How should I screen and monitor an adult patient for type 2 diabetes, including which glucose tests to use, diagnostic thresholds, and follow‑up testing intervals?

Type 2 Diabetes Screening and Monitoring

Screen adults aged 35-45 years (or earlier if overweight/obese with risk factors) using fasting plasma glucose (FPG ≥126 mg/dL) or A1C (≥6.5%), and repeat screening every 3 years if normal. 1

Who to Screen

Priority populations requiring immediate screening:

• Adults with hypertension or hyperlipidemia should be screened regardless of age, as detecting diabetes substantially improves cardiovascular risk stratification and enables aggressive blood pressure and lipid management that reduces cardiovascular events and mortality 2

• Adults aged 40-70 years with BMI ≥25 kg/m² should be screened 1, 3

• Adults aged 35-45 years or older without additional risk factors should begin screening 1

Screen earlier than age 35-45 if BMI ≥25 kg/m² (≥23 kg/m² for Asian Americans) PLUS any of these risk factors:

• First-degree relative with diabetes 1, 4
• High-risk ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 1, 4
• History of gestational diabetes or delivered baby >9 lb 1
• Polycystic ovary syndrome (PCOS) 1
• Physical inactivity 1, 4
• HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 1
• Previous impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) 1
• History of cardiovascular disease 1

Recommended Screening Tests

Fasting plasma glucose (FPG) is the preferred screening test because it is the most practical, reproducible, cost-effective, has less intraindividual variation, and is more convenient for patients than other options 2, 1

A1C is an acceptable alternative when fasting is impractical, as it requires no fasting and is convenient 1, 4

Important limitations to avoid:

• Do NOT use random blood glucose alone for screening—sensitivity is only 39-55% and it is poorly standardized 1
• Random capillary blood glucose has only 75% sensitivity even when interpreted by age and time since last meal 2
• The 2-hour oral glucose tolerance test (OGTT) is more cumbersome, less reproducible, and less acceptable to patients than FPG, though it may identify more individuals as diabetic 2

Diagnostic Thresholds

Diabetes is diagnosed when ANY of the following criteria are met:

• FPG ≥126 mg/dL (7.0 mmol/L) after 8-hour fast 2, 1, 3, 5
• A1C ≥6.5% 1, 3, 5
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during 75-g oral glucose tolerance test 2, 3, 5
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia (polydipsia, polyuria) 3, 5

Pre-diabetes categories (increased risk for progression):

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

Confirmation of Diagnosis

Always confirm abnormal screening results with repeat testing on a separate day, especially for patients with borderline results or those without symptoms 2, 1, 3

Exception: A single random plasma glucose ≥200 mg/dL with unequivocal hyperglycemic symptoms (polydipsia, polyuria, unexplained weight loss) is sufficient for diagnosis without confirmatory testing 1, 3

Screening Frequency

Standard interval: Every 3 years if initial screening is normal 2, 1, 3

More frequent screening (annually or more often) is warranted for:

• Patients with borderline results (pre-diabetes range) 1
• Patients with multiple risk factors 1
• High-risk populations 2

For adolescents with risk factors, if initial screening is normal, repeat at minimum 2-3 year intervals 4

Critical Pitfalls to Avoid

• Do NOT wait until age 45 for overweight/obese patients with risk factors—screen immediately 1
• Do NOT rely on A1C alone to rule out diabetes—values <6.5% do not exclude diabetes if FPG or OGTT are elevated 1
• Do NOT perform community screening outside healthcare settings—this fails to ensure proper follow-up and appropriate targeting of high-risk populations 1
• Do NOT screen healthy-weight adolescents without risk factors—universal adolescent screening leads to unnecessary testing and false positives 4
• Do NOT base diagnosis on a single abnormal value unless the patient has unequivocal hyperglycemic symptoms 1, 3, 6

Integration with Cardiovascular Risk Management

For patients with hypertension or hyperlipidemia, diabetes screening should be part of an integrated cardiovascular risk reduction strategy, as detecting diabetes enables:

• Lower blood pressure targets (diastolic BP <80 mm Hg) 2
• Lower LDL cholesterol targets 2
• More aggressive management of other modifiable risk factors 2