In a red urine specimen that tests positive for blood but shows no red blood cells on microscopy, what is the most likely cause of the abnormal color if ammonium sulfate precipitation leaves the supernatant red?

In a red urine specimen that tests positive for blood but shows no red blood cells on microscopy, what is the most likely cause of the abnormal color if ammonium sulfate precipitation leaves the supernatant red?

• A. Pyridium
• B. Porphyrins
• C. Hemoglobin
• D. Myoglobin

Answer: (D)

The test is distinguishing the source of a red urine color when no red blood cells are seen. In urine, a positive blood dipstick can come from free hemoglobin or myoglobin. When ammonium sulfate is added, larger proteins like hemoglobin precipitate, while the smaller myoglobin tends to stay in solution. If the supernatant remains red after precipitation, the pigment responsible for the color is the soluble myoglobin, pointing to myoglobinuria—from muscle breakdown or rhabdomyolysis—as the most likely cause. Hemoglobin would be expected to precipitate and be removed with the pellet, not stay in the red supernatant. Porphyrins and pyridium produce color changes by different mechanisms and wouldn’t fit this precipitation pattern.

The test is distinguishing the source of a red urine color when no red blood cells are seen. In urine, a positive blood dipstick can come from free hemoglobin or myoglobin. When ammonium sulfate is added, larger proteins like hemoglobin precipitate, while the smaller myoglobin tends to stay in solution. If the supernatant remains red after precipitation, the pigment responsible for the color is the soluble myoglobin, pointing to myoglobinuria—from muscle breakdown or rhabdomyolysis—as the most likely cause. Hemoglobin would be expected to precipitate and be removed with the pellet, not stay in the red supernatant. Porphyrins and pyridium produce color changes by different mechanisms and wouldn’t fit this precipitation pattern.