Leukocytes in Urine

White blood cells — your immune system’s first responders.

Leukocytes are white blood cells — the cells your immune system sends wherever it detects trouble. Finding them in urine usually means the immune system is responding to something in the urinary tract, most often an infection. A test strip detects them through an enzyme they release, called leukocyte esterase.

What leukocytes are

White blood cells are produced in the bone marrow and travel wherever the immune system needs them. In urine, more than a small number — a finding called pyuria — means immune cells are active somewhere along the urinary tract.

Why they appear in urine

The most common reason is a urinary tract infection — of the bladder, urethra, or kidneys. Other causes include kidney stones, inflammation without infection, and some medications. Sample contamination can also produce a positive, which is why collection technique matters.

What a result can tell you

Leukocytes are most informative read alongside nitrite. Both positive, with symptoms, points fairly clearly toward a UTI — useful information to bring to a provider promptly, since early treatment is simpler treatment. Leukocytes alone, without symptoms, are worth a careful retest with a clean-catch sample before drawing conclusions.

What it can’t tell you

A strip can’t tell where the immune response is happening or what’s causing it, and it can’t confirm infection — only a urine culture can identify bacteria. Persistent leukocytes without an explanation deserve a provider’s evaluation, even when you feel well.