If a urine dipstick is positive for blood but there are no red blood cells on microscopy, which tests are most appropriate to differentiate the cause?

• A. Hemolysis and rhabdomyolysis tests
• B. Urinalysis
• C. Urine culture
• D. Serum calcium

Answer: (A)

When a urine dipstick is positive for blood but there are no red blood cells seen on microscopy, the heme detected by the dipstick is coming from a source other than intact RBCs. The two main culprits are free hemoglobin from intravascular hemolysis and myoglobin from muscle breakdown (rhabdomyolysis). To determine which is causing the positive result, you test for both possibilities.

Hemolysis tests look for signs that red cells are being destroyed in circulation. You’d evaluate markers like low haptoglobin, elevated LDH, and an increased indirect bilirubin, along with a red cell count trend or smear if needed. If these point to hemolysis, the pigment in the urine is likely hemoglobin rather than whole cells.

Rhabdomyolysis tests assess muscle injury and the release of myoglobin into the bloodstream and urine. Key tests include an elevated creatine kinase (CK/CPK) level and, if needed, myoglobin levels. These findings support myoglobinuria as the source of the dipstick positivity.

Urinalysis or urine culture don’t specifically distinguish between hemoglobinuria and myoglobinuria, and serum calcium is not helpful for this differentiation, so they’re not the most informative next steps in this scenario.

By ordering both hemolysis and rhabdomyolysis testing, you directly differentiate the cause of the positive dipstick without assuming which pigment is present, guiding appropriate management.