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Difficulty: MediumMeasures of Disease Frequency and Association

Researchers are conducting a case-control study to investigate a potential association between the use of a newly approved oral biologic for severe plaque psoriasis and the subsequent development of drug-induced lupus erythematosus (DILE). The investigators identify 150 patients from a national rheumatology database who were recently diagnosed with DILE. They also randomly select 150 healthy volunteers without DILE from a primary care registry, matched for age and sex, to serve as a comparison group. A detailed review of the participants' pharmacological histories reveals that 100 of the patients with DILE had been prescribed the new oral biologic prior to their diagnosis. Among the comparison group, 50 subjects had a history of taking the same medication. Based on the data collected, which of the following is the odds ratio for developing DILE associated with the use of this oral biologic?

  1. A
    0.250.25
  2. B
    0.330.33
  3. C
    2.02.0
  4. D
    2.52.5
  5. 4.04.0Answer

Answer

The odds ratio for developing DILE associated with the oral biologic is 4.04.0.
The correct answer accurately applies the odds ratio formula for a case-control study. The odds of exposure in the case group is 100/50=2.0100/50 = 2.0. The odds of exposure in the control group is 50/100=0.550/100 = 0.5. The odds ratio is the ratio of these two odds: 2.0/0.5=4.02.0 / 0.5 = 4.0. Alternatively, using the cross-product ratio OR=a×db×c=100×10050×50=100002500=4.0OR = \frac{a \times d}{b \times c} = \frac{100 \times 100}{50 \times 50} = \frac{10000}{2500} = 4.0.

Step-by-Step Solution

1
Identify the study design and organize the data into a 2x2 contingency table.
Cases (DILE) = 150, Controls (No DILE) = 150. Exposed cases = 100, Unexposed cases = 50. Exposed controls = 50, Unexposed controls = 100.
This is a case-control study (selected based on outcome), which means the appropriate measure of association is the odds ratio.
2
Calculate the odds of exposure among the cases.
Oddscases=Exposed casesUnexposed cases=10050=2.0Odds_{cases} = \frac{\text{Exposed cases}}{\text{Unexposed cases}} = \frac{100}{50} = 2.0
The odds of exposure for cases compares those who took the biologic to those who did not within the DILE group.
3
Calculate the odds of exposure among the controls.
Oddscontrols=Exposed controlsUnexposed controls=50100=0.5Odds_{controls} = \frac{\text{Exposed controls}}{\text{Unexposed controls}} = \frac{50}{100} = 0.5
The odds of exposure for controls compares those who took the biologic to those who did not within the healthy group.
4
Calculate the odds ratio (OR).
OR=OddscasesOddscontrols=2.00.5=4.0OR = \frac{Odds_{cases}}{Odds_{controls}} = \frac{2.0}{0.5} = 4.0
The odds ratio quantifies the strength of the association between the exposure and the disease in a case-control study.

Key Concept

In a case-control study, the odds ratio (OR) is the primary measure of association. It is calculated by dividing the odds of exposure among cases by the odds of exposure among controls.
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Topics

Measures of Disease Frequency and Association

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