Aims: 1) To compare diagnoses for threshold and subthreshold depressive and manic syndromes based on direct interviews to those relying on information from relatives; 2) to test the appropriateness of lowering the diagnostic threshold using the family history method in order to obtain comparable prevalence estimates regardless of the source of information; 3) identify factors that influence the likelihood of reporting disorders by informants.
Background: Given the widespread use of diagnostic information obtained from relatives, the assessment of agreement between diagnoses derived from a direct interview and family history information is crucial in order to estimate the risk of potential bias attributable to the use of surrogate information. Moreover, as unavailable relatives may be non-random with respect to psychopathological outcomes, the family history method is also frequently used in family studies for non-participating relatives, and therefore could also bias the results of these studies. Methods: Within a family study, 1625 distinct informant- index subject pairs were identified. Diagnoses from direct interviews of index subjects were compared with those derived from family history information provided by their first-degree relatives and spouses.
Results: 1) Inter-informant agreement was acceptable for mania, but low for all other threshold and subthreshold mood syndromes. 2) Except for mania, the family history method provided significantly lower prevalence estimates for mood syndromes. This gap also remained significant after lowering the diagnostic threshold for the mood syndromes using the family history method. 3) Except for mania, individuals who themselves had a history of a mood disorder were more likely to detect this disorder in their relatives.
Conclusions: Inter-informant reliability is acceptable only for mania, but not for other mood syndromes. Moreover, the higher likelihood of detecting disorders by affected informants, entails the risk of overestimation of the size of familial aggregation of mood disorders.