Classification and Diagnostic Criteria
Key Takeaways
- The ADA diagnoses diabetes with an A1C of 6.5% or higher, a fasting plasma glucose of 126 mg/dL or higher, a 2-hour 75-g OGTT glucose of 200 mg/dL or higher, or a random glucose of 200 mg/dL or higher with classic hyperglycemia symptoms.
- Prediabetes is defined by an A1C of 5.7%-6.4%, a fasting glucose of 100-125 mg/dL (IFG), or a 2-hour OGTT glucose of 140-199 mg/dL (IGT).
- LADA is slow-onset autoimmune diabetes presenting in adulthood, usually GAD65-antibody positive, while MODY is monogenic diabetes with autosomal dominant inheritance and onset typically before age 25.
- The one-step gestational diabetes approach diagnoses GDM from a 75-g OGTT if any one value meets fasting 92 mg/dL, 1-hour 180 mg/dL, or 2-hour 153 mg/dL or higher.
- The two-step gestational diabetes approach requires at least 2 of 4 values on a 100-g OGTT (fasting 95, 1-hour 180, 2-hour 155, 3-hour 140 mg/dL or higher) to diagnose GDM after a positive 50-g glucose challenge test.
Diagnostic Criteria for Diabetes
The ADA Standards of Care in Diabetes diagnoses diabetes using any one of four criteria; in the absence of unequivocal hyperglycemia, the result should be confirmed by repeat testing, ideally with the same test on a separate day.
| Test | Diagnostic Threshold |
|---|---|
| A1C | ≥6.5% (≥48 mmol/mol) |
| Fasting plasma glucose (FPG) | ≥126 mg/dL (≥7.0 mmol/L); no caloric intake for at least 8 hours |
| 2-hour plasma glucose, 75-g OGTT | ≥200 mg/dL (≥11.1 mmol/L) |
| Random plasma glucose | ≥200 mg/dL plus classic hyperglycemia symptoms (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis |
If two different tests from the same sample (for example, A1C and FPG) are both above threshold, the diagnosis is confirmed without further repeat testing. If two different tests are discordant, the test that is above the diagnostic threshold should be repeated. A1C can be unreliable in conditions that affect red-cell turnover — hemolytic anemia, recent blood loss or transfusion, hemoglobinopathies, and pregnancy — in which case glucose-based criteria should be used instead of A1C.
Prediabetes
Prediabetes is defined by meeting any of the following; none requires symptoms:
| Test | Range |
|---|---|
| A1C | 5.7%-6.4% |
| Impaired fasting glucose (IFG) | FPG 100-125 mg/dL |
| Impaired glucose tolerance (IGT) | 2-hour plasma glucose, 75-g OGTT, 140-199 mg/dL |
A person can meet more than one prediabetes criterion at once, for example both IFG and IGT. Meeting multiple criteria does not change the diagnosis but does correlate with a higher risk of progressing to type 2 diabetes and often prompts more assertive lifestyle counseling and National DPP referral, covered in section 2.3.
Classifying the Types
- Type 1 diabetes — autoimmune beta-cell destruction; absolute insulin deficiency; typically autoantibody-positive (GAD65, IA-2, IAA, ZnT8)
- Type 2 diabetes — insulin resistance plus relative, progressive insulin deficiency; the most common form
- LADA (Latent Autoimmune Diabetes in Adults), sometimes called "type 1.5" — slower-onset autoimmune beta-cell destruction presenting in adulthood; usually GAD65-antibody positive; may be manageable without insulin at diagnosis but progresses to insulin dependence as autoimmune destruction continues
- MODY (Maturity-Onset Diabetes of the Young) — monogenic, single-gene defect inherited in an autosomal dominant pattern; onset typically before age 25 with a strong multigenerational family history; the most common subtypes are GCK-MODY, which is mild and stable and often needs no pharmacologic treatment, and HNF1A-MODY, which is progressive and characteristically sulfonylurea-sensitive
- Secondary diabetes — hyperglycemia caused by another condition or agent: pancreatic disease (pancreatitis, cystic fibrosis-related diabetes, pancreatectomy, hemochromatosis), endocrinopathies (Cushing syndrome, acromegaly, pheochromocytoma), or drug- or chemical-induced hyperglycemia (glucocorticoids, atypical antipsychotics, post-transplant immunosuppressants such as tacrolimus)
Diabetes diagnosed in the first trimester of pregnancy, or before 24 weeks' gestation, using the standard non-pregnancy criteria above is classified as overt or pregestational diabetes, not gestational diabetes — an important distinction for both risk stratification and postpartum follow-up.
Distinguishing type 1 from type 2 diabetes at diagnosis relies on clinical context plus targeted testing: a low or undetectable C-peptide level indicates minimal endogenous insulin production, consistent with type 1 diabetes or long-standing type 2 diabetes, while a normal-to-high C-peptide alongside obesity, insulin resistance, and negative autoantibodies points to type 2 diabetes. Islet autoantibody panels (GAD65, IA-2, IAA, ZnT8) help identify type 1 diabetes and LADA when the clinical picture is ambiguous, such as an adult presenting with a normal BMI, rapid progression to insulin dependence, or a personal or family history of other autoimmune disease.
Gestational Diabetes Mellitus: One-Step vs. Two-Step Screening
Gestational diabetes mellitus (GDM) is screened between 24 and 28 weeks' gestation in people not already known to have diabetes, earlier if risk factors suggest pre-existing, undiagnosed diabetes. The ADA accepts either of two screening strategies; institutions choose one.
One-step approach (IADPSG criteria): a fasting 75-g OGTT; GDM is diagnosed if any one value meets or exceeds:
| Timepoint | Threshold |
|---|---|
| Fasting | ≥92 mg/dL |
| 1-hour | ≥180 mg/dL |
| 2-hour | ≥153 mg/dL |
Two-step approach: Step 1 is a non-fasting 50-g glucose challenge test (GCT); a 1-hour value above the institution's chosen cutoff, commonly in the 130-140 mg/dL range, is a positive screen requiring Step 2, the diagnostic 100-g OGTT performed fasting. GDM is diagnosed if at least 2 of the 4 values meet or exceed the Carpenter-Coustan thresholds:
| Timepoint | Threshold |
|---|---|
| Fasting | ≥95 mg/dL |
| 1-hour | ≥180 mg/dL |
| 2-hour | ≥155 mg/dL |
| 3-hour | ≥140 mg/dL |
The one-step approach diagnoses more people with GDM, since lower thresholds and a single abnormal value both increase sensitivity, and reflects the IADPSG framework the ADA endorses. The two-step approach remains widely used clinically in the United States and is endorsed by ACOG and an NIH consensus panel. A CDCES should be able to explain either pathway accurately to a person being screened, since the pathway used depends on the practice or institution, not on the person's individual risk.
Regardless of which screening pathway was used during pregnancy, everyone diagnosed with GDM should be tested for persistent diabetes with a 75-g OGTT at 4-12 weeks postpartum, applying the standard non-pregnancy diagnostic criteria from the top of this section rather than the pregnancy-specific gestational thresholds.
Which fasting plasma glucose value meets the ADA diagnostic threshold for diabetes?
A pregnant patient undergoes the two-step gestational diabetes screening approach. Using the Carpenter-Coustan criteria for the 100-g OGTT, how many of the four glucose values (fasting, 1-hour, 2-hour, 3-hour) must meet or exceed threshold to diagnose GDM?
Which type of diabetes is characterized by monogenic, autosomal dominant inheritance, onset typically before age 25, and a strong multigenerational family history, most commonly involving GCK or HNF1A gene mutations?