Fasting plasma glucose (FPG) and A1C both describe glycemia, but they reflect different time frames and have different testing limitations. A laboratory reference interval is not the same thing as a diagnostic rule, and one result should not be used for self-diagnosis.
What does fasting glucose show?
FPG measures plasma glucose after at least eight hours without caloric intake. It is a point-in-time measurement and can be affected by recent illness, stress, and other conditions.
What does A1C show?
A1C reflects average glucose exposure over roughly the prior two to three months and does not require fasting. Conditions that alter red-cell turnover or the relationship between hemoglobin and glucose can make A1C misleading, including some anemias, recent blood loss or transfusion, pregnancy, dialysis, and certain hemoglobin variants.
Common diagnostic ranges for nonpregnant adults
- FPG 100–125 mg/dL (5.6–6.9 mmol/L): prediabetes range
- FPG ≥126 mg/dL (≥7.0 mmol/L): one diagnostic criterion for diabetes
- A1C 5.7–6.4% (39–47 mmol/mol): prediabetes range
- A1C ≥6.5% (≥48 mmol/mol): one diagnostic criterion for diabetes
Without unequivocal hyperglycemia, an abnormal result generally requires confirmation with a repeat or another diagnostic test. Pregnancy uses different testing pathways and criteria.
What if FPG and A1C disagree?
The tests reflect different aspects of glucose metabolism, so perfect agreement is not expected. Persistent or substantial discordance should prompt review for test interference or other clinical explanations.
Questions to discuss
- Was this truly a fasting sample?
- Which result should be repeated or confirmed?
- Could anemia, transfusion, kidney disease, pregnancy, or another condition affect my A1C?
- Would an oral glucose tolerance test add useful information?
Related guides
See the lab result guide for other tests. The HbA1c ↔ eAG calculator shows an average-glucose equivalent and does not replace a diagnostic test. If fasting insulin was measured in the same draw as fasting glucose, the HOMA-IR calculator explains the index and its limits.
To separate laboratory dates and fasting conditions from home readings, use the blood glucose log. A home log does not replace a diagnostic lab test.
Sources and review status
American Diabetes Association Standards of Care in Diabetes—2026 · Korea Disease Control and Prevention Agency: Diabetes. Sources checked 2026-09-25. General health information only; not a diagnosis or treatment plan. This page has not been represented as independently physician-reviewed.