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The page below is a sample from the LabCE course Chemical Screening of Urine by Reagent Strip. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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False-Positive and False-Negative Results

False-positive nitrite results may occur when urine specimens have remained at room temperature for an extended period of time, allowing bacterial contaminants to multiply and produce measurable levels of nitrites.
Interference from some medications that cause the urine to become red or orange may lead to an incorrect reading of positive for nitrite.
False-negative results may occur in urine specimens that did not remain in the bladder for the length of time needed for the bacteria to reduce a measurable quantity of dietary nitrate to nitrite. A false-negative nitrite test can also occur with severe bacterial infections. The bacteria will further reduce the nitrite to nitrogen, which does not react in the test.