Leukocyte esterase: what leukocytes in urine mean on a urine test

When a urine test says leukocytes are positive, it is usually reporting leukocyte esterase, an enzyme released by white blood cells. A positive result means white cells are, or recently were, in the urine, most often because of an infection or inflammation somewhere in the urinary tract. It is a screening signal, read together with nitrite, a microscope count of white cells and any symptoms.

What the test measures

Leukocyte esterase is produced by neutrophils, the white cells most often found in urine. The esterase pad on a dipstick changes colour when the enzyme is present, and the colour is compared with a chart, by eye or by machine, to grade the result. Of all the strip pads, this one is the most sensitive to reading time: depending on the brand, it needs between 30 seconds and two minutes to develop.

Because the pad detects the enzyme rather than counting cells, it does not always agree with the microscope. In a 2026 study of stored urine, white cell counts became unreliable within 4 hours, while the leukocyte esterase reading held steady for at least 8 hours.

Normal result

A negative result is normal. Positive results are graded by how strong the colour change is.

The test catches most infections but also flags many samples that turn out not to be infected. In one review, it picked up 72 to 97% of infections confirmed by culture, while 41 to 86% of people without infection tested negative. A meta-analysis of 70 studies found sensitivity of 48 to 86% and specificity of 17 to 93%, depending on the setting. Either way, a positive result is followed up rather than taken as a diagnosis.

Read together with nitrite, the picture sharpens. When leukocyte esterase and nitrite are both negative, a urinary tract infection (UTI) becomes much less likely, and a negative blood pad lowers the odds further. UK primary care guidance for women under 65 with urinary symptoms treats a positive leukocyte result with a negative nitrite as making infection about as likely as another explanation; in the studies behind it, about half of such women had a UTI. In that situation the guidance recommends a urine culture to settle it.

What a positive result usually points to

The commonest reason is a UTI, in the bladder or, less often, the kidney. Other causes include:

• Inflammation without a bacterial UTI, including sexually transmitted infections that inflame the urethra.
• Trichomonas infection, or vaginal secretions such as blood or heavy discharge getting into the sample.
• Kidney stones, where white cells often come with red cells, crystals and pain.
• Bacteria living in the urine without causing infection, common in older adults, who can show white cells as well.
• Less common causes, including tuberculosis, viral infections, a foreign body in the urinary tract, inflammation of the kidney's filters, recent hard exercise, and medicines such as corticosteroids and cyclophosphamide.

What a doctor typically looks at next:

• Your symptoms, and the nitrite and blood results from the same strip.
• The urine under the microscope, for white cells and other signs of infection.
• A urine culture, to see whether bacteria grow and which ones.
• Squamous epithelial cells in the same sample. More than 5 per high-power field suggests contamination from the skin or vagina, and a fresh, carefully collected sample may be asked for.
• If white cells persist but the culture grows nothing, a pattern called sterile pyuria, UK guidance suggests considering chlamydia, especially at ages 16 to 24, other vaginal infections, tuberculosis and kidney causes.

What a negative result means

A negative result is the usual finding, and together with a negative nitrite it makes a UTI much less likely. It can still miss one. High levels of sugar, protein or ketones in the urine, very concentrated urine, high doses of vitamin C and some antibiotics can all hide a true positive. If you have symptoms, the result is read with them, not instead of them.

What moves it

• How the sample is collected. Vaginal secretions and skin contamination are a common cause of a positive result without infection. For women, holding the labia apart while collecting may help.
• Antibiotics. Some antibiotics, including cephalexin, nitrofurantoin, tetracycline and gentamicin, can cause a false negative.
• Urine chemistry. Sugar, protein or ketones in the urine, very concentrated urine and vitamin C can push the result down.
• Reading time. Read too early, the pad can under-call a positive, which is why strips are read at the time the maker specifies.
• Preservatives. Boric acid, used in some urine culture bottles, may inhibit the leukocyte esterase test, so those bottles are not meant for strip testing.
• Delay. Results become unreliable when more than two hours pass between collection and testing, unless the sample is kept cold.

Preparing for the test

A urine test needs no fasting, but if blood tests that need fasting are taken at the same visit, follow the instructions for those. Collect a midstream "clean catch": wash your hands, clean the genital area, pass a little urine into the toilet, then collect the middle of the stream.

Tell the person taking the sample if you have your period or any vaginal discharge, about any urinary symptoms, if you have taken antibiotics recently, and about every medicine and supplement you take, including vitamin C. Do not stop any medicine before a test unless your doctor tells you to.

How Clare's panel handles it

Clare's panel includes a urinalysis, and in it Leukocyte Esterase is measured by the lab. The same sample gives Urine WBC, the white cell count under the microscope, along with Nitrite and Bacteria, so the enzyme reading can be checked against an actual count.

Related tests

Leukocyte esterase is read with Nitrite, the chemical sign of certain bacteria, and Urine WBC, the microscope count that some reports call pus cells. Epithelial Cells help judge whether the sample was contaminated, and Bacteria shows whether organisms were seen. For how the rest of a full panel works, see Comprehensive blood test in Dubai: what to expect.

Why the result means more over time

A positive result with no symptoms can come from contamination, and a carefully collected repeat can settle that. White cells that keep appearing while the culture grows nothing are the pattern that leads doctors to look for other causes, and that pattern is visible over time rather than in a single test. When Clare opens, each urine result will sit in one health account with its date, next to the ones before it.

Clare is opening with a waitlist at clare.health.