Mr. Downing has brought in a urine sample from Cleo, his 6-year-old female spayed cocker spaniel. The color of the urine appears red. On centrifugation of the sample, you find that the supernatant is now clear. What was the most likely cause of the red color?

• A. Hemoglobinuria
• B. Hematuria
• C. Myoglobinuria
• D. Uroglobinuria

Answer: (B)

The key idea is using centrifugation to tell whether the red color in urine comes from intact red blood cells or from dissolved pigments. When urine containing red blood cells is spun down, the cells settle into the pellet and the liquid portion (the supernatant) becomes clear. That clearing indicates the color was due to cells, not dissolved pigment. So the most likely cause of the red urine is red blood cells present in the urine, i.e., hematuria.

If the red color remained in the liquid after centrifugation, you’d suspect pigment in the urine from free hemoglobin or myoglobin. Uroglobinuria wouldn’t typically present with this clearing pattern or gross red coloration in the same way.