Because urine dipstick tests are relatively inexpensive, convenient, and simple to perform, they are often done in a physician’s office. Most urine testing dipsticks have four tests in their panel that indicate the possibility of a urinary tract infection (UTI). These are:
1. Leukocyte esterase (LE)
Approximately 96% of people with UTIs have pyuria or white blood cells (WBCs) in the urine. Both living and lysed WBCs release the enzyme leukocyte esterase. (A positive LE test is equivalent to 4 or more WBCs per high power field when urine is examined microscopically.) A positive LE is a good indicator of a UTI. High concentrations of protein (500mg or more per deciliter) or glucose (1g or more per deciliter) in the urine, however, can interfere with this test.
2. Nitrite test (NIT)
The NIT test is an indirect test for bacteria in the urine (bacteriuria). Many of the Gram-negative enteric bacilli (the Enterobacteriaceae) that cause UTIs will reduce dietary nitrate in the urine to nitrite. Pseudomonas, a non-fermentative Gram-negative bacillus that causes UTIs, reduces nitrate to nitrogen gas, not nitrite. Staphylococcus saprophyticus and Enterococcus species, common Gram-positive bacteria that cause UTIs do not reduce nitrate to nitrite. A positive NIT along with a positive LE is good indicator of a UTI. Low colony counts, however, can result in a false-negative NIT so a urine specimen should not be collected if the person has recently urinated. A first-morning urine sample or one with urinary retention of 4-8 hours is best.
A person with a UTI usually exhibits trace to 30mg of protein (mainly albumin) per deciliter indicating that protein-containing substances such as bacteria, WBCs, and mucous are in the urine.
4. Red blood cells (RBCs)
This test looks for the strong pseudoperoxidase action of RBCs and hemoglobin. Microscopic hematuria (RBCs in the urine) is common with UTIs. A trace result is equivalent to 5-10 RBCs per microliter (µl). A RBC excretion of up to 5 RBCs per µl is seen in normal urine. Levels above this warrant further diagnostic evaluation.
Microscopic analysis of urine
Microscopic analysis of urine can also be used to confirm UTIs. Along with classic symptoms of a UTI, detection of 2-5 WBCs or15 bacteria or more per high-power microscopic field is indicative of a UTI.