In a surveillance centre for Hepatitis , in a low prevalance area, the method for testing for Hepatitis B was single ELISA. This policy was changed to double testing in series. This would result in the following 2 parameters of the test being affected -a) Specificity and positive predictive valueb) Sensitivity and positive predictive valuec) Sensitivity and negative predictive valued) Specificity and negative predictive value

**Core Concept**
The question is testing the understanding of the relationship between the number of tests used in a screening protocol (in this case, single ELISA vs. double testing in series) and the resulting effects on test parameters such as sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) in the context of Hepatitis B testing.

**Why the Correct Answer is Right**
Double testing in series involves two consecutive tests: the first test (Test 1) and the second test (Test 2). Test 2 is performed only on those who test positive in Test 1. This approach increases the specificity of the overall testing protocol because it reduces the false-positive rate. The positive predictive value (PPV) is also increased because it becomes more likely that a positive result is true. However, the sensitivity of the overall protocol may decrease because some true positives may be missed in the first test and not proceed to the second test.

**Why Each Wrong Option is Incorrect**
**Option A:** Sensitivity and positive predictive value - This is incorrect because double testing in series can decrease sensitivity due to missed true positives in the first test. PPV is also not directly affected by the number of tests used.

**Option B:** Sensitivity and positive predictive value - This is a repetition of Option A, and the explanation remains the same.

**Option C:** Sensitivity and negative predictive value - This is incorrect because double testing in series does not directly affect the negative predictive value, which is the probability that a person with a negative test result is truly negative.

**Option D:** Specificity and negative predictive value - This is incorrect because double testing in series increases specificity, but it does not directly affect the negative predictive value.

**Clinical Pearl / High-Yield Fact**
In screening protocols, increasing the number of tests used can improve specificity and PPV but may decrease sensitivity. This is a trade-off that must be carefully considered in the context of the disease prevalence, test characteristics, and the consequences of false positives and false negatives.

**Correct Answer: D. Specificity and negative predictive value