Objective: To determine how, with the aid of the database of the Transition Project (www.transitionproject.nl), one can calculate posterior probabilities for general practice that provide insight into the clinical significance of the simultaneous occurrence of two events (a symptom and a diagnosis, or two diseases) in general practice.
Method: The use of the 'International classification of primary care' (ICPC) to code both the patient's reason for encounter and the diagnosis of the general practitioner in the Transition Project has resulted in a database for the period 1985-2002 with a total of 201,127 patient-years, in which the posterior probability of a diagnosis in the presence of a complaint or symptom is available in the form of an odds ratio. Also in the case of the simultaneous occurrence of 2 episodes of care (comorbidity) in a patient it is now possible to determine whether the ratio between the prior and the posterior probability indicates a clinically relevant relationship or that it is a chance finding. Such posterior probabilities have been calculated for the conditions otitis media, hypertension in diabetes mellitus, shortness of breath and heart failure. In the calculation ofthe prior and posterior probabilities, only 'certain' diagnoses were used.
Results: For the diagnosis 'otitis media' in the age group 0-4 years, otalgia had the highest posterior probability (odds ratio: 15.77), with discharge from the ear taking second place (odds ratio: 8.59). 'Fever' contributed almost nothing. The odds ratio for hypertension in 45-74-year-old women with diabetes mellitus was 3.42. When the symptom was 'shortness of breath', the prior probability of heart failure in the age group 45-64 years was relatively low (2.0) but the posterior probability was relatively high (24.2). In this way, the combination of prior and posterior probabilities can provide support for the clinical work of the general practitioner. As a predictive variable for heart failure in the age group 65-74 years, 'ankle oedema' played an important role while 'fatigue' contributed nothing to the diagnosis. It was apparent from the database that the care for patients with heart failure often coincided with that for chronic diseases such as diabetes mellitus, hypertension, ischaemic heart disease, chronic obstructive pulmonary disease and atrial fibrillation. But the question whether there is a clinically relevant relationship could only be answered on the basis of the posterior probabilities: the highest odds ratio's were found for 'atrial fibrillation/atrial flutter' (32.5), 'chronic obstructive pulmonary disease' (22.5) and 'chronic skin ulcer' (20.2).
Conclusion: The calculation of prior and posterior probabilities on the basis of the database of the Transition Project makes it possible for general practitioners to determine the clinical relevance of their observations.