How does disease prevalence affect the usefulness of a screening test for atrial fibrillation in patients with a history of transient ischemic attack (TIA) or ischemic stroke?

Understanding How Disease Prevalence Affects Screening Test Performance

Increased disease prevalence would increase the pretest probability (Option D is correct). This fundamental principle of diagnostic testing reflects that pretest probability is directly determined by disease prevalence in the population being tested.

Key Concepts in Test Performance

Pretest Probability and Prevalence

• Pretest probability is synonymous with disease prevalence in the population being screened 1
• When you expand screening from a high-risk group (patients >75 years with TIA) to all ischemic stroke patients, you are changing the prevalence of atrial fibrillation in your tested population
• The prevalence of AF varies substantially depending on the population: at least 1 in 3-4 individuals with ischemic stroke have AF, with >80% of cardioembolic strokes associated with AF 1

Why the Other Options Are Incorrect

Option A (Decreased prevalence reduces NPV) is incorrect:

• Decreased disease prevalence actually increases the negative predictive value
• When disease is rare, a negative test is more likely to be truly negative
• NPV improves as prevalence decreases because there are fewer false negatives in the population

Option B (Likelihood ratios depend on prevalence) is incorrect:

• Likelihood ratios are independent of disease prevalence - this is their primary advantage
• The positive LR of 10 and negative LR of 0.1 remain constant regardless of the population tested
• LRs are intrinsic properties of the test itself, determined by sensitivity and specificity

Option C (Increased prevalence reduces PPV) is incorrect:

• Increased disease prevalence actually increases the positive predictive value
• When disease is more common, a positive test is more likely to be truly positive
• The reported PPV of 93.7% was likely calculated in the high-risk population (>75 years with TIA)

Clinical Application to AF Screening

Population-Specific Considerations

• The original device was validated in patients >75 years discharged from stroke units with TIA - a high-prevalence population for AF 1
• Expanding to all ischemic stroke patients changes the pretest probability depending on stroke subtype 1
• Cardioembolic strokes have very high AF prevalence (>80%), while small-vessel (lacunar) strokes have much lower prevalence 1

Impact on Predictive Values

• The PPV of 93.7% will decrease when applied to lower-prevalence populations (younger patients, lacunar strokes)
• The NPV of 81% will increase in lower-prevalence populations
• Detection rates for AF vary from 6.2% in unselected stroke patients to 13.4% in selected/cryptogenic cases 2

Common Pitfalls to Avoid

• Do not confuse likelihood ratios with predictive values - only predictive values change with prevalence
• Do not assume test performance is uniform across populations - the same screening device will have different clinical utility depending on AF prevalence
• Remember that age significantly affects AF prevalence - expanding from >75 years to all ages substantially lowers pretest probability 3, 2

The fundamental principle: pretest probability = prevalence, and both predictive values (but not likelihood ratios) change with prevalence in predictable directions.