Lack of clinical utility of urine myoglobin detection by microconcentrator ultrafiltration in the diagnosis of rhabdomyolysis

BACKGROUND: In the diagnosis of rhabdomyolysis, the microconcentrator qualitative assay for urine myoglobin (uMb) is often used as a screening tool. The accuracy and clinical utility of this assay in screening patients with rhabdomyolysis have not been examined. METHODS: We conducted a retrospective analysis of the relationship between creatine kinase (CK), serum myoglobin (sMb), the urine qualitative assay for myoglobin and the semi-quantitative assay for urine haem pigments (uH) in patients evaluated for rhabdomyolysis. RESULTS: There were 673 patients with CK and uMb recorded on the same day. The uMb assay had a sensitivity of only 26.4% [95% confidence interval (CI): 23.1-29.7%] and specificity of 96.8% (95% CI: 95.5-98.1%) for the detection of severe rhabdomyolysis, defined as a CK >10 000 U/l. SMb and CK measured simultaneously in 83 patients were highly correlated (R(2) = 0.72 for log-transformed values), suggesting that the negative uMb test was not a result of the absence of sMb. In 241 patients who had CK, uMb and uH measured on the same day, the presence of 'moderate' or 'large' uH in the absence of haematuria, indicating presence of myoglobinuria, had a sensitivity of 81% (95% CI: 76-86%) for the detection of CK >10 000 U/l vs a sensitivity of 22% (95% CI: 17-27%) for the uMb assay. CONCLUSIONS: The microconcentrator-based uMb assay has a poor and clinically inadequate sensitivity in the detection and diagnosis of rhabdomyolysis.