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#1
02-23-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 284 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96
Biostatistics Question

Gary Johnson presents to the clinic with recurrent epigastric pain that occurs about two hours after eating. He attributes the pain to the stress associated with going to high school. After obtaining the relevant information from the history and physical examination, Dr. Brown suspects a helicobacter pylori infection and counsels her patient that ordering a stool antigen assay would be appropriate. Mr. Johnson consents, but admits that he is a little confused about something. What he would like to know is why people sometimes say that a negative result is a good thing? Dr. Brown explains that if an investigation is very sensitive, then a negative test simply means that the patient probably does not have the condition. In a hypothetical study, 443 out of 543 positive tests and 548 out of 566 negative tests were correct. Based on this information, what proportion of people with a helicobacter pylori infection would Dr. Campbell tell her patient are correctly identified by the test?

#2
02-23-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 284 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96

a. 443/543
b. 548/648
c. 548/566
d. 461/543
e. 443/461
#3
02-23-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 284 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96

The question can be answered by making a punnett square....

----------------------------Condition-----------------------------
--------------------Present-------Absent------------------------

Test Positive------A (443)-------B (100)-----------A + B (543)

Test Negative-----C (18)--------D (548)-----------C + D (566)

------------------A + C (461)----B + D (516)--------Total (1064)

Sensitivity is calculated by dividing the total number of patients that have the condition and have a positive test (Box A) by the total number of patients that have the condition (Box A + C).

Box A: 499
Box A + C: 508

Therefore the answer to the question is 499/508

#4
02-25-2016
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 284 Threads: 32 Thanked 86 Times in 71 Posts Reputation: 96

you can begin solving the question by entering the data into the punnet square that is immediately available in the question.

----------------------------Condition-----------------------------
--------------------Present-------Absent------------------------

Test Positive------A (443)-------B ( )-----------A + B (543)

Test Negative-----C ( )--------D (548)-----------C + D (566)

------------------A + C ( )----B + D ( )--------Total ( )

next you have to calculate how many patients with the condition tested negative (box C). to do so subtract the total number of patients with a negative test (box C + D: 566) by the number of true negatives (box D: 548). doing so you get the following data....

Test Negative-----C (18)--------D (548)-----------C + D (566)

now you can calculate the number of patients with the condition (box A + C). to do so add the number of patients with the condition who tested positive (box A: 443) to the number of patients with the condition who tested negative (box C: 18). What you find is that there were 461 patients with the condition, of which 443 tested positive. at this point its easy to calculate the sensitivity.
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