A pharmaceutical company maintains a cohort of 50,000 patients with stored serum samples collected at enrolment. Over 8 years, 300 patients develop rheumatoid arthritis. To test whether anticyclic citrullinated peptide antibodies predict future disease, investigators assay the stored samples of these 300 patients and of 600 sampled non-cases. The principal advantage of this design over a conventional case-control study is:

• A It eliminates the need for a control group
• B It is immune to all forms of selection bias
• C It allows direct calculation of relative risk for the entire cohort
• D Exposure is documented before disease onset, preserving temporal sequence ✓

Explanation

This is a nested case-control study conducted within an existing cohort. Because serum was collected before any patient developed arthritis, reverse causation and recall bias are avoided, which is its chief advantage over a conventional retrospective case-control study. Controls are still required, so option A is wrong, and incidence in the full cohort cannot be directly computed without assaying everyone. Selection bias is reduced, not abolished.

Reference: Modern Epidemiology (Rothman, Greenland, Lash), 4th ed.

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Written and medically reviewed by the StethoPrep medical team.