False negative results occur when elements present in the urine interfere with either the enzymatic reaction or prevent the oxidation of potassium iodide. Examples of such substances include:

• large quantities of ketones
• aspirin
• ascorbic acid > 50 mg/dL with some reagent strips
• levadopa
• 5-hydroxyindoleacetic acid
• homogentisic acid
• sodium fluoride ( a preservative)

A specific gravity higher than 1.020 may lower glucose reagent sensitivity, especially in the presence of a high urine pH. Exposing reagent strips to excess humidity may also reduce glucose reagent reactivity.

Check the package insert of the reagent strips used in your laboratory for interfering substances that may affect glucose results.