If a patient has more than 20 RBC/hpf but a negative urine dipstick for blood, which could explain this discrepancy?

• A. Large amounts of ascorbic acid present
• B. Presence of myoglobin in the urine
• C. Diagnosis of hemolytic anemia
• D. Specific gravity = 1.00

Answer: (A)

The key idea is that the urine dipstick test for occult blood can be chemically interfered with. The dipstick detects the heme moiety in hemoglobin (and myoglobin) via a peroxidase-like reaction that produces a color change. If there are large amounts of ascorbic acid (vitamin C) in the urine, it acts as an antioxidant and can reduce the oxidized chromogen or neutralize the peroxide driving the reaction. This prevents the color change, giving a negative dipstick result even when red blood cells are actually present on microscopic examination. So you can have many RBCs per high-power field but a negative dipstick for blood because the high ascorbic acid blocks the test signal.

As for the other possibilities, myoglobin would still participate in the same reaction and typically yield a positive dipstick; hemolytic anemia would cause free heme in the urine and also a positive result; and a specific gravity of 1.00 does not explain this chemical interference producing a discordant result.