Ketone tests on urine strips use which reagent, and under which conditions can a false positive occur?

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Ketone test strips measure acetoacetate using sodium nitroprusside. When acetoacetate is present, the nitroprusside reagent undergoes a color change, giving a positive result. False positives occur when the urine’s properties interfere with the color readout. Alkaline urine (high pH) can promote color development by the reagent even without ketones, making a result appear positive. Highly concentrated urine (high specific gravity) concentrates substances that can intensify the color and mimic a ketone signal. Pigmented urine contains colored substances that can alter the strip’s appearance, leading to an apparent positive even if ketones are not present. Old urine can lead to false negatives because acetoacetate degrades over time, so the opposite (false positive) isn’t expected there. Urea concentration alone isn’t a reliable cause of false positives for this test.