Use of Serum Myoglobin Assays for Urine Myoglobin Measurements Can Cause False-Negative Results

Several conditions, including rhabdomyolysis, trauma, and surgery, are associated with the release of large amounts of myoglobin into the circulation. Saturation of the salvage system of the kidney will produce myoglobinuria, a condition associated with acute renal failure (ARF). The exact mechanism is not known, but precipitation of myoglobin in the tubules and myoglobin-mediated formation of free radicals have been postulated (1). Early identification of severe myoglobinuria permits acute treatment, thereby avoiding ARF. Quantitative myoglobin assays have several pitfalls, however, including discrepancies in myoglobin recovery (2). The lack of quantitative information precludes differentiating mild myoglobinuria, which probably does not cause ARF, from severe myoglobinuria. Therefore, myoglobin immunoassays developed for serum applications have been adapted for use with urine samples(3)(4)(5). Prior reports have recommended alkalinizing urine specimens before storage. Because quantification of urine myoglobin might be valuable in the early evaluation of patients who might have rhabdomyolysis, a rapid in-house myoglobin assay is desirable. To this end, we evaluated 4 different commercial serum myoglobin assays for their potential to measure myoglobin in urine. These 4 assays can be run on the analyzers present in our laboratory [Integra 700 (Roche), Elecsys 1010 (Roche), Aeroset (Abbott Diagnostics), and BN ProSpec (Dade Behring)]. All 4 assays are based on immunogenic detection of myoglobin, but by different techniques, namely turbidimetry, nephelometry, and a heterogeneous immunoassay. The assays were evaluated for imprecision with control materials provided by the manufacturers. For all assays, interassay CVs were <5% for all tested concentrations (50–1100 μg/L).