WBR0391

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biostatistics/Epidemiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::Investigators of a case-control study are investigating the association between the chronic use of non-steroidal anti-inflammatory drugs (NSAIDs) over a one year duration and impaired renal function. 2000 men and women over the age of 50 are surveyed, and the results of the study are categorized in the table shown below. What is the odds ratio of having impaired renal function with chronic NSAID use based on the data below? ]]
Answer A	AnswerA::0.041
Answer A Explanation	[[AnswerAExp::0.041 represents the attributable risk, which is calculated using the following equation: Attributable Risk = (a/[a+b]) – (c/[c+d])]]
Answer B	AnswerB::0.500
Answer B Explanation	AnswerBExp::0.500 does not correspond to any relevant value.
Answer C	AnswerC::1.066
Answer C Explanation	[[AnswerCExp::1.066 represents the relative risk (RR), which is another relative measure of effect often used in cohort studies. RR is calculated using the following equation: RR = (a/[a+b]) / (c/[c+d])]]
Answer D	AnswerD::1.200
Answer D Explanation	AnswerDExp::1.200 correctly represents the OR
Answer E	AnswerE::1.500
Answer E Explanation	AnswerEExp::1.500 does not represent any relevant value.
Right Answer	RightAnswer::D
Explanation	[[Explanation:: Odds ratio is a relative measure of effect that compares the 2 groups of a case-control study (chronic NSAID use vs. no chronic NSAID use). In this case-control study, the effect the investigators are measuring is renal impairment following one year of NSAID use. Accordingly, the odds ratio (OR) is calculated using the equation: OR = ad/bc = (800 x 300)/(400 x 500) = 240000/200000 = 1.2. OR = 1 implies that there is no difference between the control group (no chronic NSAID use) and the intervention group (chronic NSAID use). In this study, had OR been = 1, then the use of chronic NSAID has no effect on renal impairment compared with no chronic NSAID use. OR > 1 implies the control group (no chronic NSAID use) is better than the intervention group (chronic NSAID use). In this study, OR = 1.2, which demonstrates that the odds of renal impairment with chronic NSAID use is 20% ([1.2 - 1.0] x 100) higher than with no chronic NSAID use. OR < 1 implies the intervention group (chronic NSAID use) is better than the control group (no chronic NSAID use). Had OR been < 1 (e.g 0.7) in this study, the odds of renal impairment with chronic NSAID use would have been 30% ([1.0 - 0.7] x 100) lower than with no chronic NSAID use. Educational Objective: Odds ratio is defined as a relative measure of effect that compares the 2 groups of the case-control study. The odds ratio (OR) is calculated using the equation: OR = ad/bc References: First Aid 2014 page 50]]
Approved	Approved::Yes
Keyword	WBRKeyword::Odds ratio, WBRKeyword::OR, WBRKeyword::Case-control, WBRKeyword::Equation, WBRKeyword::NSAID, WBRKeyword::Renal impairment
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::