Examkin

Question

Difficulty: MediumDiagnostic and Screening Test Performance

A global health organization is rolling out a new rapid serological assay for a mosquito-borne viral infection. In the initial validation study conducted in a highly endemic tropical country, the assay demonstrated a sensitivity of 85%85\% and a specificity of 90%90\%.

The organization is now deploying the identical assay to a temperate region where recent epidemiological surveys have established the disease prevalence to be 20%20\%.

Assuming the intrinsic properties of the assay remain unchanged, what is the expected positive predictive value of the assay in this temperate region?

  1. A
    32%32\%
  2. B
    60%60\%
  3. 68%68\%Answer
  4. D
    85%85\%
  5. E
    96%96\%

Answer

The expected positive predictive value in the temperate region is 68%68\%.
Positive Predictive Value (PPV) is the probability that a patient with a positive test result actually has the disease. It is calculated by dividing the number of true positives by the total number of positive results (true positives + false positives). In a hypothetical population of 100100 people with a 20%20\% disease prevalence, 2020 people have the disease and 8080 do not. A test with 85%85\% sensitivity correctly identifies 1717 of the 2020 diseased individuals (True Positives = 1717). A test with 90%90\% specificity correctly identifies 7272 of the 8080 healthy individuals, leaving 88 individuals falsely identified as positive (False Positives = 88). The total number of positive results is 17+8=2517 + 8 = 25. Therefore, the PPV is 1725\frac{17}{25}, which equals 0.680.68 or 68%68\%.

Step-by-Step Solution

1
Establish a hypothetical population to simplify calculations (e.g., 100100 individuals) and determine the number of diseased and healthy individuals based on the 20%20\% prevalence.
Diseased = 2020 individuals; Healthy = 8080 individuals.
Using a baseline population of 100100 allows for straightforward conversion of percentages into absolute numbers of true and false test results.
2
Calculate the number of True Positives (TP) and False Positives (FP) using the given sensitivity (85%85\%) and specificity (90%90\%).
TP = 85%85\% of 20=1720 = 17. FP = 10%10\% (which is 11 minus specificity) of 80=880 = 8.
Positive Predictive Value (PPV) depends on the total number of positive test results, which consists of true positives (from the diseased group) and false positives (from the healthy group).
3
Calculate the Positive Predictive Value (PPV) by dividing the True Positives by the Total Positives.
PPV=1717+8=1725=0.68PPV = \frac{17}{17 + 8} = \frac{17}{25} = 0.68, or 68%68\%.
PPV represents the clinical probability that a person with a positive test result actually has the disease.

Key Concept

The Positive Predictive Value (PPV) of a diagnostic test is heavily dependent on the prevalence of the disease in the tested population; to calculate it accurately, one must determine the absolute number of true positives and false positives in the specific population being tested.
Rate this question

Topics

Diagnostic and Screening Test Performance

More questions from this topic