Diabetes diagnosis pair
HbA1c and Fasting glucose: how to read them together
HbA1c and fasting glucose are interpreted together because they capture different time windows: HbA1c reflects average glucose over 8-12 weeks, while fasting glucose is a single-moment reading. They agree in most patients but disagree usefully in patients with rapid glucose changes, haemoglobinopathies, or recent illness.
Quick answer
HbA1c and Fasting glucose are interpreted together because hbA1c and fasting glucose are interpreted together because they capture different time windows: HbA1c reflects average glucose over 8-12 weeks, while fasting glucose is a single-moment reading. They agree in most patients but disagree usefully in patients with rapid glucose changes, haemoglobinopathies, or recent illness. UK reference: NICE NG28 (Type 2 diabetes in adults: management and diagnosis).
Why these markers are ordered together
NICE NG28 sets HbA1c 48 mmol/mol (6.5%) or fasting plasma glucose 7.0 mmol/L as the diabetes diagnostic threshold (typically requiring confirmation on a second occasion unless the patient is symptomatic). At-risk (non-diabetic hyperglycaemia) is HbA1c 42-47 mmol/mol or fasting glucose 5.6-6.9 mmol/L. HbA1c is more convenient (no fasting required, single visit) but is unreliable in conditions affecting red cell turnover (sickle cell, recent transfusion, haemodialysis, pregnancy). Fasting glucose remains the back-up when HbA1c is unreliable or when the clinical picture is acute (recent steroid use, hospital admission).
Four scenarios, four interpretations
The 2x2 interpretation matrix our clinicians use when these markers come back together.
HbA1c at or above 48 mmol/mol (6.5%) + Fasting glucose at or above 7.0 mmol/L
What it likely means: Diabetes, both markers above diagnostic threshold. NICE NG28 confirmed.
What to do next: Confirm with a repeat on a separate occasion unless the patient is symptomatic. Refer for diabetes care (NHS diabetes care pathway or private endocrinology). Lifestyle and pharmacological treatment per NICE NG28.
HbA1c 42-47 mmol/mol (6.0-6.4%) + Fasting glucose 5.6-6.9 mmol/L
What it likely means: Non-diabetic hyperglycaemia / at-risk for type 2 diabetes. NICE PH38 prevention category.
What to do next: Lifestyle intervention with structured education (NHS Diabetes Prevention Programme is one route). Annual HbA1c monitoring. Address modifiable risk: weight, activity, diet, sleep.
HbA1c above 48 mmol/mol but Fasting glucose normal
What it likely means: Either diabetes with predominantly post-prandial pattern (eat-then-spike, fast-then-recover), or HbA1c artefactually raised by red cell condition (low red cell turnover, iron deficiency anaemia, splenectomy).
What to do next: Confirm HbA1c is reliable (FBC, ferritin). If reliable, consider OGTT (oral glucose tolerance test) to characterise. Diabetes diagnosis is still valid based on the HbA1c.
HbA1c normal but Fasting glucose above 7.0 mmol/L
What it likely means: Recent-onset diabetes (HbA1c lags by 8-12 weeks), acute stress hyperglycaemia (steroids, infection, MI), or HbA1c artefactually low (haemoglobinopathy, recent transfusion, pregnancy late-stage).
What to do next: Repeat fasting glucose, ideally with an OGTT. Review recent illness, steroid use, transfusion history. Consider haemoglobin electrophoresis if HbA1c reliability is in doubt.
When the pair is ambiguous
In pregnancy, HbA1c underestimates true glucose; fasting glucose and OGTT are preferred. In anaemia, low red cell turnover can falsely raise HbA1c; high red cell turnover (treatment of iron deficiency, recent transfusion, haemolysis) falsely lowers it. In acute illness, both markers can be transiently disturbed, repeat 4-6 weeks after recovery for a baseline.
Individual markers
Read about each marker in isolation:
Get the pair tested together
Both markers in one blood draw at 134 Harley Street. Custom panels designed by a GMC-registered doctor from £275, or fixed panels including this pair from £249.