A 35-year-old woman visits her physician in New York after returning from a month-long tri… — USMLE Step 2 CK

A 35-year-old woman visits her physician in New York after returning from a month-long trip to a region endemic for malaria. She complains of intermittent fevers, chills, and myalgias. A newly commercialized rapid diagnostic assay for *Plasmodium falciparum* antigen is considered. The manufacturer's insert states that during clinical validation in the highly endemic tropical region, the assay demonstrated a sensitivity of $95\%$ , a specificity of $90\%$ , and a positive predictive value of $85\%$ . The physician is evaluating the utility of this same assay for symptomatic returning travelers at the New York clinic, where the pre-test probability of malaria is substantially lower than in the validation setting. Assuming the intrinsic accuracy of the assay remains constant, which of the following is the most likely performance characteristic of this assay in the New York clinic?

The positive predictive value will be less than $85\%$ Answer
B
The negative predictive value will be lower than in the validation setting
C
The sensitivity of the assay will fall below $95\%$
D
The specificity of the assay will rise above $90\%$
E
The false-positive rate will exceed $10\%$

Answer

The positive predictive value will be less than

85\%

The correct answer is that the positive predictive value (PPV) will be less than

85\%

. PPV is defined as the probability that a patient with a positive test result actually has the disease. Unlike sensitivity and specificity, which are intrinsic properties of a diagnostic test, predictive values are highly dependent on the disease prevalence (pre-test probability) in the tested population. When a test is applied in a setting with a lower disease prevalence (such as the New York clinic compared to the highly endemic region), the absolute number of false-positive results increases relative to the number of true-positive results. Consequently, the PPV decreases.

Step-by-Step Solution

Identify the test characteristics provided and how the clinical settings differ.

The assay has an intrinsic sensitivity of

95\%

and specificity of

90\%

. The new clinical setting (New York) has a much lower disease prevalence than the validation setting (endemic region).

Understanding which variables are changing is the first step in determining test performance outcomes.

Determine the effect of decreased prevalence on intrinsic versus predictive test values.

Intrinsic values (sensitivity, specificity, and false-positive rate) remain unchanged. Predictive values (PPV, NPV) shift based on disease prevalence.

Sensitivity and specificity are independent of prevalence, while predictive values are heavily prevalence-dependent.

Calculate or deduce the directional change in positive predictive value (PPV).

Because the prevalence of malaria is lower in New York, the proportion of false positives relative to true positives will mathematically increase, causing the PPV to drop below its original

85\%

Lower pre-test probability mathematically leads to a lower PPV, even if the test's diagnostic accuracy remains exactly the same.

Key Concept

Positive predictive value (PPV) is directly proportional to disease prevalence, whereas sensitivity and specificity are intrinsic test properties that remain constant regardless of population prevalence.

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