What does a fasting glucose test measure?
A fasting glucose test measures the sugar in your blood at a single moment, after a period without food. Because eating raises glucose, fasting is what makes results comparable between people and between visits.
It answers a different question from an A1c. Fasting glucose is a snapshot; A1c reflects an average over roughly three months. Someone can have a reasonable fasting result and a raised average, or the reverse, which is why both are sometimes used together.
A single raised result is usually repeated before anything is concluded, because illness, stress, poor sleep and some medicines all affect it. Interpretation belongs with the clinician who ordered it.