**Question:** All comprise inherent properties of a screening test except -

A. Sensitivity
B. Specificity
C. Positive predictive value
D. Negative predictive value

**Correct Answer:** C. Positive predictive value

**Core Concept:**
A screening test is a diagnostic test performed on a large group of individuals who are deemed to be at risk for a particular disease or condition. The purpose of a screening test is to identify individuals who may have the disease or condition and require further evaluation and confirmatory tests. In this context, we discuss four key properties that characterize a screening test:

1. Sensitivity: The proportion of truly positive individuals who test positive. In other words, it is the ability of the test to correctly identify individuals with the disease or condition.
2. Specificity: The proportion of truly negative individuals who test negative. It represents the test's ability to correctly identify individuals without the disease or condition.
3. Positive predictive value (PPV): The probability that a person who tests positive actually has the disease or condition. It is calculated as the ratio of true positives to the sum of true positives and false positives. A high PPV indicates a more reliable positive test result.
4. Negative predictive value (NPV): The probability that a person who tests negative does not have the disease or condition. It is calculated as the ratio of true negatives to the sum of true negatives and false negatives. A high NPV indicates a more reliable negative test result.

**Why the Correct Answer is Right:**

The correct answer, Positive Predictive Value (PPV), is not one of the inherent properties of a screening test. PPV is determined by the prevalence of the disease or condition in the population being tested. If the prevalence is high, the PPV will be higher, making a positive screening test result more reliable. Conversely, if the prevalence is low, the PPV will be lower, making a positive screening test result less reliable. In contrast, the other three properties (sensitivity, specificity, and negative predictive value) are inherent properties of the test itself and do not depend on the prevalence of the disease or condition.

**Why Each Wrong Option is Incorrect:**

A. Sensitivity: Sensitivity is the proportion of true positives, which represents the test's ability to correctly identify individuals with the disease or condition. A high sensitivity is desirable, indicating that the test does not miss many true positives.

B. Specificity: Specificity is the proportion of true negatives, representing the test's ability to correctly identify individuals without the disease or condition. A high specificity is desirable, indicating that the test does not falsely label many true negatives as positives.

C. Negative Predictive Value (NPV): The NPV is the proportion of true negatives, representing the test's ability to correctly identify individuals without the disease or condition. A high NPV is desirable, indicating that the test does not falsely label many true negatives as positives.

D. Positive Predictive Value (PPV): As explained above, PPV depends on the prevalence of the disease or condition in the population being tested. A high PPV indicates that a positive test result is more likely to