Pearl(s):

  1. If the nitrite and LE results are concordant (both + or both – ), the test is accurate ~90% of the time.
  2. Leuk esterase has a lower PPV (higher FP rate) than nitrites, especially in low prevalence populations and in the ER
  3. In general, nitrites have better PPV but lower NPV
  4. Give everyone a wipe. Clean catches reduce LE false positives.

Context: We use urine dipsticks in clinical assessment every day. Maybe, some of us, don’t understand them as well as we should.

Question: What are the text characteristics of the urine dipstick in predicting UTI?

Answer: These data are sourced from a meta-analysis of 70 publications (1).

  • Nitrites:
    • Sensitivity: 50-55%
    • Specificity: 90-99%
    • PPV (Family Practice, pre-test prob 50%): 0.84
    • NPV (Family Practice, pre-test prob 50%): 0.61
    • PPV (ER, pre-test prob 15%): 0.62
    • NPV (ER, pre-test prob 15%): 0.92
  • Leukocyte Esterase:
    • Sensitivity: 50-87% (highest in studies of family practice clinics)
    • Specificity: 46-86%  (highest in ER)
    • PPV (Family Practice, pre-test prob 50%): 0.62
    • NPV (Family Practice, pre-test prob 50%): 0.69
    • PPV (ER, pre-test prob 15%): 0.41
    • NPV (ER, pre-test prob 15%): 0.92
  • Both Positive:
    • PPV (Gen Pop, pre-test prob 15%): 0.89
    • NPV (Gen Pop, pre-test prob 15%): 0.91

In a separate study (2), good clean catch technique reduced the number of false positives for LE but not nitrites by around 50%.

Sources:

(1) Deville WL, Yzermans JC, van Duijn NP, Bezemer PD, van der Windt DA, Bouter LM. The urine dipstick test useful to rule out infections. A meta-analysis of the accuracy. BMC Urol. 2004;4:4.

(2) J Emerg Med. 2015 Jun;48(6):706-11. doi: 10.1016/j.jemermed.2015.02.020. Epub 2015 Apr 1