Blood glucose: a snapshot
Plasma glucose measures the glucose concentration at the time the sample is collected. Fasting status, recent food intake, stress, acute illness, physical activity, and medications can influence the result.
A1C: a longer-term view
A1C, also called hemoglobin A1C or HbA1c, reflects average glucose exposure over roughly the past two to three months. It does not require fasting, but it does not show daily highs and lows and may not match glucose measurements in every person.
When can A1C be misleading?
Conditions that change red blood cell lifespan or hemoglobin can produce falsely high or low results. Examples include certain anemias and hemoglobin variants, recent blood loss, transfusion, erythropoietin treatment, and hemodialysis. The laboratory method also matters.
What about fasting insulin?
Insulin helps regulate glucose. Measurement can be useful for selected clinical questions, but it is not one of the standardized tests used to diagnose diabetes. The result varies with fasting conditions, laboratory method, medications, and metabolic stage.
Does HOMA-IR establish insulin resistance?
HOMA-IR is an estimate calculated from fasting glucose and insulin. It can be useful in research and selected settings, but it does not have one universal cutoff across populations and laboratories. It should not be treated as a stand-alone diagnosis.
How is diabetes diagnosed in the United States?
Recognized tests include fasting plasma glucose, A1C, a two-hour plasma glucose after a 75-gram oral glucose tolerance test, or a random plasma glucose in a compatible clinical setting. Unless hyperglycemia is unequivocal, an abnormal result generally requires confirmation according to current professional guidance.