Diagnosing mental illness is a central role for psychiatrists. Correct diagnosis informs both treatment and prognosis, and facilitates accurate communication. We sought to explore how psychiatrists distinguished two common psychiatric diagnoses: bipolar disorder (BD) and borderline personality disorder (BPD).

We conducted a qualitative study of psychiatrists to explore their practical experience. We then sought to validate these results by conducting a questionnaire study testing the theoretical knowledge and practical experience of a large number of UK psychiatrists. Finally we studied the assessment process in NHS psychiatric teams by analysing GP letters, assessments by psychiatrists, and assessment letters.

There was broad agreement in both the qualitative and questionnaire studies that the two diagnoses can be difficult to distinguish. The majority of psychiatrists demonstrated in survey responses a comprehensive understanding DSM-IV-TR criteria although many felt that these criteria did not necessarily assist diagnostic differentiation. This scepticism about diagnostic criteria appeared to strongly influence clinical practice in the sample of clinicians we observed. In only a minority of assessments were symptoms of mania or BPD sufficiently assessed to establish the presence or absence of each diagnosis.

Clinical diagnostic practice was not adequate to differentiate reliably BD and BPD. The absence of reliable diagnostic practice has widespread implications for patient care, service provision and the reliability of clinical case registries.