What are the diagnostic criteria for diabetes mellitus in a patient with a family history, obesity, or a history of gestational diabetes?

Diagnostic Criteria for Diabetes Mellitus

Diabetes mellitus is diagnosed when fasting plasma glucose is ≥126 mg/dL (7.0 mmol/L), hemoglobin A1C is ≥6.5% (48 mmol/mol), 2-hour plasma glucose during a 75-g oral glucose tolerance test is ≥200 mg/dL (11.1 mmol/L), or random plasma glucose is ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia. 1, 2

Standard Diagnostic Criteria

The diagnosis requires confirmation on a subsequent day unless unequivocal hyperglycemia with acute metabolic decompensation is present 1:

• Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) - fasting defined as no caloric intake for at least 8 hours 1
• Hemoglobin A1C ≥6.5% (48 mmol/mol) - using a standardized assay 1
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during 75-g oral glucose tolerance test 1
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms (polyuria, polydipsia, unexplained weight loss) 1

Prediabetes Categories

Individuals with intermediate hyperglycemia who do not meet diabetes criteria are classified as having prediabetes 1:

• Impaired fasting glucose (IFG): fasting plasma glucose 100-125 mg/dL (5.6-6.9 mmol/L) 1
• Impaired glucose tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT 1
• A1C 5.7-6.4% (39-47 mmol/mol) 1

Those with A1C >6.0% (>42 mmol/mol) or both IFG and IGT represent very high risk and warrant aggressive intervention 1.

Screening Recommendations for High-Risk Populations

Adults with Risk Factors

Testing should begin at age 35 years for all adults, or earlier in those with risk factors, and repeated at minimum 3-year intervals if normal 1:

• Overweight or obesity (BMI ≥25 kg/m²) with additional risk factors 1
• First-degree relative with diabetes (parent or sibling) 1
• History of gestational diabetes mellitus - requires testing at least every 1-3 years 1
• Physical inactivity 1
• High-risk race/ethnicity (African American, Hispanic/Latino, Native American, Asian American, Pacific Islander) 1
• Hypertension (≥130/80 mmHg or on antihypertensive therapy) 1
• HDL cholesterol <35 mg/dL (0.90 mmol/L) and/or triglycerides >250 mg/dL (2.82 mmol/L) 1
• Polycystic ovary syndrome 1
• History of cardiovascular disease 1

Gestational Diabetes Screening

Women with obesity (BMI ≥30 kg/m²), prior gestational diabetes, or strong family history should be screened at the first prenatal visit (12-14 weeks) using standard diagnostic criteria 1, 3:

• If negative, mandatory repeat screening at 24-28 weeks of gestation 1, 3, 4
• Early screening detects pre-existing undiagnosed type 2 diabetes requiring intensive management 3

For average-risk pregnant women, screen at 24-28 weeks using either 1:

Two-step approach (preferred by ACOG):

• 50-g glucose challenge test (non-fasting), threshold ≥130-140 mg/dL 1, 4
• If positive, proceed to diagnostic 100-g OGTT 4
• Diagnosis requires 2 or more values meeting thresholds: fasting ≥95 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥155 mg/dL, 3-hour ≥140 mg/dL 1, 4

One-step approach (IADPSG criteria):

• 75-g OGTT (fasting required) 1, 4
• Diagnosis if any single value meets threshold: fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, 2-hour ≥153 mg/dL 5, 4

Interpretation of Early Pregnancy Glucose Testing

• Fasting glucose <92 mg/dL: Normal, but repeat screening at 24-28 weeks is mandatory 5
• Fasting glucose 92-125 mg/dL: Early gestational diabetes, initiate immediate management 5
• Fasting glucose ≥126 mg/dL: Overt pre-existing diabetes, not gestational diabetes 5

Critical Testing Considerations

The fasting plasma glucose test is preferred over OGTT for routine diagnosis due to ease of administration, patient convenience, and lower cost 1. However, OGTT identifies additional cases that fasting glucose alone would miss 6.

A1C has advantages including no fasting requirement and less day-to-day variability, but may be less sensitive in certain populations and should not be used during pregnancy 1.

Common Pitfalls to Avoid

• Never rely on fasting glucose alone for gestational diabetes diagnosis - a full OGTT is required 3, 5
• Do not skip the 24-28 week rescreen in high-risk women with negative early testing - insulin resistance increases exponentially in later pregnancy 3
• Confirm all positive screening tests on a subsequent day unless the patient has unequivocal hyperglycemia with symptoms 1
• Do not use A1C for gestational diabetes diagnosis - pregnancy-specific glucose criteria must be used 1, 5

Testing Conditions

For accurate OGTT results 1:

• Perform after 8-14 hour overnight fast
• Ensure at least 3 days of unrestricted diet (≥150g carbohydrate daily)
• Patient should remain seated and not smoke during test
• Maintain normal physical activity before testing