Introduction: At times the specific assignment of a patient in a "double blind" trial can be disclosed, which may influence the final results of the study.

Objective: To evaluate the level of knowledge that investigators have about the treatment that each patient receives.

Methods: In the international clinical trial BRILLIANT (Blood pressure, Renal effects, Insulin control, Lipids Lisinopril And Nifedipine Trial), that attempts to compare the effects of lisinopril with those of nifedipine retard in hypertensive patients with diabetes mellitus type II and microalbuminuria over a period of 12 months, the medications are administered according to a classic double blind concealment procedure. During the study a basal level of the excretion of microalbuminuria is determined, and again at 6 and 12 months. A clinical evaluation is made at each visit. Upon completion of the study, and before being informed of the real aleatorial assignment, four of the participating investigators tried to predict the medication that a patient received based on the adverse reactions detected and the modifications of renal excretion of albumin based on pre-established criteria. A total of 33 patients were analyzed, and followed actively for 12 months, after which the aleatorization code was revealed.

Results: In 16 patients treated with lisinopril there was agreement with the predictions in 12-14 cases (75-87.5%). Of the 17 patients treated with nifedipine, it was predicted that 15 were treated with this drug. These results indicate that the double blind could have been revealed in 27-29 of the 33 randomized patients (82-88%).

Discussion: In order to improve the double blind method, one alternative is prevent investigators from knowing the clinical data relevant to the trial -in this case the microalbuminuria levels. Another alternative would be to separate the clinical management of the patient from the evaluation of the treatments.