Leukocytes in urine: do you have a bladder infection?

The test strip checks whether your urine contains leukocyte esterase. That is a substance white blood cells carry and release when they break down at an infection. So the strip does not see the cell itself but what the cell leaves behind. That explains something that can be confusing: the strip can be positive while the sediment under the microscope counts almost no cells. The cells have already fallen apart, for instance because your urine was very dilute or because the container stood too long.

So there are two ways of looking at the same thing. The strip is quick and rough. The urine sediment actually counts the cells, in the lab, and is more precise.

You do not get a number with a unit back but a scale. Negative is the expected result; without symptoms you then need to do nothing. Trace is a borderline case, which you repeat on a cleanly collected morning sample. At 1+ or 2+ immune cells are present and you read the result together with your symptoms. At 3+ they are clearly present and you have the result assessed by a doctor. That scale rises with the amount of substance in your urine and says nothing about how serious something is: a 3+ deserves a doctor's explanation, but on its own it is no diagnosis.

The honest summary: this strip is sensitive but imprecise. It rarely misses a real infection, but it also turns positive without infection. So the answer to "do I have a bladder infection" with a positive strip is usually: maybe. What makes that more or less likely can be spelled out.

The first thing is your symptoms. Pain or a burning feeling on passing urine, a strong urge and cloudy urine make a positive strip a good deal more convincing than a positive strip without symptoms.

The second is the nitrite on the same strip. Nitrite only forms once bacteria in your urine convert nitrate, so a positive nitrite test is less common but far more convincing. Leukocytes and nitrite together, with symptoms, point strongly to a bacterial bladder infection. If the leukocyte pad is positive but the nitrite negative, it stays open, because by no means every bacterium makes nitrite.

That is why your doctor often requests a urine culture anyway. It shows which bacterium is involved and which antibiotic works, and a strip can never do that. The culture gives the answer; the strip only asks the question. If the culture stays negative while leukocytes are present, that is called sterile pyuria and investigation in a different direction is needed: an STI test, or a look at kidney stones and medication.

If a bladder infection goes untreated, it can travel up through the ureters to your kidneys. That is rare, but it is the reason to take fever and pain in your side seriously.

One thing is important to know: a positive strip is never on its own a reason for antibiotics. In older people a positive result without symptoms is common, and it usually needs no treatment. A doctor looks at the strip, your symptoms and the culture together.

Have this test done if you have symptoms that fit the urinary tract: pain or a burning feeling on passing urine, going more often, urgency without result, cloudy urine or pain low in your abdomen. It is also a standard part of a wider urine screen, for instance when your kidneys are checked.

How you collect matters more than the strip itself. Clean the area first, let the first flow pass and only then collect; that first flow carries away the bacteria that entered the bladder through the urethra and say nothing about your bladder. Do not touch the inside of the container. Morning urine is the most concentrated and therefore usually the most useful.

Deliver your container within two hours or keep it in the fridge until then, because urine that stands for long changes. If you are on your period, postpone the test for a few days; blood in the sample makes the result unusable.

If you have fever, chills, nausea or pain in your side, contact a doctor the same day. You do not wait for a test strip for that.