A two-stage screening strategy was applied in a single-handed general practice in northern Italy. The study was designed so that all the second-stage psychiatric interviews were conducted during the first phase of the study (group A patients), while self-report questionnaire (GHQ) and GP assessments only were collected during the second phase (group B patients).
The estimated true case rate of psychiatric disorder (36%) did not differ between the two phases of the study, but there were marked differences in the general practitioner's diagnostic behaviour. During the first phase (when psychiatric interviews were being conducted in the practice), her behaviour was heavily biased towards diagnosing psychiatric disorder; 61% of the patients were regarded by her as psychiatric cases and the hidden psychiatric morbidity rate was only 9%. During the second phase of the study (no psychiatric interviews in the practice), only 37% of the patients were regarded as psychiatric cases by the GP, and the hidden morbidity rate was 59%.
The results are interpreted as demonstrating that the presence of a psychiatrist conducting a two-stage screening survey in the practice had a profound effect on the diagnostic threshold. They also provide support for the use of the GHQ as a case-finding instrument in general practice.