Urine, by nature is sterile. There maybe a few leukocytes present in the urine, however they do not come up in chemical tests and usually give a negative result. Leukocytes are enzymes which are present in the white blood cells. Also, our body has been fashioned in such a way that there is no direct contact between blood and urine. So the possibility of blood mixing with urine is also extremely rare, unless the body is affected with some disease.
Inspite of this, it may be possible to sometimes discover white blood cells in your urine test results. The most likely cause of this is infection. Any infection present in the kidneys or an inflammation of the bladder can show up as a high white blood cell count in your test results.
The most common cause is usually bacterial urinary tract infection or commonly known to us as UTI. Apart from this, inflammation can also due to be other factors like presence of kidney stones, immune disorders, allergies, growth in the genitourinary system etc.
The work of white blood cells is to protect our body against what it perceives to be harmful foreign invaders like infections or disease. Imagine them to be like a little white army, being sent forth by the body whenever it feels that the immune system of some part of the body has been compromised.
An easy example to understand this is the white spots that we get in case of acne or the white spots that form in case of sore throat. It means that the body has sent its forces to fight against the invading infection.
Having many white blood cells indicates an infection. A prudent step forward would be to go in for a detailed urine culture report – both routine and microscopic. General urine reports which are routine may or may not be able to detect an infection, but a detailed microscopic report will definitely give you the reasons for your concern.
Urinary tract infection (UTI) is a possibility too. It is not a big issue if treated on time. In fact, in most cases, it can be solved with some simple OTC drugs and following some measures like drinking plenty of fluids, lots of cranberry juice (make sure it is pure and not the sugar loaded commercial junk), urinating frequently, dressing comfortably and avoiding jeans or any other tight clothing because bacteria thrives in moist and dark places, and maintaining good intimate hygiene.
If it's not urinary tract infection, further tests to map the kidney function and a cystoscopy may be needed so that the doctors can have a look at the bladder to ensure that everything is okay.
Another unlikely but not entirely impossible angle is to recheck any medications one is consuming . Certain drugs, for example those that control arthritis can sometimes also cause UTI. Provide a full disclosure of your daily routine and of any present and past medical history to your physician and I am sure he/she will be able to help you more.