Fasting Plasma Glucose (≥126 mg/dL) — Type 2 Diabetes

Test Characteristics

Metric	Value
False-negative rate	41% (sensitivity 59%)
False-positive rate	1.2% (specificity 98.8%)
Bayes factor (positive test)	50×
Bayes factor (negative test)	1/2×
Base rate	5% undiagnosed diabetes among US adults

Interpreting Results

Scenario	Prior	+ Result	− Result
Unselected adult	5%	50 × 5% ≥ 100%5%× 50100%	1/2 × 5% = 2.5%5%÷ 22.5%
Low-risk young adult	1%	50 × 1% = 50%1%× 5050%	1/2 × 1% = 0.5%1%÷ 20.5%

50 × 5% ≥ 100%5%× 50100%: exact posterior is 72%. Even more convincing than A1c when positive (50× vs. 10×), but the same weak negative Bayes factor (1/2×). A normal fasting glucose does not rule out diabetes.

• + result: risk goes from vegetarian (5%) to 72%
• − result: same weak negative as A1c

Sources:

• Systematic review, PLOS One, 2020. PLOS One.
• Frontiers in Medicine, 2023. Network meta-analysis.