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False-Positive and False-Negative Results

A false-positive result for blood on the urine reagent strip can occur if the collection container or reagent strip is contaminated with oxidizing agents, such as hypochlorite (bleach) or if the specimen is contaminated with povidone-iodine, a strong oxidizing agent used in surgical procedures. Microbial peroxide found in association with urinary tract infections may also cause false-positive results.

The muscle tissue form of hemoglobin, myoglobin is a well-known cause of false-positive blood reactions on the reagent strip. When tissue hemoglobin is present, the urine specimen has a clear red appearance. Patients suffering from muscle-wasting disorders or muscular destruction due to trauma, prolonged coma, or convulsions or individuals engaging in extensive exertion may have myoglobin in their urine. Specific tests for myoglobin, such as immunodiffusion techniques or protein electrophoresis, are needed to confirm the presence of this substance in a urine specimen.
Captopril can cause a falsely-decreased test result.
If the urine specimen is not mixed before testing, blood cells may settle in the sediment and produce a false-negative result for blood.