Pseudoproteinuria in tolmetin-treated patients.

In a clinical study of the new nonsteroidal anti-inflammatory drug tolmetin, the urines of 5 patients reacted positively when tested for protein by the standard sulfosalicylic acid method. Pseudoproteinuria, due to the precipitation by acid of the major metabolite of tolmetin, was suspected. The urines were subsequently tested for protein by the following alternate methods: solubility of acid precipitate test, heat coagulation test, Upjohn modification of the sulfosalicylic acid test, biuret test, Albustix, and electrophoresis. All were negative for protein. These results confirmed that the reactions with the sulfosalicylic acid test did not indicate true proteinuria but pseudoproteinuria. It is suggested that patients who receive tolmetin be tested for proteinuria by methods other than those involving acid precipitation.