In general, screening methods are insensitive at levels below 10* CFU/mL. Therefore, they are not acceptable for urine specimens collected by suprapubic aspiration, catheterization, or cystoscopy. Screening methods may also fail to detect a significant number of infections in symptomatic patients with low colony counts (102 to 103 CFU/mL) such as young, sexually active females with acute urethral syndrome. Further complicating the laboratory's decision whether to adopt a screening method is whether screening results will be used to rule out infection in asymptomatic patients. Under these circumstances, testing for pyuria is essential.
Therefore, given the importance of the 102 CFU/mL count and the PMN count, no screening test should be used indiscriminately. Selecting a screening method largely depends on the laboratory and the patient population being served by the laboratory. For example, there will be a cost advantage in screening urine in laboratories that receive many culture-negative specimens. On the other hand, urine from patients with symptoms of UTI plus a selected group expected to have asymptomatic bacteriuria should be cultured. For example, patients in their first trimester of pregnancy should be cultured because these women might appear asymptomatic but have a covert infection and become symptomatic later; UTIs in pregnant women may lead to pyelonephritis and the likelihood of a premature birth. Other situations in which patients with no symptoms of UTI might be cultured include the following:
Other factors that must be considered when selecting a rapid urine screen include accuracy, ease of test performance, reproducibility, turn-around time, and whether bacteriuria and/or pyuria are detected.
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