The presence of leukocytes in a urine sample is a common finding that indicates the body is mounting an immune response. This finding, known medically as pyuria, signals inflammation or infection somewhere within the genitourinary tract. While pyuria is often associated with common urinary tract infections (UTIs), sexually transmitted diseases (STDs) are also a cause. These infections trigger a localized inflammatory reaction, which then results in immune cells shedding into the urine. Understanding this connection guides the necessary next steps for accurate diagnosis and effective treatment.
What Leukocytes in Urine Indicate
The detection of leukocytes in urine confirms the presence of pyuria, a non-specific sign of underlying inflammation or infection. Leukocytes are the body’s primary immune cells, and their appearance in the urine means they have migrated to the urinary tract to fight off an invading pathogen or respond to tissue irritation. A count of ten or more white blood cells per cubic millimeter of urine is considered the threshold for pyuria.
The most common trigger is a bacterial urinary tract infection, but in sexually active populations, STDs must be considered major factors. When pyuria is present without the typical bacterial growth on a standard urine culture, it is termed “sterile pyuria,” a finding highly suggestive of infections caused by organisms that are difficult to grow in a lab, including many STDs. Pyuria can occur with or without noticeable symptoms, meaning its detection during a routine test can be the first clue to an asymptomatic infection.
STDs That Drive Leukocyte Migration
Certain STDs are particularly effective at causing inflammation in the genital tract, leading to the migration of leukocytes into the urine. The primary mechanism involves the infection of the mucosal linings of the urethra and cervix. When these tissues become infected, the resulting conditions are called urethritis in men and cervicitis in women.
Chlamydia trachomatis is one of the most common bacterial STDs and a leading cause of sterile pyuria. The organism infects the cells lining the urethra or cervix, which initiates a localized immune defense that recruits a significant number of white blood cells to the site. These immune cells then mix with the urine as it passes through the inflamed tract, leading to a positive pyuria result. This process often occurs even when the infected individual feels completely healthy.
Neisseria gonorrhoeae, the bacterium responsible for gonorrhea, works similarly by provoking an intense inflammatory response in the same areas. The resulting urethritis or cervicitis generates pus—which is primarily made up of dead and active leukocytes—that is then excreted. Trichomonas vaginalis, a parasitic STD, also causes inflammation of the urethra and vagina, contributing to pyuria in a substantial number of cases. In all these instances, the presence of leukocytes in the urine is a direct biological consequence of the immune system’s attempt to clear the STD pathogen from the genitourinary tract lining.
Interpreting the Test Results
A finding of pyuria, especially sterile pyuria, in a sexually active person with urinary symptoms or risk factors suggests the presence of an STD. Therefore, a positive leukocyte result necessitates further, more targeted testing to identify the exact microbial culprit.
The next step in the diagnostic process involves specific tests, such as Nucleic Acid Amplification Tests (NAATs), which are highly sensitive and can accurately detect the genetic material of STD pathogens like Chlamydia trachomatis and Neisseria gonorrhoeae. For men, a first-catch urine specimen is the preferred sample type for NAAT testing, while women can use a urine sample, a vaginal swab, or a cervical swab. Once the specific STD is confirmed, treatment can be initiated with the appropriate antibiotics or antiparasitic medications.