Changes that occur in Na+K+ ATPase regulation and therefore in the membrane potential in cells from bipolar individuals, as compared to cells from unaffected control individuals, are utilized to provide a diagnostic assay for a bipolar disorder. The diagnostic assay may also or instead exploit the similarity of cells from bipolar patients to those of people already known to have a bipolar disorder. A similar diagnostic assay is provided for diagnosing unipolar disorder. The diagnostic assays may further involve manipulation of membrane potential by incubation of cells in K+-free buffer and/or incubation with one or more compounds that alter Na+K+ ATPase activity. Although a variety of cells may be used, the diagnostic assays preferably employ lymphoblasts or whole blood cells.