A screening test for gestational diabetes mellitus (GDM) has a sensitivity of 85% and specificity of 90%. In a population where the prevalence of GDM is 5%, which of the following best distinguishes the clinical utility of this test for ruling IN disease versus ruling OUT disease?

Question

Accepted Answer

B. Positive Predictive Value (PPV) is lower than Negative Predictive Value (NPV) due to low disease prevalence, limiting its ability to rule in disease. ## Understanding Test Utility in Low-Prevalence Populations

**Key Point:** In low-prevalence populations, PPV is substantially lower than NPV, even when sensitivity and specificity are reasonable. This is the critical discriminator between ruling-in and ruling-out utility.

### Calculation of Predictive Values

Given: Sensitivity = 85%, Specificity = 90%, Prevalence = 5%

Using Bayes' theorem for a population of 10,000:
- True Positives (TP) = 5% × 85% = 425
- False Positives (FP) = 95% × 10% = 950
- True Negatives (TN) = 95% × 90% = 8,550
- False Negatives (FN) = 5% × 15% = 75

$$PPV = \frac{TP}{TP+FP} = \frac{425}{425+950} = \frac{425}{1375} ≈ 31\%$$

$$NPV = \frac{TN}{TN+FN} = \frac{8,550}{8,550+75} = \frac{8,550}{8,625} ≈ 99\%$$

### Likelihood Ratios

$$LR+ = \frac{Sensitivity}{1-Specificity} = \frac{0.85}{0.10} = 8.5$$

$$LR- = \frac{1-Sensitivity}{Specificity} = \frac{0.15}{0.90} = 0.17$$

**Clinical Pearl:** A positive test result only increases the probability of disease to 31% (PPV). A negative test result decreases it to <1% (1 − NPV = 1%). This test is far superior for **ruling out** GDM than for **ruling it in**.

**High-Yield:** In low-prevalence settings, even good tests have poor PPV. The low PPV (31%) is the distinguishing feature that limits this test's ability to confidently diagnose disease when positive.

### Why This Matters Clinically

A positive screening test requires confirmatory testing (e.g., oral glucose tolerance test) because only 31% of positive screens truly have GDM. A negative test is highly reassuring (99% NPV) and effectively rules out disease.

**Mnemonic:** **SnNOut, SpPIn** — High Sensitivity rules out (Negative result is reassuring); High Specificity rules in (Positive result is diagnostic). This test has moderate sensitivity (85%) and high specificity (90%), making it better for ruling out.

[cite:Park 26e Ch 10]

Answer

A. Negative Likelihood Ratio (LR−) is lower than Positive Likelihood Ratio (LR+), making it better for ruling out disease

Answer

C. Sensitivity is higher than specificity, indicating the test is equally useful for both ruling in and ruling out disease

Answer

D. Positive Likelihood Ratio (LR+) is higher than Negative Likelihood Ratio (LR−), making it better for ruling in disease

A screening test for gestational diabetes mellitus (GDM) has a sensitivity of 85% and specificity of 90%. In a population where the prevalence of GDM is 5%, which of the following best distinguishes the clinical utility of this test for ruling IN disease versus ruling OUT disease?

Explanation

Understanding Test Utility in Low-Prevalence Populations

Calculation of Predictive Values

Likelihood Ratios

Why This Matters Clinically

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