A gastroenterology research network is evaluating a novel fecal biomarker to detect advanc… — USMLE Step 2 CK

A gastroenterology research network is evaluating a novel fecal biomarker to detect advanced colorectal adenomas. A prospective validation study enrolls $1,200$ asymptomatic adults undergoing screening colonoscopy, which serves as the gold standard. Colonoscopy confirms the presence of advanced adenomas in $200$ of the participants. The novel biomarker test correctly identifies $160$ of these participants. Additionally, the biomarker test yields a positive result in $90$ participants who are confirmed to have no adenomas on colonoscopy. What is the positive predictive value of the novel biomarker test in this study population, expressed as a percentage?

Answer: 64 %

Answer

The positive predictive value (PPV) is calculated by dividing the number of true positives (TP) by the total number of positive test results (

TP + FP

). In this scenario,

TP = 160

and

FP = 90

. Therefore,

PPV = \frac{160}{160 + 90} = \frac{160}{250} = 0.64

, which is

64\%

Step-by-Step Solution

Determine the number of True Positives (TP).

TP = 160

The study states that out of the

200

patients with confirmed adenomas, the biomarker correctly identified

160

Determine the number of False Positives (FP).

FP = 90

The study explicitly states the biomarker test yielded a positive result in

90

participants who did not have adenomas.

Calculate the Positive Predictive Value (PPV).

PPV = \frac{160}{160 + 90} = \frac{160}{250} = 0.64

PPV is the probability that a person with a positive test result actually has the disease, calculated as the ratio of true positives to total positive test results.

Convert the resulting probability to a percentage.

0.64 \times 100 = 64

The question specifically asks for the value to be expressed as a percentage.

Key Concept

The positive predictive value (PPV) represents the proportion of positive test results that are true positives, indicating the probability that a patient with a positive test actually has the disease.

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