Serum versus Urinary Antimicrobial Concentrations in Cure of Urinary-Tract Infections

The relative importance of antimicrobial concentrations obtained in the serum and urine of broad-spectrum antibiotics in the treatment of urinary infections remains controversial. In a prospective study, 33 consecutive patients were treated with oral oxytetracycline, 250 mg four times a day. Antimicrobial activity could not be detected in the serum of either the 20 patients cured or the 13 regarded as failures. Bactericidal activity in the urine, however, was observed in 18 of 20 cured and five of 13 considered failures. The minimal inhibitory concentrations of the infecting organism to oxytetracycline correlated well with the clinical course of the patient. We conclude that cure of urinary infections depends upon antimicrobial concentrations in the urine rather than in the serum and that testing for antimicrobial sensitivity based on urinary levels should be available in clinical practice. (N Engl J Med 291:1159–1163, 1974)