Although the dipstick method of detecting blood in the urine is convenient, it cannot differentiate between hemoglobin, myoglobin, or red blood cells. ¹

Several reviews suggest that urine myoglobin is unstable with subpar performance in rhabdomyolysis¹, often defined as creatine kinase (CK) elevation 5 times the upper limit of normal in the proper context (eg, crush injury, hypoxic/ischemic or drug injury). ²  A sensitivity of 71% and a specificity of 54% for urine hemoglobin by dipstick, and a sensitivity of 25% and specificity of 75%  for urine myoglobin  has been reported in patients with serum CK >10,000 U/L. ³  

So while a positive dipstick for blood with few or no RBCs in the urine may make us think about rhabdomyolysis, its absence should not be used to exclude it in a susceptible host.

Bonus Pearl: Did you know that consumption of quail has been associated with rhabdomyolysis, possibly due to their feeding on poisonous plants such as hemlock?

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References

1. Rodriguez-Capote Karina, Balion CM, Hill SA, et al. Utility of urine myoglobin for the prediction of acute renal failure in patients with suspected rhabdomyolysis: A systematic review. Clin Chem 2009;55:2190-97. https://www.ncbi.nlm.nih.gov/pubmed/19797717
2. Nance JR, Mammen AL. Diagnostic evaluation of rhabdomyolysis. Muscle Nerve 2015;51:793-810. https://www.ncbi.nlm.nih.gov/pubmed/25678154
3. Grover DS, Atta MG, Eustace JA, et al. Lack of clinical utility of urine myoglobin detection by microconcentrator ultrafiltration in the diagnosis of rhabdomyolysis. Nephrol Dial Transplant 2004;19:2634-38. https://www.ncbi.nlm.nih.gov/pubmed/15280520