A screening test for gestational diabetes mellitus (GDM) in Indian pregnant women is being validated. The test has a sensitivity of 88% and a specificity of 92%. All of the following statements about this test are true EXCEPT:

Question

Accepted Answer

A. The test is more useful for ruling in GDM than for ruling out GDM. ## Likelihood Ratios and Clinical Utility

**Key Point:** Likelihood ratios quantify how much a test result changes the probability of disease. A test with high sensitivity but lower specificity is better for ruling OUT disease (high NPV), not ruling IN disease.

### Calculating Likelihood Ratios

Given:
- Sensitivity = 88%
- Specificity = 92%

**Positive Likelihood Ratio (LR+)** = $\frac{	ext{Sensitivity}}{1 - 	ext{Specificity}} = \frac{0.88}{1-0.92} = \frac{0.88}{0.08} = 11$

**Negative Likelihood Ratio (LR−)** = $\frac{1 - 	ext{Sensitivity}}{	ext{Specificity}} = \frac{1-0.88}{0.92} = \frac{0.12}{0.92} ≈ 0.13$

### Interpretation of Likelihood Ratios

| LR+ Value | Clinical Interpretation |
|-----------|------------------------|
| > 10 | Strong evidence to rule IN disease |
| 5–10 | Moderate evidence to rule IN disease |
| 1–5 | Weak to moderate evidence to rule IN disease |
| 1 | No diagnostic value |
| 0.1–1 | Weak to moderate evidence to rule OUT disease |
| < 0.1 | Strong evidence to rule OUT disease |

**High-Yield:** 
- **LR+ = 11** → Strong evidence to RULE IN GDM (good for confirmation)
- **LR− = 0.13** → Moderate evidence to RULE OUT GDM (good for exclusion)

This test is actually **better for ruling IN disease** (LR+ = 11 is strong) than for ruling OUT (LR− = 0.13 is only moderate). Option 3 states the opposite.

### Verification of Other Options

**Option 1:** False Negative Rate = 100% − Sensitivity = 100% − 88% = 12%
- Out of 100 GDM patients, 12 will test negative ✓ (Correct)

**Option 2:** False Positive Rate = 100% − Specificity = 100% − 92% = 8%
- Out of 100 non-GDM individuals, 8 will test positive ✓ (Correct)

**Option 4:** LR− = 0.12/0.92 ≈ 0.13 ✓ (Correct)

**Clinical Pearl:** A test with LR+ > 10 and LR− > 0.1 is useful for both ruling in and ruling out. However, this test's LR+ (11) is much stronger than its LR− (0.13), making it primarily a confirmatory test, not an exclusionary one.

**Mnemonic:** **SnNOut, SpPIn**
- **Sn**Nsitive test, **N**egative result → rules **Out** disease
- **Sp**ecific test, **P**ositive result → rules **In** disease

This test has moderate sensitivity (88%) and good specificity (92%), so it's better for ruling IN (SpPIn) than ruling OUT.

Answer

B. The negative likelihood ratio is approximately 0.13

Answer

C. If 100 pregnant women without GDM are tested, approximately 8 will test positive (false positives)

Answer

D. If 100 pregnant women with GDM are tested, approximately 12 will test negative (false negatives)

A screening test for gestational diabetes mellitus (GDM) in Indian pregnant women is being validated. The test has a sensitivity of 88% and a specificity of 92%. All of the following statements about this test are true EXCEPT:

Explanation

Likelihood Ratios and Clinical Utility

Calculating Likelihood Ratios

Interpretation of Likelihood Ratios

Verification of Other Options

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LR+ Value	Clinical Interpretation
> 10	Strong evidence to rule IN disease
5–10	Moderate evidence to rule IN disease
1–5	Weak to moderate evidence to rule IN disease
1	No diagnostic value
0.1–1	Weak to moderate evidence to rule OUT disease
< 0.1	Strong evidence to rule OUT disease