What are the diagnostic criteria (fasting plasma glucose diagnostic level, 2‑hour oral glucose tolerance test diagnostic level, HbA1c diagnostic level) for type 2 diabetes and the recommended screening guidelines for adults?

Type 2 Diabetes Diagnosis Guidelines

Type 2 diabetes is diagnosed when any one of four criteria is met: fasting plasma glucose ≥126 mg/dL (after ≥8 hours fasting), 2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test, HbA1c ≥6.5% (using NGSP-certified laboratory method), or random plasma glucose ≥200 mg/dL with classic hyperglycemic symptoms. 1

Diagnostic Thresholds

Primary Diagnostic Criteria

Any ONE of the following confirms diabetes:

• Fasting Plasma Glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after at least 8 hours without caloric intake 2, 3, 1
• 2-hour Plasma Glucose ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test (OGTT) performed per WHO protocol 2, 3, 1
• HbA1c ≥6.5% measured in a laboratory using an NGSP-certified method standardized to the DCCT assay 2, 3, 1
• Random Plasma Glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) 2, 3, 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia with acute metabolic decompensation, abnormal results must be confirmed by repeat testing on a subsequent day. 2, 1

• If two different tests are both above diagnostic thresholds, diagnosis is confirmed 1
• If results are discordant between two different tests, repeat the test that exceeded the diagnostic threshold 1
• No confirmation is needed when a patient has classic hyperglycemic symptoms with random plasma glucose ≥200 mg/dL 3, 1

Prediabetes Criteria

Prediabetes is diagnosed when results fall within these ranges:

• FPG: 100-125 mg/dL (5.6-6.9 mmol/L) 2, 3, 1
• 2-hour OGTT: 140-199 mg/dL (7.8-11.0 mmol/L) 2, 3, 1
• HbA1c: 5.7-6.4% 2, 3, 1

Screening Guidelines for Adults

Who to Screen

Screen all adults aged 35 years and older, regardless of risk factors. 3 The American Diabetes Association updated this recommendation from age 45 to age 35 in recent guidelines. 3

Screen adults of any age with BMI ≥25 kg/m² (or ≥23 kg/m² in Asian Americans) who have one or more additional risk factors: 3, 1

• First-degree relative with diabetes 3
• High-risk race/ethnicity (Hispanic, American Indian/Alaska Native, Asian American, Black/African American, Native Hawaiian, Pacific Islander) 3
• History of cardiovascular disease 3
• Hypertension (≥140/90 mmHg or on antihypertensive therapy) 3
• HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 3
• Women with polycystic ovary syndrome 3
• Physical inactivity 3
• History of gestational diabetes 3, 1

Screening Frequency

• Repeat screening every 3 years if results are normal 3
• Screen annually in patients with prediabetes 2, 3
• Women with history of gestational diabetes require lifelong testing at least every 3 years 3

Test Selection and Practical Considerations

Preferred Testing Approach

FPG is the preferred initial test due to ease of administration, convenience, patient acceptability, and lower cost compared to OGTT. 1 Medicare reimbursement for FPG is $6 versus $19 for OGTT. 2

HbA1c offers the advantage of not requiring fasting, making it more convenient for patients. 2 However, it has lower sensitivity than glucose-based tests. 2

When NOT to Use HbA1c

Do not use HbA1c for diagnosis in conditions affecting red blood cell turnover: 2, 1

• Hemoglobinopathies (sickle cell disease, thalassemia) 2, 1
• Hemolytic anemias 2, 3, 1
• Pregnancy (second and third trimesters) 2, 3
• Recent blood loss or transfusion 2, 1
• Hemodialysis 2, 1
• Erythropoietin therapy 2, 1

In these conditions, use only plasma glucose criteria (FPG or OGTT). 2, 1

HbA1c Quality Requirements

• HbA1c must be performed in a laboratory using an NGSP-certified method 2, 1
• Point-of-care HbA1c testing should be used with caution for diagnosis, though it may be acceptable in CLIA-certified settings meeting quality standards 1
• Marked discordance between HbA1c and plasma glucose should raise suspicion for hemoglobin variants interfering with the assay 2, 1

Common Pitfalls and How to Avoid Them

Critical Diagnostic Errors

Do not rely on a single abnormal test without confirmation (except when random glucose ≥200 mg/dL with classic symptoms). 1 This is the most common diagnostic error—always confirm with repeat testing on a different day. 2

Do not use capillary blood glucose testing as a stand-alone diagnostic test—it performs poorly for diagnosis. 1

Recognize that type 2 diabetes frequently goes undiagnosed for many years because hyperglycemia develops gradually and may not initially cause symptoms. 3, 1 The duration of undiagnosed glycemic burden strongly predicts adverse outcomes. 3

Population-Specific Considerations

Do not overlook ethnic-specific BMI thresholds. Asian Americans have increased diabetes risk at BMI ≥23 kg/m² rather than ≥25 kg/m². 3, 1 Failing to apply this lower threshold leads to delayed diagnosis in this population.

Consider islet autoantibody testing in younger patients or those without traditional risk factors to exclude type 1 diabetes or latent autoimmune diabetes in adults (LADA). 3, 1 Up to 40% of adults with new type 1 diabetes are initially misdiagnosed as having type 2 diabetes. 1

Test Concordance Issues

Understand that FPG, 2-hour OGTT, and HbA1c do not necessarily detect diabetes in the same individuals—the concordance between these tests is imperfect. 2, 1 In high-risk individuals with FPG 100-125 mg/dL, nearly 50% may have diabetes by OGTT criteria. 4 This explains why confirmation testing is essential.

Diagnostic Algorithm

1. Assess risk factors and symptoms 3, 1

2. Perform one diagnostic test:

   • FPG (preferred for cost and convenience) 1
   • HbA1c (if fasting not feasible and no contraindications) 2
   • Random glucose if symptomatic 1
   • OGTT (most sensitive but least practical) 2

3. If results are abnormal:

   • Confirm with repeat testing on a different day 2, 1
   • Exception: No confirmation needed if random glucose ≥200 mg/dL with classic symptoms 3, 1

4. If confirmed, diagnose type 2 diabetes and begin management 3

5. If results indicate prediabetes, implement preventive measures and rescreen annually 3