Which microscopic structure in urine can indicate an infection?

Leukocytes, or white blood cells, are a critical indicator of infection when found in urine. The presence of leukocytes in a urinalysis typically suggests that there is an inflammatory response in the urinary tract, often due to a urinary tract infection (UTI). These cells play a key role in the immune response, combating pathogens such as bacteria. A higher number of leukocytes in the urine can signal the body is actively trying to fight off an infection, making them a reliable marker in diagnosing UTIs or other urinary infections.

In the context of this question, while crystals, erythrocytes, and epithelial cells may appear in urine, they do not specifically indicate infection. Crystals might be associated with metabolic disturbances or kidney stones, erythrocytes (or red blood cells) often suggest bleeding or injury, and epithelial cells typically reflect normal shedding of cells from the lining of the urinary tract but are not definitive signs of infection. Thus, leukocytes are uniquely significant in the context of infection identification.