From the Guidelines
Myoglobin does not appear as blood in urinalysis, but rather can cause a false positive result for blood due to its peroxidase-like activity. This is a crucial distinction, as myoglobinuria indicates muscle damage, such as rhabdomyolysis, which is associated with increased morbidity and mortality, particularly in patients with severe limb trauma 1. In such cases, raising CPK levels above five times normal is a sign of rhabdomyolysis, and myoglobin is responsible for intraluminal kidney tubular obstruction, resulting in reduced glomerular filtration rate. To accurately diagnose myoglobinuria, additional testing is required, such as:
- Microscopic examination to rule out red blood cells
- Specific immunoassays for myoglobin This is important because myoglobinuria requires different management approaches compared to hematuria, and prompt identification can help reduce the risk of acute kidney injury and associated mortality 1. Key laboratory tests to complement the assessment of myoglobinuria include plasma potassium tests for hyperkalemia and measurements of kidney function by means of plasma creatinine tests.
From the Research
Myoglobin and Urinalysis
- Myoglobin can react positively for blood by urine dipstick, which can serve as an indicator of myoglobinuria 2
- Urinalysis results can reliably predict the absence of myoglobinuria and could be used to avert overtesting for urine myoglobin while also providing useful diagnostic information when urine myoglobin test results are not immediately available 2
Myoglobinuria Detection
- Urine myoglobin detected by dipstick/ultrafiltration was positive in only 19% of cases 3
- The absence of urine myoglobin, by qualitative assay, does not exclude rhabdomyolysis 3
Rhabdomyolysis and Myoglobin
- Rhabdomyolysis is a clinical and biochemical syndrome that occurs when skeletal muscle cells disrupt and release creatine phosphokinase (CK), lactate dehydrogenase (LDH), and myoglobin into the interstitial space and plasma 4
- Myoglobinuria is the most significant consequence of rhabdomyolysis, leading to acute renal failure (ARF) in 15%-33% of patients with rhabdomyolysis 3
Diagnostic Value of Urine Myoglobin
- There is inadequate evidence evaluating the use of urine myoglobin as a predictor of acute renal failure in patients with suspected rhabdomyolysis 5
- Sensitivity of the urine myoglobin test was 100% in 5 of the 8 studies, specificity varied widely (15% to 88%), and CIs around these measures were high 5