Urine Dip Leukocyte Esterase and Nitrites

Nitrites are found in urine after bacteria reduce urinary nitrates to nitrites. The test is specific (95%–98%) but not highly sensitive. The usefulness of nitrite positivity in isolation is limited because the uropathogens S saprophyticus, Pseudomonas, and enterococci do not reduce nitrate. The nitrite dipstick reagent is sensitive to air exposure, so containers should be closed immediately after removing a strip. If the container remains open then false positives will result.

Leukocyte esterase (LE) is an enzyme in neutrophils and is a measure of pyuria. Dipstick LE has a specificity of 59-96%. It may be falsely positive when there is contamination by bacteria in vaginal fluid, as occurs in vaginitis or cervicitis.To detect significant pyuria accurately, five minutes should be allowed for the dipstick reagent strip to change color. LE sensitivity is decreased by high levels of protein or glucose in the urine.

If nitrite or leukocyte esterase is positive, the dipstick has a sensitivity of 75% (67%–100%) and specificity of 82% (67%–98%) for UTI. If both a positive leukocyte esterase and nitrite is used to diagnose UTI, the specificity improves to 98%-100%, but the sensitivity declines to 35% to 84%.

If a dip is used to help diagnose a urinary tract infection it should be evaluated in context of the clinical presentation.

Interestingly, in a study in Annals in 2001, if urine dipstick results are defined as positive when leukocyte esterase or nitrite is positive or blood is more than trace, the over treatment rate is 47% and the under treatment rate is 13%. Comparing this to urinalysis results, defined as positive when WBCs are more than 3 per high-power field or RBCs are more than 5 per high-power field, the over treatment rate is 44% and the under treatment rate is 11%. A post on urinalysis for UTI to come.

Jeff Simerville, et. al. Urinalysis: A Comprehensive Review. American Family Physician – Volume 71, Issue 6 (March 2005)

David R. Lane, Sukhjit S. Takhar. Diagnosis and Management of Urinary Tract Infection and Pyelonephritis. Emergency Medicine Clinics of North America – Volume 29, Issue 3 (August 2011)

Lammers RL, Gibson S, Kovacs D, Sears W, Strachan G. Comparison of test characteristics of urine dipstick and urinalysis at various test cutoff points. Ann Emerg Med. 2001 Nov;38(5):505-12

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