Normal Blood Sugar Values for Adults
For a typical adult without diabetes, normal fasting plasma glucose is <100 mg/dL, random glucose is <140 mg/dL, 2-hour oral glucose tolerance test is <140 mg/dL, and HbA1c is <5.7%. 1, 2
Fasting Plasma Glucose (FPG)
- Normal: <100 mg/dL (5.6 mmol/L) 1, 2
- Prediabetes (Impaired Fasting Glucose): 100-125 mg/dL (5.6-6.9 mmol/L) 1, 2, 3
- Diabetes: ≥126 mg/dL (7.0 mmol/L) 1, 2
- Fasting is defined as no caloric intake for at least 8 hours 1, 4
Random (Non-Fasting) Plasma Glucose
- Normal: Generally <140 mg/dL in healthy individuals 2
- Diabetes threshold: ≥200 mg/dL (11.1 mmol/L) with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) 1, 2
- Random glucose can be drawn at any time of day without regard to meals 1
2-Hour Oral Glucose Tolerance Test (OGTT)
- Normal: <140 mg/dL (7.8 mmol/L) 1, 5
- Prediabetes (Impaired Glucose Tolerance): 140-199 mg/dL (7.8-11.0 mmol/L) 1, 2, 3
- Diabetes: ≥200 mg/dL (11.1 mmol/L) 1, 5
- The test uses a 75-gram glucose load after an 8-hour fast, with blood drawn at baseline and 2 hours post-load 1, 5
Hemoglobin A1c (HbA1c)
- Normal: <5.7% 1, 2
- Prediabetes: 5.7-6.4% 1, 2
- Diabetes: ≥6.5% 1, 2
- HbA1c can be measured at any time without fasting, offering greater convenience 2, 4
- Must be performed in an NGSP-certified laboratory using methods standardized to the DCCT assay 1, 2
Important Clinical Considerations
Day-to-Day Variability
- Substantial intraperson variability exists in fasting glucose measurements. Research using continuous glucose monitoring shows that fasting glucose standard deviation averages 7.52 mg/dL day-to-day in the same individual 6
- Among individuals with an initial normal fasting glucose, 40% would be reclassified as prediabetes and 3% as diabetes based on sequential measurements over multiple days 6
- This variability underscores why diagnostic criteria require confirmation with repeat testing on a separate day 1, 2
When HbA1c Should NOT Be Used
HbA1c is completely unreliable and should not be used in conditions affecting red blood cell turnover: 2, 4
- Hemoglobinopathies (including sickle cell disease)
- Hemolytic anemias
- Pregnancy (second/third trimester)
- Recent blood loss or transfusion
- Hemodialysis
- Erythropoietin therapy
In these situations, only plasma glucose criteria (FPG or OGTT) should be used for assessment 2, 4
Test Concordance Issues
- FPG, 2-hour OGTT, and HbA1c do not identify the same individuals—concordance among these tests is imperfect 2
- The OGTT identifies more individuals with diabetes than either FPG or HbA1c cutpoints alone 2
- In high-risk individuals with FPG 100-125 mg/dL, nearly 50% may have diabetes when assessed by OGTT 7