For many years it has been felt that only the presence of at least 105 bacteria per ml in a clean-catch midstream urine specimen can provide acceptable laboratory support for a diagnosis of urinary tract infection. Recently this assumption has been challenged; some experts feel that 104 bacteria per ml or even fewer are an adequate indication of infection. Others believe that the presence of polymorphonuclear leukocytes plays an important role in the pathology and clinical manifestations of UTI. Still others point to cases of UTI in which the patient is asymptomatic and may or may not have leukocytes in the urine, but still has significant bacteriuria. It is not possible to define with complete assurance the minimum number of bacteria per millilitre of urine that is definitely associated with UTI, nor is it possible to assert that leukocytes must be present. General recommendations for reporting are given below.
Category 1: fewer than 104 bacteria per ml. Report as probable absence of UTI. (Exception: if fewer than 104 bacteria per ml are present in urine taken directly from the bladder by supra-pubic puncture or cystoscopy, report the identification and the result of the susceptibility test.)
Category 2: 104-105 bacteria per ml. If the patient is asymptomatic, request a second urine specimen and repeat the count. If the patient has symptoms of UTI, proceed with both identification and susceptibility tests if one or two different colony types of bacteria are present. Bacterial counts in this range strongly suggest UTI in symptomatic patients, or in the presence of leukocyturia. If the count, the quality of the urine specimen, or the significance of the patient’s symptoms is in doubt, a second urine specimen should be obtained for retesting.
Category 3: more than 10s bacteria per ml. Report the count to the physician and proceed with identification and susceptibility tests if one or two different colony types of bacteria are present. These bacterial counts are strongly suggestive of UTI in all patients, including asymptomatic females.
If more than two species of bacteria are present in urine samples in categories 2 and 3, report as “Probably contaminated; please submit a fresh, clean-catch specimen”.