Question:

A new biomarker has been identified that allows for the early detection of invasive gastric carcinoma. It has a sensitivity of 89% and a specificity of 85% when compared to endoscopy with multiple biopsies. The test is used in 2 groups: a population in the United States, where 5 out of 100,000 people have gastric cancer, and a population in China, where 100 out of 100,000 people have been diagnosed with the disease. Which of the following is the most accurate statement concerning this new test?

Negative predictive value of the test is lower in the American population

Positive predictive value of the test is higher in the Chinese population

Sensitivity of the test is higher in the American population

Specificity of the test is higher in the Chinese population

The test is not reliable in the American population

Welcome to USMLEPREPS!

Master your USMLE exams with our comprehensive question bank.

Subscribe to our Life Time Plan for unlimited access to Step 1, Step 2, and Step 3 preparation materials.

Don't miss this offer!

Hurry up!

: : Get The Offer

Unlimited Access Step ( one, two and three ).

Priority Access To New Features.

Free Lifetime Updates Facility.

Dedicated Support.

Explanation:

There are many explanatory sources, such as pictures, videos, and audio clips to explain these explanations and questions and explain the answers, but you must subscribe first so that you can enjoy all these advantages. We have many subscription plans at the lowest prices. Don't miss today's offer. Subscribe

	Positive condition	Negative condition
Positive test result	TP	FP	PPV = TP / (TP + FP)
Negative test result	FN	TN	NPV = TN / (TN + FN)
	Sensitivity = TP / (TP + FN)	Specificity = TN / (TN + FP)
FN = false negative; FP = false positive; NPV = negative predictive value; PPV = positive predictive value; TN = true negative; TP = true positive.

The positive predictive value (PPV) is defined as the proportion of individuals with positive test results who actually have the disease. This concept can be understood in terms of probability: given that a patient has a positive test result, what is the likelihood that he or she actually has the disease? PPV depends not only on the sensitivity and specificity of a test, but also on the prevalence of the disease in the population being tested. The more common the disease in the population, the more likely it is that a patient with a positive test result actually has the disease.

If the disease is relatively common (eg, gastric cancer in China), there is a higher probability that a patient who tests positive actually has the disease (ie, more likely to be a true positive). However, if the disease is relatively uncommon (eg, gastric cancer in the United States), a patient who tests positive will have a lower probability of actually having the disease (ie, more likely to be a false positive).

(Choice A) Like PPV, the negative predictive value (NPV) depends on disease prevalence. However, NPV is inversely associated with the prevalence: NPV increases as the disease prevalence decreases. This is because the probability of a true negative result is higher in a population with low disease prevalence (eg, gastric cancer in the United States).

(Choices C and D) Sensitivity and specificity are intrinsic test parameters that do not depend on the prevalence of disease in the tested population.

(Choice E) Reliability is a measure of the consistency (reproducibility) of a diagnostic test result (ie, if the test is taken multiple times, will the results be the same?). No information is provided to suggest that the gastric cancer biomarker is unreliable.

Educational objective:
Positive and negative predictive values depend on disease prevalence in the tested population. The sensitivity and specificity of a diagnostic test are not affected by disease prevalence.