Which condition causes a false negative on ketone testing in urine (Sodium Nitroprusside) strips?

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Ketone test strips using sodium nitroprusside detect acetoacetate, one of the ketone bodies, but they do not detect beta-hydroxybutyrate or acetone. This means the result depends on which ketone bodies are present in the urine.

In fresh urine, ketosis may be characterized by a predominance of beta-hydroxybutyrate rather than acetoacetate. Since the strip reacts with acetoacetate, if most ketones are in the beta-hydroxybutyrate form, the color change can be weak or absent, giving a false negative even though ketosis is occurring. Over time, or as the balance of ketone bodies shifts, acetoacetate levels can rise and the test may become positive. That’s why a false negative can occur in fresh urine due to the test’s limitation to detect only acetoacetate.