A 6-month-old male presents for a routine visit to his pediatrician. Two months ago, the patient was seen for tachypnea and wheezing, and diagnosed with severe respiratory syncytial virus (RSV) bronchiolitis. After admission to the hospital and supportive care, the patient recovered and currently is not experiencing any trouble breathing. Regarding the possibility of future reactive airway disease, which of the following statements is most accurate?

“Your child has a greater than 20% chance of developing asthma”

“There is no clear relationship between RSV and the development of asthma.”

“Your child’s risk of asthma is less than the general population.”

“Your child has a less than 5% chance of developing asthma”

“Your child’s risk of asthma is the same as the general population.”

A research fellow proposes a nested case-control study within an existing cohort examining antibiotic exposure and C. difficile infection. The mentor suggests this design wastes the cohort structure and that relative risk should be calculated instead. The fellow argues that odds ratios from nested case-control studies approximate relative risk while being more efficient. Evaluate the validity of each position and synthesize the optimal approach.

Use nested case-control with OR since it's mathematically equivalent to cohort RR with proper sampling

Calculate RR from full cohort since all data are available and RR is more interpretable

Use nested case-control only if computational resources are limited, otherwise use full cohort

Perform both analyses and compare results to validate the nested case-control approach

Calculate hazard ratios using Cox regression as a compromise between efficiency and accuracy

Application in different study designs — USMLE Lesson

OR vs. RR - A Tale of Two Ratios

Calculations for both ratios are derived from a 2x2 contingency table, which categorizes individuals by exposure and disease status.

	Disease +	Disease -
Exposed	a	b
Unexposed	c	d

Relative Risk (RR): Compares the incidence (risk) of disease between exposed and unexposed groups. Primarily used in cohort studies.
- RR = $[a/(a+b)] / [c/(c+d)]$
Odds Ratio (OR): Compares the odds of prior exposure between the diseased group and the non-diseased group. The main measure for case-control studies.
- OR = $(a/c) / (b/d) = ad/bc$
- 📌 Mnemonic: OR is the 'cross-product' of the 2x2 table.

⭐ For rare diseases (low prevalence), the OR provides a good approximation of the RR.

Interpreting Ratios - What Do the Numbers Mean?

The value of an Odds Ratio (OR) or Relative Risk (RR) quantifies the association between an exposure and an outcome.

Ratio Value	Interpretation of Association
> 1.0	↑ Increased odds/risk. The exposure is a risk factor.
= 1.0	No association. Exposure does not alter outcome odds/risk.
< 1.0	↓ Decreased odds/risk. The exposure is a protective factor.

Study Designs - The Right Tool for the Job

Study Design	Temporal Direction	Appropriate Measure
Cohort Study	Prospective or Retrospective	Relative Risk (RR)
Case-Control	Retrospective Only	Odds Ratio (OR)
Cross-Sectional	Snapshot in Time	Odds Ratio (OR)

⭐ Case-control studies are retrospective and cannot be used to calculate incidence. Therefore, you MUST use the odds ratio as the measure of association.

Case-control study design flow chart

High‑Yield Points - ⚡ Biggest Takeaways

Odds ratio (OR) is the primary measure of association in case-control studies.

Relative risk (RR) is the standard for cohort studies (prospective or retrospective) and RCTs.

Incidence cannot be calculated in case-control designs, so RR is not applicable.

Cross-sectional studies also utilize the OR.

The OR approximates the RR when the disease is rare (low prevalence).

An OR > 1 indicates increased odds of exposure in cases.

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