Unipolar relatives in bipolar pedigrees: a search for indicators of underlying bipolarity.

In an effort to identify features indicative of underlying bipolarity within the unipolar relatives of bipolar probands, we compared unipolar relatives of bipolars with unipolar relatives of unipolars. Using data from the Collaborative Study of the Psychobiology of Depression, we compared a number of demographic and clinical features individually, and then developed a logistic regression model for the differences found. Unipolar relatives of bipolars were somewhat more likely to be male and to have subthreshold bipolar features, and less likely to have panic symptoms. In addition, they had a small but significant decrease in the number of depressive symptoms and a large decrease in all treatment indicators. A multiple logistic regression model for these differences was highly significant, but had limited ability to discriminate between the two groups. These differences are not large enough to effectively discriminate between the groups for the purposes of classification. These particular results may result from a number of factors, most likely the choice of comparison group. Nonetheless, the work demonstrates a potential method for the construction of caseness indices for use in genetic studies of bipolar and other psychiatric disorders.