Examkin

Question

Difficulty: MediumDiagnostic Testing Performance

A 62-year-old man presents to the clinic to discuss pancreatic cancer screening. He has no current symptoms of abdominal pain, jaundice, or weight loss. Vital signs are within normal limits, and physical examination is unremarkable. His father and paternal uncle both died of pancreatic adenocarcinoma, prompting his enrollment in a high-risk familial surveillance registry. Epidemiological data indicates that the prevalence of early-stage pancreatic adenocarcinoma in this specific surveillance cohort is 10%10\%.

As part of a new clinical protocol, the patient's blood is tested using a novel multi-target serum biomarker panel. The assay has a validated sensitivity of 90%90\% and a specificity of 90%90\% for detecting early-stage disease.

The patient's biomarker panel returns a positive result.

Based on the provided test characteristics and cohort prevalence, what is the post-test probability (in percent) that this patient actually has early-stage pancreatic adenocarcinoma?

Answer: 50 %

Answer

50
The question asks for the post-test probability of disease following a positive test result, which is the positive predictive value (PPV) of the assay. PPV depends on the test's sensitivity and specificity, as well as the disease prevalence in the tested population.

Using a hypothetical cohort of 10001000 patients:
- Number of patients with disease: 1000×0.10=1001000 \times 0.10 = 100
- Number of patients without disease: 1000×0.90=9001000 \times 0.90 = 900

Next, calculate the number of positive test results in each group:
- True positives (TP) = 100×0.90 (sensitivity)=90100 \times 0.90 \text{ (sensitivity)} = 90
- False positives (FP) = 900×(1−0.90 specificity)=90900 \times (1 - 0.90 \text{ specificity}) = 90

The total number of positive test results is 90 (TP)+90 (FP)=18090 \text{ (TP)} + 90 \text{ (FP)} = 180.

PPV = TPTP+FP=90180=0.50\frac{\text{TP}}{\text{TP} + \text{FP}} = \frac{90}{180} = 0.50 (or 50%50\%).

Therefore, the probability that the patient has early-stage pancreatic adenocarcinoma is 50%50\%.

Step-by-Step Solution

1
Determine the disease prevalence in the cohort.
The prevalence is 10%, meaning in a population of 1000, 100 have the disease and 900 do not.
Prevalence establishes the pre-test probability necessary for calculating predictive values.
2
Calculate the true positive (TP) and false positive (FP) counts.
TP = 100 × 0.90 = 90. FP = 900 × 0.10 = 90.
Using absolute numbers derived from sensitivity and specificity clarifies the ratio of true to false positive results.
3
Calculate the Positive Predictive Value (PPV).
PPV = TP / (TP + FP) = 90 / (90 + 90) = 0.50, or 50%.
The PPV directly represents the post-test probability of disease following a positive test result.

Key Concept

Positive predictive value (PPV) and its dependence on disease prevalence.
Rate this question

Topics

Diagnostic Testing Performance

More questions from this topic