Investigators conduct a retrospective study utilizing electronic health records from a lar… — USMLE Step 3

Investigators conduct a retrospective study utilizing electronic health records from a large integrated healthcare system to explore the relationship between cumulative exposure to first-generation H1-antihistamines and the subsequent diagnosis of mild cognitive impairment (MCI) in older adults. The researchers identify a group of patients recently diagnosed with MCI and match them to a group of healthy controls with normal cognition, based on age, sex, and socioeconomic status. A review of pharmacy dispensing records over the preceding $10$ years is performed to determine the history of regular, high-dose H1-antihistamine use.

The investigators compile the following data:

Patient Group	History of Regular H1-Antihistamine Use	No History of Regular H1-Antihistamine Use	Total
MCI (Cases)	$180$	$120$	$300$
Normal Cognition (Controls)	$200$	$400$	$600$

Based on these findings, which of the following is the best estimate of the odds ratio for developing mild cognitive impairment associated with regular use of first-generation H1-antihistamines?

A
0.33
B
0.75
C
1.50
D
1.80
3.00Answer

Answer

3.00

The study design presented is a case-control study, as patients were identified based on the presence or absence of a disease (mild cognitive impairment) and then assessed for prior exposure (first-generation H1-antihistamines). The appropriate measure of association for a case-control study is the odds ratio (OR). The odds ratio determines if the odds of prior exposure are different between cases and controls.

Using a standard 2x2 contingency table, the formula is OR = (a / c) / (b / d), which algebraically simplifies to the cross-product ratio (a × d) / (b × c).
-

a

= exposed cases =

180

c

= unexposed cases =

120

b

= exposed controls =

200

d

= unexposed controls =

400

Plugging these values into the formula:

OR = \frac{180 \times 400}{200 \times 120}

OR = \frac{72,000}{24,000}

OR = 3.00

This means that the odds of having a history of regular H1-antihistamine use were 3 times higher in patients who developed mild cognitive impairment compared to the healthy controls.

Step-by-Step Solution

Identify the study design and the appropriate measure of association.

The scenario describes a case-control study (starting with disease status and looking back at exposure). The appropriate measure of association is the odds ratio (OR).

Case-control studies cannot directly measure incidence, making the odds ratio the standard metric for determining association.

Extract the values from the contingency table to set up the OR formula.

Exposed cases (a) = 180, unexposed cases (c) = 120, exposed controls (b) = 200, unexposed controls (d) = 400.

These values populate the cells of a standard 2x2 contingency table needed for calculating the cross-product.

Apply the odds ratio formula.

OR = (a × d) / (b × c) = (180 × 400) / (200 × 120) = 72,000 / 24,000 = 3.00.

The odds ratio compares the odds of exposure in cases (a/c) to the odds of exposure in controls (b/d).

Key Concept

The odds ratio (OR) is the primary measure of association in case-control studies, calculated as the odds of exposure among cases divided by the odds of exposure among controls: (a × d) / (b × c).

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