How do urine tests detect STIs?

Dear Ani,

Various types of sexually transmitted infections (STIs) require different tests. A urine test can be used to check for chlamydia and gonorrhea, two of the most common bacterial STIs.

A urine test or urinalysis involves collecting a urine sample which is then sent to a laboratory to be analyzed. At the lab, technicians look for evidence of chlamydia or gonorrhea in the urine using various tests. The lab chooses the correct test based on technical capabilities of the lab, cost, and other factors. One option is a nucleic acid amplification test (NAAT), which is used to make extra copies of any nucleic acids or genetic material present in the urine sample. By amplifying the genetic material, lab technicians can more easily identify any parts that belong to chlamydia or gonorrhea bacteria. Another test called nucleic acid hybridization (NAH) works kind of like Velcro by using a DNA probe to seek out and attach to the rRNA material of the chlamydia or gonorrhea bacteria. A third test called enzyme immunoassay (EIA) checks for antigens produced by the body's immune system to fight the STI.

In the past, a swab test was the only way to check for chlamydia and gonorrhea. In a swab test, a swab similar to a q-tip is used to collect fluids from the vagina or penis. Then, the fluid sample is tested for STIs by growing a culture at a lab. For women, a swab is inserted into the vagina to collect fluids, similar to the procedure for a Pap smear. For men, the swab must be inserted into the urethra, which may be painful. In general, male and female patients tend to prefer the new urine test since it is not invasive. Urine tests for chlamydia are comparable to swab tests when it comes to sensitivity and specificity, producing similar numbers of false positives and false negatives. However, the new urine tests are more expensive.

As a side note, you have right to know the reason for any tests your doctor or health care provider performs. If a doctor or nurse asks you to fill up the pee cup or give a blood sample without explaining why, it's your prerogative to ask questions and be an informed patient and consumer. You are your strongest healthcare advocate!