Drug-Resistant Tuberculosis: Inconsistent Results of Pyrazinamide Susceptibility Testing

To the Editor. —Pyrazinamide, in combination with isoniazid and rifampin, often plays a prominent role in the therapy of tuberculosis.1Frequently, pyrazinamide susceptibility testing is performed, and the results generally dictate if pyrazinamide will or will not be used as a component of treatment of or prophylaxis for drug-resistant tuberculosis. Recently, our institution had a nosocomial outbreak of drug-resistant tuberculosis. When identically paired isolates were sent to two reference laboratories, however, discordant pyrazinamide susceptibility results were reported. From 1991 to 1993, a total of 55 patients with tuberculosis resistant to rifampin, isoniazid, streptomycin sulfate, and ethambutol hydrochloride were identified by our institution.2Ten identically paired isolates ofMycobacterium tuberculosisfrom 10 different patients were sent to two reference laboratories for susceptibility testing. Restriction fragment length polymorphism (RFLP) analyses were performed by the Centers for Disease Control and Prevention (CDC), the Public Health Research Institute, and/or the Albert Einstein