Fast Five Quiz: Urine Discoloration

Bradley Schwartz, DO


October 22, 2021

Hematuria, or blood in the urine, is likely the most common cause of red urine. Directly visualizing erythrocytes via microscopy is the best means of identifying true hematuria, because dipstick testing can produce false-positive results. The presence of heme in the urine also produces red discoloration and generates a positive dipstick reading; however, this should not be considered hematuria in the absence of microscopic confirmation.

The differential diagnoses for gross hematuria are broad and include, but are not limited to, infection, stone disease, cancer, trauma, fistulas, medical renal disease, and contamination (eg, due to menstruation). Gross hematuria warrants a thorough diagnostic evaluation owing to the poor prognosis associated with some of its underlying etiologies.

Red urine discoloration due to hemoglobinuria may present in hemolytic disorders, as in "march hematuria" observed in troops or "runner's hematuria." Likewise, myoglobinuria due to myocyte destruction (eg, caused by rhabdomyolysis after crush injury) can also result in red urine discoloration. Similarly, the disordered heme production in porphyria can result in red urine discoloration that may change to brown or purple with sunlight exposure. Both mercury and lead poisoning have also been implicated in red urine discoloration.

Numerous medications can cause red urine discoloration. Most commonly, rifampin or phenazopyridine is the culprit, with tears and other bodily fluids generally also discolored. Others include chlorpromazine, thioridazine, senna, and laxatives with a phenolphthalein component.

Other more benign causes of red urine discoloration are related to the ingestion of certain pigment-containing foods. Consumption of a large amount of beets can result in red urine with no observable erythrocytes on microscopy or heme on dipstick testing. Heavy consumption of rhubarb or carrots can also cause red urine discoloration.

Read more on common causes of hematuria.


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