Image Image Image Image Image Image  
 
 

Positive Nitrites and Negative Leukocytes?

Larry Mellick, MD, MS, FAAP, FACEP
Professor of Emergency Medicine and Pediatrics
Medical College of Georgia

Pearl: Every once in a while one sees a urine test with a positive nitrite test and a negative leukocyte test. While the temptation is to ignore the result or consider it a false positive, the overwhelming evidence suggests that making such an assumption would be a mistake.

(See Figure 1.)

Presentation: The dipstick of the urine of a middle-aged female without clear urinary tract infection symptoms demonstrated a positive nitrite test but a negative leukocyte esterase test. The urine culture subsequently grew out greater than 100,000 bacteria of the Escherichia coli species per milliliter of urine.

Discussion: When some bacteria in the urine reduce nitrates to nitrites, the urine nitrite test on urine test strips will turn positive. In clinical practice, just how reliable are those dipstick tests? And how reliable is the nitrite test when it alone is positive?

A number of published papers have studied the sensitivity and specificity of the urine dipstick tests. In fact, several useful summaries of the published studies can be found and their findings are presented in table format. Based on a review of 1489 publications (with 26 studies meeting criteria for inclusion) published in 1999, it appears that nitrite tests are pretty reliable.1

Sensitivity Specificity Positive likelihood ratio Negative likelihood ratio
Leukocyte esterase (LE) 84% 78% 4 0.2
Nitrite (Only) 50% 98% 25 0.5
Nitrite or LE 88% 93% 13 0.1
Nitrite and LE 72% 96% 18 0.3

Reference1

Another table based on only seven but more current references reported similar findings.

Results Sensitivity (%) Specificity (%) PPV NPV
Leukocyte esterase (LE) 72 to 97 41 to 86 43 to 56 82 to 91
Nitrite (Only) 19 to 48 92 to 100 50 to 83 70 to 88
LE or Nitrite 46 to 100 42 to 46 51 88

Table adapted from: Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. Am Fam Physician 2005;71(6):1153-62.2

These summaries show that a nitrite test is not very sensitive, but when it is positive, one can be comfortable that the positive results are correct.

However, there are known causes of false-positive nitrite tests. Of course, a false-positive nitrite test result may occur when a urine specimen has remained at room temperature for an extended period of time and bacterial contaminants are allowed to multiply and produce measurable levels of nitrites. Other cause of false-positive results have been reported. Hyperbilirubinemia is a potential cause of a false-positive nitrite test.3 Additionally, Gallagher et al. demonstrated that the nitrite reagent, in contrast to the other eight reagents on the Chemstrip-9 dipstick (Biodynamics, Indianapolis, IN), rapidly loses accuracy when stored in uncapped vials.4 By the end of a week of exposure, one-third of the nitrite tests gave false-positive readings. At the end of a second week, nearly three-quarters gave false-positive readings, for a specificity of only 28%.4

In summary, as shown in the tables above, the sensitivity of nitrite tests is not great. The low sensitivity has been attributed to bacteria causing infection that are deficient in the nitrate reducing enzyme, nitrate reductase, or low-grade bacteriuria such as might be seen in young children who empty their bladder frequently. The specificity, on the other hand, is excellent. So, when the test is positive, strongly consider the possibility of a urinary tract infection.

References:

  1. Gorelick MH, Shaw KN. Screening tests for urinary tract infection in children: A meta-analysis. Pediatrics 1999;104(5):e54.
  2. Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. Am Fam Physician2005;71(6):1153-62.
  3. Watts S, Bryan D, Marill K. Is there a link between hyperbilirubinemia and elevated urine nitrite. Am J Emerg Med 2007;25(1):10-4.
  4. Gallagher EJ, Schwartz E, Weinstein RS. Performance characteristics of urine dipsticks stored in open containers. Am J Emerg Med1990;8(2):121-3.

Online Sources:

  1. http://www.aafp.org/afp/2005/0315/p1153.pdf
  2. http://emedicine.medscape.com/article/233101-overview

Back to Pearls & Pitfalls

 
© AHC Media. All rights reserved. Terms of Use | Privacy Policy