WBR0038

Author	[[PageAuthor::Associate Editor-In-Chief: Gonzalo A. Romero, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Behavioral Science/Psychiatry
Sub Category	SubCategory::Infectious Disease
Prompt	[[Prompt::A study is trying to prove different parameters of a new test used in diagnosing an infectious endemic disease in developing countries. Underserved areas’ tribes have have called the disease “Ztika”. For years physicians have struggled to diagnose it early enough to treat it soon so the infectious organism is controlled with antimicrobial therapy with the final goal of decreasing mortality. The disease is asymptomatic until severe symptoms develop and is exactly when the immune response is triggered making the final diagnosis possible through IgM specific serologic testing. A group of investigators had the initiative to develop a new test X-22231 which screens for the serum marker Z-AWS (a component of the infectious organism), present in early phases of this infectious disease called “Ztika” and decide to run a Phase I trial which proved to have promising results. The researchers move onto a Phase II trial recruiting more patients. The Institutional Review Board meets with the Sponsors and Data Monitoring Safety Committee in order to establish that a proper study design is being created and conducted. The study starts having an initial sample which involves 500 subjects and followed overtime which helped gathering data. The pre-test probabilities were determined; Sensitivity 90% and Specificity 80%. It was determined that 100 patients did have the disease “Ztika”. What is the probability that a patient with a positive result from the X-22231 test really had the disease?]]
Answer A	AnswerA::90/100
Answer A Explanation	[[AnswerAExp::Sensitivity TP/ (TP + FN)]]
Answer B	AnswerB::320/ 400
Answer B Explanation	[[AnswerBExp::Specificity or TN / (TN + FP)]]
Answer C	AnswerC::90/170
Answer C Explanation	AnswerCExp::PPV. Correct answer. See the explanation above.
Answer D	AnswerD::320 / 330
Answer D Explanation	[[AnswerDExp::Negative Predictive Value (NPV); or the probability that a negative test result is truly negative. TN / (TN + FN)]]
Answer E	AnswerE::90 + 320 / 500
Answer E Explanation	[[AnswerEExp::Accuracy “Trues divided by the sample size” or (TN + TP) / (TP + TN + FP + FN)]]
Right Answer	RightAnswer::
Explanation	[[Explanation::Test taking strategy tip: It is important to read throughly this question scanning for the data that is vital which will help in answering the question correctly. USMLE Step 1 more often is including long stems, so being able to read quickly, yet carefully will enable the examinee to screen for the information necessary to aswer correctly. This question tests for basic statistical knowledge which is commony tested on STEP 1 and other USMLE examinations. A great tip is to read at the final lines to find what the question is really looking for. Explanation: At the end of the long stem the data is provided directly in a way that will enable construct the table which will orient the test taker to calculate what’s being asked; the Positive Predictive Value. The sample size is 500, it is given the number of people with the disease 100, which is the True Positive + False Negative (TP + FN). Also the sensitivity or the proportion of truly diseased persons is given 90%. From there it can be assumed that the True positives were 90 and False negatives were 10. The remainder 400 did not have the disease. The specificity (proportion of truly non-diseased persons who are identified as non diseased) is also given in the stem which is 80% so it can be established that the True Negatives (TN) is the 80% of this 400 or 320 and the False Positives were 80. The Positive Predictive Value is the probability that a positive test result is actually positive, being TP/(TP + FP) being calculated as 90 / 170. Educational Objective: References: ]]
Approved	Approved::Yes
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