A positive finding of the enzyme leukocyte esterase in urinalysis indicates the presence of white blood cells in the urine. This normally suggests inflammation of some sort, often from a urinary tract infection. A simple dipstick test as part of a urinalysis can detect the presence of this enzyme.
Leukocyte esterase in urinalysis functions as a screening test. A negative result makes it unlikely an infection exists, thus making further testing of the urinary with a microscopic examination and urine culture for bacteria usually unnecessary. 
Causes of Positive Results
Although a urinary tract infection is the most common cause of leukocyte esterase in urinalysis some other and rarer causes for the positive result with a negative microorganism culture do exist.  These include:
- Infection with a sexually transmitted microorganism called ureaplasma urealyticum
- Balanitis, a skin disease occurring at the head of the penis
- Cancer of the bladder
- Kidney stones
- A foreign body in the urinary tract
- Glomerulonephritis, an inflammation of small blood vessels in the kidneys
- Prescription drugs such as corticosteroids
False Negative Results
Sometimes a test for leukocyte esterase produces negative results but later follow-up indicates a disease condition like a urinary tract infection. This could happen from a number of causes a simple as the laboratory technician not allowing enough time before reading the dipstick. For an accurate test, the reading should wait five minutes after moistening the end in urine.
Other causes are: 
- A higher than normal specific gravity of the urine
- Glucose or ketones in the urine
- Urinary protein
Drugs or supplements: Keflex, Nitrofurantoin, Tetracycline, Gentamicin, Vitamin C
Reliability of Leukocyte Esterase Dipstick Tests
A literature review covering the period 1966 to 2003 found 30 studies on using urinary leukocyte esterase or nitrite dipstick tests to exclude infections of the urinary track. Based on the pooled results, the reviewers concluded in certain circumstances, there could be a 5 percent probability of a negative result. 
However, another meta-analysis stressed the importance of using both the leukocyte esterase and a nitrite test to improve sensitivity, in other words, reduce the possibility of false results. This review found the accuracy of nitrite tests highest in pregnant women and elderly patients. The accuracy of leukocyte esterase in urinalysis was best in urology patients and the highest sensitivity (86 percent) occurred when the test was run in family medicine practice. Combining both tests gave sensitivities between 68% and 88%. 
Problems with This Dipstick Test
A blinded test of urological patients in 2010 called into question the accuracy of leukocyte esterase tests for urinary tract infections in patients with these symptoms but without acute frequency and painful urination. According to this report, midstream collection of urine testing for leukocyte esterase, nitrites and pyuria (pus) found sensitivities respectively of 56, 10 and 56 percent. Catheter specimen collection gave slightly better results—59, 20 and 56 percent. Specificities for the same tests with catheter collection respectively were 84, 97, and 73 percent. The study calls into question the widespread use of such tests in some cases. 
Specificity vs. Sensitivity
The difference between these concepts is important in understanding medical tests. Specificity refers to the degree of false positive results. The test reported the patient had a disease or other condition where in reality this was not true. The sensitivity of a test measures the degree of false negatives in the test, in other words, how many people with a disease were not diagnosed. The higher the sensitivity, the more likely it catches everyone. The lower the specificity, the less reliable the test is because it will give too many errors.
.University of Utah Health Sciences: Urinalysis
 Family Practice Notebook: Urine Leukocyte Esterase
. St. John, A., et al., “The use of urinary dipstick tests to exclude urinary tract infection: a systematic review of the literature," American Journal of Clinical Pathology, 2006 Sep;126(3):428-36.
. Devillé, W.L., et al., “The urine dipstick test useful to rule out infections," BMC Urology, 2004 Jun; 2:4:4
. Khasriya, R., et al., “The inadequacy of urinary dipstick and microscopy as surrogate markers of urinary tract infection in urological outpatients with lower urinary tract symptoms without acute frequency and dysuria," The Journal of Urology, 2010 May;183(5):1843-7. Epub 2010 Mar 29.
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