HbA1c (Glycated Haemoglobin)

Glucose binds non-enzymatically to the N-terminal valine of haemoglobin beta chains in a process called glycation (not to be confused with glycosylation, which is enzyme-mediated). The rate of glycation is proportional to prevailing blood glucose; thus, over a red blood cell's 120-day lifespan, HbA1c accumulates in proportion to average glycaemia.

Standardised HbA1c assays align to IFCC (International Federation of Clinical Chemistry) or NGSP (National Glycohemoglobin Standardization Program) methods. Results may be expressed as a percentage (NGSP) or in mmol/mol (IFCC). Conversion: NGSP% = (0.0915 × IFCC mmol/mol) + 2.15.

Conditions that affect red blood cell turnover can distort HbA1c: haemolytic anaemia, iron deficiency anaemia (falsely raises HbA1c), haemoglobinopathies (including sickle cell trait), and recent blood transfusion can all impair interpretation. In these settings, fructosamine or continuous glucose monitoring may be preferred.

Non-diabetic (normal): HbA1c <39 mmol/mol (<5.7% NGSP). Prediabetes: 39-47 mmol/mol (5.7-6.4% NGSP) — WHO criteria, or 42-47 mmol/mol (6.0-6.4%) per some UK guidelines. Type 2 diabetes: ≥48 mmol/mol (≥6.5% NGSP) on two separate occasions, or once with symptoms.

Optimal management target in diagnosed type 2 diabetes is generally HbA1c <53 mmol/mol (<7.0%), though targets should be individualised based on hypoglycaemia risk, age, comorbidities, and patient preference.

Each 1% (11 mmol/mol) reduction in HbA1c is associated with approximately 37% reduction in microvascular complication risk (UKPDS data), establishing HbA1c reduction as a meaningful surrogate endpoint in clinical trials.

In men, rising HbA1c is associated with progressive testosterone decline, partly mediated by insulin resistance and its downstream effects on the HPG axis. The EMAS (European Male Ageing Study) and other cohort studies consistently show inverse associations between glycaemic measures and testosterone.

Elevated HbA1c is also associated with elevated SHBG in some models (particularly where weight is controlled for), but in clinical practice, men with metabolic syndrome and high HbA1c more commonly present with low SHBG — reflecting the dominant effect of hyperinsulinaemia suppressing SHBG production.

Erectile dysfunction (ED) is strongly correlated with glycaemic status: men with HbA1c in the diabetic range have roughly 3-4 times the prevalence of ED compared to normoglycaemic men, driven by both vascular and neuropathic mechanisms.

HbA1c screening is recommended in all adults with overweight/obesity, hypertension, dyslipidaemia, family history of diabetes, or age ≥45. For men presenting with low testosterone, erectile dysfunction, or unexplained fatigue, HbA1c should be part of the standard metabolic screen.

Limitations of HbA1c: reflects a 3-month average and misses glycaemic variability. A man can have a 'normal' HbA1c but severe post-prandial hyperglycaemia (detected by continuous glucose monitoring or 2-hour OGTT). The first-morning fasting glucose and insulin (for HOMA-IR calculation) provide complementary information.

HbA1c trajectory matters as much as absolute value: an HbA1c rising from 34 to 42 mmol/mol over 2 years (still 'normal' range) represents meaningful progression toward prediabetes and should prompt proactive lifestyle intervention.