**Core Concept:**
The sensitivity and specificity of a diagnostic test refer to its ability to accurately detect true positives and true negatives, respectively, in a population. In a diagnostic test, sensitivity is the probability of correctly identifying a patient with the disease (positive predictive value), while specificity is the probability of correctly identifying a patient without the disease (negative predictive value). Repeating a test increases its reliability by reducing false-positive and false-negative results.

**Why the Correct Answer is Right:**
The hemoccult test is used to detect occult blood in stool samples, which is indicative of colorectal cancer. In this scenario, there are two options for test outcomes: positive (indicating blood in the stool) or negative (no blood detected). Repeating the test increases its reliability by reducing false-positive and false-negative results. This is because repeating the test reduces the likelihood of random errors or inter-observer discrepancies.

**Why Each Wrong Option is Incorrect:**
A. Incorrect because repeating the test will not improve its sensitivity if performed only on positive results.
B. Incorrect because repeating the test does not affect its specificity when performed on negative results.
C. Incorrect because repeating the test can lead to an increased number of false-positive results if performed on positive results, thereby reducing specificity.
D. Incorrect because repeating the test does not improve its sensitivity or specificity when performed on all results, but only reduces false-positive and false-negative results when performed on specific results (positive or negative).

**Clinical Pearl:**
In a diagnostic test scenario, the "rule of 95%" is often used to determine the optimal number of tests to be repeated. The test should be repeated until the number of true positives and true negatives reach at least 95% of the total number of tests performed. This ensures that false-positive and false-negative results are minimized, improving the overall diagnostic accuracy of the test.

**Correct Answer:**
The correct answer is D. Repeating the hemoccult test improves its sensitivity and specificity when performed on all results (positive and negative). When performed on positive results only, it reduces false-positive results, but does not improve sensitivity. When performed on negative results only, it reduces false-negative results, but does not improve specificity.