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BioStat Question for Today
Good morning everyone, i leave you with a question that i found hard but is good cause that way we master biostat.
if anyone has more questions please add the links or put them here cause id like to practice since im terrible at biostat good day to everyone!! A new screening test for the development of mitral regurgitation in the setting of rheumatic fever is created. A study of 1000 patients with a history of s.pyogenes infection and a diagnosis of rheumatic fever is performed using this test which has 90% sensitivity and 85% specificity, the prevalence of mitral regurgitation in this population is estimated to be 40%. what is the positive predictive value of this test? A) 8% B) 20% C) 80% D) 85% E) 93%
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Borzoo400 (02082014) 
#2




Quote:
Diseased poeple = 40% of 1000 = 400 Therefore. .................... ..... MR+. ............ MR Positive result ...... 360. ............ 90 Neg result. ........ 40. ............. 510 NPV. 360/(360+90)= 80% Ans: ppv= 80%
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In USMLE, “Any fool can know. The point is to understand.” Last edited by nsesereso; 01292014 at 08:38 AM. 
The above post was thanked by:  
Borzoo400 (02072014) 
#3




i think is 85 % but i didnt use the prevelance in my calculations. i used 2x2 box a= 900 b=150 c=100 d=850 ppv= a/(a+b)

#4




The prevalence given is not for fun my friend.
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In USMLE, “Any fool can know. The point is to understand.” 
#6




nsesereso and Doc889 very good!! please pass me some of your
biostat knowledge!! this question made me sweat lol. thanks for the explanation also.
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#7




Then u also pass some of the other subjects to me. Am sweating too my friend, it's not easy.
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In USMLE, “Any fool can know. The point is to understand.” 
#8




Here is a youtube video with an animated explanation on how to calculate the positive predictive value... https://www.youtube.com/watch?v=Xhm2Ida053Y
The positive predictive value is the proportion of positive tests that occur in individuals that have the condition. In order to solve this question you need to know the number of true positives (i.e., people with mitral regurgitation who tested positive) and the total number of positive tests. These values are not given, but can be determined from the information provided. Sample Table Disease PresentAbsent Test Positive A (360)B (90)A+B (450) Test Negative C (40)D (510)C+D (550) A+C (400)B+D (600)(1000) How To Fill The Table A + C: The number of people with mitral regurgitation is 40% * 1000, which is 400. B + D: The number of people without mitral regurgitation is 1000  400, which is 600. A: The number of true positives [(A+C) * sensitivity] is 600 * 90%, which is 360. D: The number of true negatives [(B+D) * specificity] is 400 * 85%, which is 510. B: The number of false positives [(B+D)D] is 600  510, which is 90. [B]A + B: The total number of positive tests is 360 + 90, which is 450. Answer The positive predictive value is calculated by dividing the number of true positives by the total number of positive tests. Positive Predictive Value [A/(A+B)] = 360/450....... 80%
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