WBR0147

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Will Gibson and Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biostatistics/Epidemiology
Sub Category	SubCategory::Endocrine
Prompt	[[Prompt::Two medical teams are conducting identical studies that aim to detect an association between vitamin K deficiency and osteoporosis in France and Russia. Both teams are using the same test kit that is designed to measure vitamin K concentration in the blood. The sensitivity of the test is 90% and its specificity is 85%. According to previous studies, the prevalence of vitamin K deficiency is higher in Russia than it is in France. Based on the previous information, which of the following statements is correct?]]
Answer A	[[AnswerA::The ability of the test to detect vitamin K deficiency among people who actually have a low vitamin K level is higher in Russia than it is in France.]]
Answer A Explanation	[[AnswerAExp::The ability of the test to detect vitamin K deficiency among people who actually have a low vitamin K concentration refers to the sensitivity of the test. The sensitivity of the test is 90%, and this value is constant for a particular test. The sensitivity of this test is independent of the prevalence of vitamin K deficiency.]]
Answer B	AnswerB::The positive predictive value of the test is the same in France and Russia.
Answer B Explanation	[[AnswerBExp::The positive predictive value (PPV) refers to the probability that in the case of a positive test, the patient actually has the specified disease. This term is affected by the pre-test probability of a true positive based on the prevalence within a population. Since the prevalence of vitamin K deficiency is different in Russia from its prevalence in France, the PPV is not the same in these two countries.]]
Answer C	AnswerC::The positive predictive value of the test is higher in Russia than it is in France.
Answer C Explanation	[[AnswerCExp::The positive predictive value (PPV) refers to the probability that in the case of a positive test, the patient actually has the specified disease. A higher population prevalence leads to a higher positive predictive value.]]
Answer D	AnswerD::If the test is positive, the probability that the person truly has vitamin K deficiency is higher in France than it is in Russia.
Answer D Explanation	[[AnswerDExp::The PPV depends on the sensitivity, specificity, and prevalence of the tested item. Since the prevalence of vitamin K deficiency is lower in France than it is in Russia, then the PPV of this test is lower in France than it is in Russia.]]
Answer E	AnswerE::The positive predictive value of the test does not depend on the sensitivity and specificity.
Answer E Explanation	[[AnswerEExp::The PPV depends on the sensitivity, specificity, and prevalence of the tested item.]]
Right Answer	RightAnswer::C
Explanation	[[Explanation::The positive predictive value (PPV) is used to indicate the probability that in the case of a positive test, the patient actually has the specified disease. PPV is a function of the population sampled because if the pre-test probability of positive test is higher, a positive prediction is more likely to be correct. Since the prevalence of vitamin K deficiency is higher in Russia than it is in France, the PPV would be higher in Russia. Educational Objective: The PPV depends on the sensitivity, specificity, and prevalence of the tested item. References: First Aid 2014 page 51]]
Approved	Approved::Yes
Keyword	WBRKeyword::Statistics, WBRKeyword::Sensitivity, WBRKeyword::Specificity, WBRKeyword::Positive predictive value, WBRKeyword::Testing, WBRKeyword::Epidemiology
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::