The NHS describes HbA1c as a health-professional blood test that checks blood sugar level over the last two or three months.[1] Diabetes UK explains that this differs from a finger-prick glucose test, which shows blood sugar at one moment in time.[3] A private HbA1c test may be useful when it answers a clear question, such as diabetes risk, follow-up after a previous result, or monitoring in someone already diagnosed.
Do not use routine HbA1c testing for severe high-blood-sugar symptoms
Call 999 or go to A&E if you think you have high blood sugar with vomiting, stomach pain, fast breathing, a fast heartbeat, drowsiness, confusion, fruity-smelling breath, difficulty concentrating or high ketones in blood or urine. NHS guidance treats these as possible signs of serious illness.[1]
What HbA1c can and cannot tell you
HbA1c can show a longer-term pattern of glucose exposure. It cannot explain every symptom, capture day-to-day glucose swings, or replace urgent assessment when symptoms suggest acute illness. NICE CKS also warns that HbA1c needs caution, or should not be used for diagnosis, in several clinical situations.[2]
Useful for
Risk assessment, follow-up of raised glucose, and monitoring diabetes when HbA1c is appropriate.
Limited by
Red-blood-cell disorders, pregnancy context, acute illness, recent transfusion and other factors.
Needs follow-up
An abnormal result should lead to a documented plan rather than isolated reassurance or alarm.
How HbA1c results are usually discussed
The numbers below are common UK interpretation bands, but they are not a diagnosis by themselves. NICE CKS says symptomatic adults can sometimes be diagnosed from one abnormal HbA1c or fasting glucose result, although repeat testing is sensible; adults without symptoms should not be diagnosed from one abnormal result alone.[2]
| HbA1c result | Common discussion point | Important caution |
|---|---|---|
| Below 42 mmol/mol | Often treated as below the high-risk range | This does not rule out short-term glucose problems or symptoms that need assessment. |
| 42 to 47 mmol/mol | Often described as non-diabetic hyperglycaemia or high risk | The next step depends on risk factors, symptoms, previous results and whether the result needs repeating. |
| 48 mmol/mol or above | May support a diabetes diagnosis in the right context | NICE CKS says repeat testing is needed if the person has no symptoms; some situations need a different glucose test. |
When HbA1c may be the wrong test
If HbA1c is difficult to interpret, NICE CKS recommends using fasting plasma glucose for diagnosis and seeking specialist diabetes-team or clinical-biochemistry advice where uncertainty remains.[2]
Pregnancy or within 2 months after birth
NICE CKS advises against using HbA1c to diagnose diabetes in this context.
Symptoms present for less than 2 months
HbA1c reflects several weeks of glucose exposure, so early symptoms may need another test.
Acute illness or medication-related hyperglycaemia
A high result during stress, infection or steroid use may need careful interpretation.
Anaemia, haemoglobin variants or recent transfusion
Red-blood-cell changes can make HbA1c falsely high or low.
End-stage renal disease or acute pancreatic damage
Specialist advice or alternative tests may be needed.
Questions to ask before or after testing
- Why is HbA1c being checked for me now?
- Do my symptoms mean I need a same-day glucose or ketone assessment instead?
- Could anaemia, pregnancy, recent transfusion or another condition affect the result?
- If the result is abnormal, who will explain it and what happens next?
- Would another test, such as fasting plasma glucose, answer the question more safely?
Testing, results and follow-up at booking
The clinic confirms whether HbA1c testing is suitable for your symptoms and clinical context, the appropriate appointment route, any referral requirements, preparation, reporting and follow-up arrangements at booking. A result should be reviewed in context, especially if you have symptoms, a previous abnormal result, diabetes medication, pregnancy-related concerns or a condition that may affect red blood cells.
Related guidance and services
References
- [1] NHS. High blood sugar (hyperglycaemia). Page last reviewed 17 March 2026. View source
- [2] NICE Clinical Knowledge Summaries. Diabetes type 2: diagnosis in adults. Last revised July 2026. View source
- [3] Diabetes UK. What is HbA1c? View source