A screening test was carried out among 120 people and 20 had the disease. 40 showed test positive out of which 15 had the disease. What is the specificity of the test?
**Core Concept:** Test specificity is a measure of how well a test identifies people who do not have a disease. It is calculated as the proportion of people without the disease who test negative.
**Why the Correct Answer is Right:** We are given a screening test and the number of true negatives (TN), true positives (TP), false positives (FP), and false negatives (FN). The specificity (Sp) can be calculated using the formula: Sp = (TN / (TN + FP)). In this case, TN = 100 (120-20), TP = 15, and FP = 40 - TN - TP = 25. So, Sp = (100 / (100 + 25)) = 80%.
**Why Each Wrong Option is Incorrect:**
A. False: Specificity calculation involves true positives, true negatives, false positives, and false negatives, not just the number of true negatives.
B. False: The formula given in the explanation is correct for calculating specificity.
C. False: Specificity is calculated using the given formula, not just the number of true positives.
D. False: Specificity is calculated using the formula provided in the explanation.
**Clinical Pearl / High-Yield Fact:** In interpreting test results, specificity is important because it informs the probability that a positive test result indicates the absence of the disease. A high specificity indicates a reliable test.
**Correct Answer:** D. 80% (The specificity of the test is 80%)