🔬 HbA1c

HbA1c (Glycated Haemoglobin) measures the average plasma glucose concentration over the preceding 2–3 months. It is used as a primary diagnostic tool for Type 2 Diabetes and as the main longitudinal marker for monitoring glycaemic control in patients with known diabetes. Unlike fasting glucose, it does not require the patient to be nil-by-mouth and provides a stable measure of long-term sugar levels. Results in the UK are reported in mmol/mol.

Indicated for the diagnosis of Type 2 Diabetes in asymptomatic adults (requires two tests) or symptomatic adults (one test). It is used every 3-6 months to monitor glycaemic control in patients with established Type 1 and Type 2 Diabetes. It is also used to screen high-risk individuals (e.g., those with obesity, PCOS, or a strong family history) for prediabetes. It is not used for the diagnosis of Type 1 Diabetes (where presentation is usually acute) or Gestational Diabetes (where OGTT is used).

A venous blood sample is taken in an EDTA tube (lavender top), similar to an FBC. The laboratory typically uses High-Performance Liquid Chromatography (HPLC) or immunoassay techniques to separate and quantify the glycated fraction of haemoglobin. In the UK, results are reported in International Federation of Clinical Chemistry (IFCC) units (mmol/mol). The test measures the amount of glucose permanently bonded to the haemoglobin molecule within red blood cells throughout their 120-day lifespan.

A normal HbA1c in a non-diabetic individual is below 42 mmol/mol (6.0%). This indicates that blood glucose levels have remained within the physiological range over the past 8 to 12 weeks. In patients with established diabetes, 'normal' is replaced by 'target' ranges, which are often around 48–53 mmol/mol depending on the duration of disease and comorbidities. Normal findings exclude a diagnosis of diabetes or pre-diabetes at the time of testing.

Increased HbA1c directly correlates with the average plasma glucose over the preceding 2-3 months. For diagnosis, if the patient is asymptomatic, a second confirmatory HbA1c must be performed. If the two results are in different categories, the higher result is used. HbA1c should be interpreted cautiously in patients with conditions affecting red blood cells. For example, in iron-deficiency anaemia, HbA1c can be falsely elevated, while in haemolytic anaemias, it can be falsely low because cells do not live long enough to be glycated.

An HbA1c value ≥48 mmol/mol (6.5%) is diagnostic of Type 2 Diabetes Mellitus. Values between 42 and 47 mmol/mol indicate 'prediabetes' or non-diabetic hyperglycaemia, signifying a high risk of progression to diabetes. In known diabetics, a rising HbA1c suggests poor glycaemic control, non-adherence to medication, or disease progression. Conversely, a very low HbA1c in a diabetic patient may indicate over-treatment and a high risk of hypoglycaemia. Significant discrepancies between finger-prick glucose readings and HbA1c should prompt investigation into factors affecting red cell lifespan.

HbA1c is the gold standard for monitoring long-term glycaemic control and predicting the risk of microvascular complications (retinopathy, nephropathy, neuropathy). It is used to set individualized targets for patients; for many adults with Type 2 Diabetes, the target is 48-53 mmol/mol. It is a key metric in the Quality and Outcomes Framework (QOF) for UK GP practices. It provides a more reliable picture of average glycaemia than a single glucose test because it is not affected by recent meals or short-term stress.

Using HbA1c to diagnose diabetes in a patient with symptoms of Type 1 Diabetes (ketoacidosis) leads to dangerous delays. Misinterpreting a low HbA1c in a patient with haemolysis as 'good control'. Not repeating the test for an asymptomatic patient with a first-time high result. Failing to adjust targets for the elderly, which can lead to life-threatening hypoglycaemia. Forgetting that iron deficiency can cause a falsely high HbA1c, leading to a misdiagnosis of diabetes.抽血管选择错误（应为EDTA管）。

HbA1c is unreliable in conditions with high red cell turnover (anaemia, splenomegaly, pregnancy, recent haemorrhage, or erythropoietin therapy). It cannot be used to diagnose diabetes in children, in suspected Type 1 Diabetes, or in patients with rapid onset of symptoms (where glucose must be used). It is also invalid in patients with certain haemoglobinopathies (e.g., Sickle cell) depending on the lab method used. It does not reflect glycaemic variability (the 'highs and lows') which may be important in Type 1 Diabetes management.

Memorise the diagnostic thresholds: <42 (Normal), 42-47 (Prediabetes), ≥48 (Diabetes). Understand that HbA1c is not recommended for diagnosing diabetes in pregnancy (use OGTT) or in cases of acute illness. Be aware of the 'individualised target' principle in NICE NG28 (e.g., relaxing targets for the frail elderly to avoid hypos). Know that if HbA1c is unreliable, alternative monitoring includes capillary blood glucose profiles or fructosamine.