Differential Diagnosis for Positive Occult Blood in Urine with No RBCs Present

Single Most Likely Diagnosis

• Myoglobinuria: This condition, where myoglobin is present in the urine, can cause a positive occult blood test due to the chemical similarity between myoglobin and hemoglobin. Myoglobinuria often occurs after muscle injury (rhabdomyolysis) and does not involve the presence of red blood cells (RBCs) in the urine.

Other Likely Diagnoses

• Hemoglobinuria: Similar to myoglobinuria, hemoglobinuria involves the presence of hemoglobin in the urine, which can be detected by occult blood tests. This can occur due to hemolysis (breakdown of RBCs) without the actual presence of intact RBCs in the urine.
• Contamination: Certain substances or contaminants, such as menstrual blood or food (e.g., beets, rhubarb), can falsely elevate occult blood test results without the presence of RBCs in the urine.

Do Not Miss Diagnoses

• Urothelial Carcinoma: Although less common, it's crucial not to miss a diagnosis of urothelial carcinoma (cancer of the bladder or urinary tract), which can present with microscopic or occult hematuria. The absence of visible RBCs does not rule out this serious condition.
• Kidney or Bladder Trauma: Trauma to the kidneys or bladder can cause internal bleeding that might not always result in visible RBCs in the urine but could still yield a positive occult blood test.

Rare Diagnoses

• Porphyrins: Certain porphyrin disorders can lead to the excretion of porphyrins in the urine, which might react with occult blood tests, giving a false-positive result.
• Food or Drug Interference: Certain foods or drugs can interfere with the chemical assay used in occult blood tests, leading to false-positive results without the presence of RBCs or actual blood in the urine.