HbA1c reflects average blood glucose over roughly two to three months. Fasting insulin measures the amount of insulin in a blood sample after fasting. They answer different questions, so their results do not have to move together.[1][2]

A1c looks back over time

Glucose attaches to hemoglobin, a protein in red blood cells. The A1c test measures the proportion of hemoglobin with glucose attached. Because red blood cells circulate for months, the result provides a longer view than a single glucose measurement.[1]

A1c is used to screen for or diagnose prediabetes and type 2 diabetes, and to monitor blood glucose in people with diabetes. An abnormal screening result often requires confirmation with another test.[1]

Fasting insulin is a snapshot

Insulin is the hormone that helps cells take glucose from the bloodstream. A fasting insulin result describes how much is circulating at collection, not the average glucose level over recent months. Providers interpret it with glucose and the reason for testing.[2]

For example, insulin may rise when cells become less responsive to it. The extra insulin can keep glucose within the usual range for a while. That explains how elevated insulin can coexist with an A1c that has not risen.[2]

Which result is more useful?

It depends on the question. Standard prediabetes assessment uses A1c, fasting plasma glucose or, in some circumstances, an oral glucose tolerance test. Fasting insulin is not a required companion to every A1c test.[3]

An insulin test may help answer more specific questions, including why someone has low blood glucose or whether insulin production is appropriate for the glucose result.[2]

When A1c needs extra context

Conditions that affect red blood cells can make A1c misleading. Anemia, kidney failure and liver disease are examples. When A1c does not fit other findings, a provider may use different glucose tests.[1]