Double-blind procedure: an assessment in a study of lithium prophylaxis

Fifty-seven patients with primary affective disorder who were in a double-blind outpatient study to evaluate the prophylaxis of lithium were questioned as to whether they believed they were receiving lithium or placebo. Research nurses who were 'blind' to the patients' medication and a close relative living with each patient were also questioned. Nearly all patients (96%) said they beleived they were receiving lithium, ascribing this belief in 63% of the cases to a perceived improvement in condition. In only 14% of the cases on lithium was the presence of side-effects implicated in the patients' beliefs. One of the three nurses had a correct guess rate in excess of chance expectancy. All nurses tended to be more accurate in their guesses in the cases of patients who had been in prophylactic trials of over 15 months' duration than for patients with briefer periods in the study. Patients' relatives were, as a group, extremely accurate, their correct guess rate exceeding chance expectancy at the 0-001 level.