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Interpretation of a Hypothetical Leukocyte Esterase (LE) Test Result in a Cohort of 1000 Women Assuming a 30% Prevalence of UTI (Pre-Test Probability), Test Sensitivity 71%, and Specificity 85%*

Results

UTI Present

UTI Absent

300 patients with UTI

700 patients without UTI

LE test positive

213 patients (TP)

105 patients (FP)

LE test negative

87 patients (FN)

595 patients (TN)

* Bayes theorem can be simplified to allow the calculation of post-test probability when pre-test probability is known:

  • Post-test probability when the test is positive = TP/(all with a positive test) = TP/(TP + FP) = 213/(213 + 105) =67%.

  • Post-test probability when the test is negative = TN/(all with a negative test) = FN/(FN + TN) = 87/(87 + 595) =13%.

FN = false negative; FP = false positive; TN = true negative; TP = true positive.