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The nitrite test on urine dipstick is commonly used for diagnosis of UTI. However, the test is only reliable in those bacteria that convert nitrates to nitrites, which primarily includes enterobaceriaceae. However, elderly patients often develop UTIs with Staph saprophyticus, pseudomonas, and enterococcus, none of which produce positive nitrites on dipstick testing. The takeaway point here is very simply....don't assume you've excluded UTI (esp. in elderly populations) just because the nitrite test is negative.
reference: Anderson RS, Liang SY. Infections in the elderly. Critical Decisions in Emergency Medicine, April 2010.