This patient likely has exercise-induced hemolysis, also known as march hemoglobinuria or runner's hemolysis. This is caused by erythrocyte damage through repetitive mechanical trauma such as running or marching, resulting in intravascular hemolysis. Hemolysis leads to increased levels of free hemoglobin in the plasma, which is filtered by the kidneys, resulting in hemoglobinuria. Urinalysis findings show evidence of blood in the urine by dipstick but no erythrocytes, as seen in this patient. With prolonged intravascular hemolysis, patients may become secondarily iron deficient through iron loss in the urine. Patients with this entity may show iron deficiency anemia with evidence of urine iron loss, indicated by the presence of hemosiderin in sloughed tubular cells by Prussian blue staining. Exercise-induced hemolysis is considered a benign condition and is treated by removal or reduction of the traumatic cause of erythrocyte injury.

Strenuous exercise is a well-described cause of either gross or microscopic hematuria. Although the mechanism for hematuria associated with exercise has not been fully elucidated, it is considered a benign condition and should resolve with termination of exercise. This patient has evidence of hemoglobinuria but not of hematuria given the lack of erythrocytes in her urine, making this a less likely diagnosis.

An inflammatory myopathy, such as polymyositis, may cause muscle damage and the finding of positive blood on the urine dipstick without significant hematuria. However, inflammatory myopathies are usually associated with muscle weakness and tenderness and an elevated creatine kinase level, which are absent in this patient.

Rhabdomyolysis is caused by injury and necrosis of muscle with release of intracellular muscle contents into the circulation; myoglobin may be detected in the urine as hemoglobin in the absence of erythrocytes. However, rhabdomyolysis is usually associated with muscle pain and significant elevations in circulating muscle enzyme levels, including creatine kinase. This patient has no muscle tenderness and her creatine kinase level is normal, making this diagnosis unlikely.