Value and use of urinalysis for myoglobinuria

Context.—Urine myoglobin testing is primarily indicated for diagnosis and risk assessment of kidney injury in patients with rhabdomyolysis. However, its utility is limited by a lack of rapid and reliable results. Myoglobin reacts positively for blood by urine dipstick, which can serve as an indicator of myoglobinuria. Objective.—To evaluate the performance and value of blood and red cell measurements by urinalysis as a surrogate test for myoglobinuria in routine clinical practice. Design.—This study is a retrospective observational study involving analysis of hemoglobin and red blood cell results by urinalysis in patients tested for urine myoglobin. Results.—A total of 13 139 urine myoglobin results from 88 Veterans Affairs facilities during a 15-year period ending in October 2014 were evaluated. Among methods used by each laboratory, qualitative urine myoglobin tests declined from 25 of 53 (47.1%) in 2000 to 5 of 77 (6.4%) in 2013. Of 7311 tests (55.6%) performed by quantitative methods with concomitant urinalysis, 3915 (53.5%) showed negative to trace blood results, of which myoglobin was 1000 lg/L or greater in 17 (0.4%). Among 1875 (25.5%) with 3+ (large) blood results, urine myoglobin was ≥1000 lg/L in 273 of 1533 (17.8%) with hematuria (≥5 red blood cells per microliter) and 109 of 342 (31.9%) without hematuria. Conclusions.—Urinalysis results reliably predicted 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.