Calculate positive predictive value of ELISA test for HIV with sensitivity 99%, specificity 99%. Prevalence of HIV in population is 5/1000.

**Core Concept**
The positive predictive value (PPV) of a diagnostic test is the probability that a patient with a positive test result actually has the disease. This requires knowledge of the test's sensitivity, specificity, and the prevalence of the disease in the population.

**Why the Correct Answer is Right**
To calculate PPV, we use the formula: PPV = (Sensitivity * Prevalence) / ((Sensitivity * Prevalence) + ((1 - Specificity) * (1 - Prevalence))). Given sensitivity = 99% (0.99), specificity = 99% (0.99), and prevalence = 5/1000 (0.005), we can plug these values into the formula.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the prevalence of HIV in the population, which is a crucial factor in calculating PPV.
**Option B:** This option is incorrect because it incorrectly states that PPV is calculated as (Sensitivity + Specificity) / (Prevalence + (1 - Prevalence)), which is not the correct formula.
**Option C:** This option is incorrect because it does not provide a numerical value for PPV and is incomplete.
**Option D:** This option is incorrect because it does not provide a numerical value for PPV and is incomplete.

**Clinical Pearl / High-Yield Fact**
Remember that PPV increases with increasing prevalence of the disease and decreases with decreasing specificity of the test.

**Correct Answer:**
To calculate PPV: PPV = (0.99 * 0.005) / ((0.99 * 0.005) + ((1 - 0.99) * (1 - 0.005))) = 0.00495 / (0.00495 + 0.00095) = 0.00495 / 0.0059 ≈ 0.835.