A new test to diagnose urinary tract infections in women is being evaluated. The comparison gold standard is positive urine dipstick plus urine culture. Results of the study are given below. What is the new test's specificity?

Question

A new test to diagnose urinary tract infections in women is being evaluated.  The comparison gold standard is positive urine dipstick plus urine culture.  Results of the study are given below.  What is the new test's specificity?

Quizplus · Accepted Answer

11ec160b_3724_267e_9249_29d5257e778d_MD0001_00 The specificity of a test refers to its ability to correctly identify individuals without the disease; it is the number of true negatives (TN) divided by all those without the disease. Specificity should be high in confirmatory tests to decrease the number of false positives (FP). Using a generic 2×2 (contingency) table, specificity can be calculated from: TN / (TN + FP). In this case: (Choice A) In this example, FP / (TN + FP) = 20 / (180 + 20) = 20/200 = 10%. This value is called the false positive rate, which is equal to (1 - specificity). It represents the probability of a false positive test result in a patient who in reality is free of disease. (Choices B and D) Sensitivity (a test's ability to correctly identify individuals with the disease) can be given by TP / (TP + FN), where TP is true positives and FN is false negatives. In this example, sensitivity = TP / (TP + FN) = 60 / (60 + 140) = 60/200 = 0.30 (or 30%). The false negative rate (= 1 - sensitivity) is given by FN / (TP + FN) = 140 / (60 + 140) = 140/200 = 0.70 (or 70%). It is the probability of a false negative test result in a patient who in reality has the disease. (Choice C) From a total of 400 patients (= 60 + 140 + 20 + 180), there are 200 patients (= 60 + 140) who have a urinary tract infection (UTI) based on the gold standard. The prevalence of UTIs is therefore 200/400 = 0.50 (or 50%). Educational objective: The specificity of a test is its ability to correctly identify individuals without the disease. Specificity should be high in confirmatory tests to decrease false positives.

A New Test to Diagnose Urinary Tract Infections in Women