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Zorluk: OrtaMeasures of Disease Frequency and Association

A prospective cohort study is conducted to evaluate the association between a history of prior abdominal surgery and the development of acute small bowel obstruction (SBO) over a 55-year follow-up period. The investigators enroll 2,0002,000 healthy adult volunteers. Among the 400400 participants with a history of prior abdominal surgery, 3232 develop an SBO during the study period. Among the 1,6001,600 participants with no history of abdominal surgery, 1616 develop an SBO.

Based on these findings, which of the following is the most accurate epidemiologic conclusion that can be drawn from this study?

  1. A
    The relative risk cannot be directly calculated because this is a case-control study; the appropriate measure is the odds ratio.
  2. B
    The positive predictive value of prior abdominal surgery for developing an obstruction is 8%8\%, which would increase if the population prevalence of obstruction decreases.
  3. The relative risk of developing a small bowel obstruction is 8.08.0 for participants with a history of prior abdominal surgery.Cevap
  4. D
    The calculated association is invalid because prior abdominal surgery acts as a confounder rather than a direct risk factor for obstruction.
  5. E
    The high absolute risk indicates that conservative management is contraindicated and all patients with prior surgeries presenting with obstruction signs require immediate laparotomy.

Cevap

The relative risk of developing a small bowel obstruction is 8.0 for participants with a history of prior abdominal surgery.
This study is a prospective cohort study because participants were categorized by their exposure status (history of prior abdominal surgery) and followed over time to observe the development of an outcome (small bowel obstruction). In cohort studies, the appropriate measure of association is the Relative Risk (RR). Relative risk is the ratio of disease incidence in the exposed group to the disease incidence in the unexposed group. The incidence in the exposed group (IEI_E) is 32/400=0.0832 / 400 = 0.08. The incidence in the unexposed group (IUI_U) is 16/1600=0.0116 / 1600 = 0.01. The calculated RR is 0.08/0.01=8.00.08 / 0.01 = 8.0. This demonstrates that individuals with a history of abdominal surgery are 88 times more likely to develop a small bowel obstruction over a 55-year period compared to those without prior surgery.

Adım Adım Çözüm

1
Identify the study design based on subject enrollment.
Participants were enrolled based on their exposure status (prior surgery vs. no prior surgery) and followed forward in time.
This confirms a prospective cohort study, which allows for the direct calculation of incidence and Relative Risk (RR).
2
Calculate the incidence of the outcome in the exposed group (IEI_E).
IE=32400=0.08I_E = \frac{32}{400} = 0.08 (or 8%8\%).
Incidence is defined as the number of new cases divided by the total population at risk within a specific time period.
3
Calculate the incidence of the outcome in the unexposed group (IUI_U).
IU=161600=0.01I_U = \frac{16}{1600} = 0.01 (or 1%1\%).
This determines the baseline risk of the disease occurring without the exposure.
4
Calculate the measure of association (Relative Risk).
RR=IEIU=0.080.01=8.0RR = \frac{I_E}{I_U} = \frac{0.08}{0.01} = 8.0.
Relative Risk is the ratio of the incidence in the exposed to the incidence in the unexposed.

Anahtar Kavram

In a cohort study, the primary measure of association is the Relative Risk (RR), calculated as the incidence rate of the outcome in the exposed group divided by the incidence rate in the unexposed group.
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Measures of Disease Frequency and Association

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