factors independently associated with ascertainment were: i) vomiting only as oppo sed to diarrhoea with and without vomiting (OR 0.37) and ii) consultation i n the surgery as opposed to at home (OR 2.18). Practice-related factors ind ependently associated with ascertainment were: i) participation in the enum eration 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 e xperience (OR 1.92). Predicted ascertainment percentages were calculated ac cording to practice characteristics. Conclusions 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 identi fy adjustment factors which could be applied to individual practices. Resea rchers need to be aware of factors which influence ascertainment in acute e pidemiological studies based in general practice.