# HbA1c (HbA1c)

The share of your red blood cells’ hemoglobin with sugar stuck to it: a three-month average of your blood glucose.

Type: lab · Unit: % · Domain: Metabolic Health · Accuracy: Laboratory · Importance: Core

Also called: glycated hemoglobin, glycated haemoglobin, hemoglobin A1c, three-month glucose, long-term glucose, A1c

## Ranges

- Laboratory range: 20–42 mmol/mol (4.0–6.0%)
- Working optimal: ≤ 38 mmol/mol (≤ 5.6%) · ref. 4

## In one sentence

The share of your red blood cells' hemoglobin that has sugar stuck to it, which, because red cells live about three months, is a three-month average of your blood glucose.

## Why it matters

It is the one glucose number that does not depend on what you ate this morning. Laboratories use it to classify glucose control, and its relationship with outcomes runs right through the range that most people sit in. In 10,232 adults aged 45 to 79 in Norfolk, followed for six to eight years, the relationship between HbA1c and both cardiovascular events and all-cause mortality was continuous across the whole distribution, including in people without diabetes: each percentage point of HbA1c was associated with 24 percent higher all-cause mortality in men and 28 percent in women after adjustment, and 72 percent of the deaths occurred in people with an HbA1c between 5 and 6.9 percent (Khaw 2004). For someone without diabetes it is the slow, stable counterpart to a sensor's fast picture, and the laboratory number the Vitality Score weighs most.

## What moves it

Three months of glucose, mainly, weighted toward the most recent month. But also anything that changes how long red cells live or how much hemoglobin they carry: iron deficiency and some anemias push it up, conditions that shorten red-cell life pull it down, and pregnancy, kidney disease and some inherited hemoglobin variants shift it; the diagnostic standards list these as reasons the test can mislead (ADA 2024). Two people with the same average glucose can differ by several mmol/mol for these reasons, which is why VT shows the gap between your laboratory HbA1c and your sensor's estimate rather than assuming they match.

## How to measure it well

A venous blood sample at any time of day; no fasting needed. New Zealand laboratories report it in mmol/mol; VT records it as a percentage, so convert before you enter it: % = (mmol/mol + 23.5) ÷ 10.93. Re-test no sooner than three months, because sooner measures the same red cells; every three months while you are changing something, every six to twelve otherwise. The laboratory prints its reference range and, where it applies, the classification cut-points the diagnostic standards set (48 mmol/mol, 6.5 percent, for diabetes; 39 to 47, 5.7 to 6.4 percent, for the band the ADA calls prediabetes; ADA 2024); those are the laboratory's and your doctor's. The app shades a band of 4.5 to 5.7 percent on its HbA1c chart and marks readings outside it in amber, as a guide, not a verdict.

## How Vitality Tracker uses it

The laboratory value carries 40 percent of the glucose area of the Vitality Score, counting in full for 90 days and up to 180 only while your weight and habits show nothing has changed, with the score's confidence shown as low; after that it drops out, and when the sensor's estimated A1c is more recent, the score uses that instead. VT keeps the laboratory result apart from the sensor-based estimate (the estimated A1c, from your last 90 days of readings), and shows the GMI-versus-HbA1c gap on the labs dashboard and in the pre-visit report when both exist. The estimated average glucose that laboratories print beside the result uses the ADAG line (see [*Estimated A1c*](https://vitalitytracker.app/knowledge/estimated-a1c)).

## How far to trust the number

Laboratory: a standardized assay with small analytical error; the biological reasons above are the larger source of difference between people, not the test.

## Method

Unit conversion (IFCC–NGSP master equation): HbA1c (mmol/mol) = 10.93 × HbA1c (%) − 23.5. GMI from a sensor: GMI (mmol/mol) = 12.71 + 4.70587 × mean glucose (mmol/L), from at least 14 days with 70 percent coverage (Bergenstal 2018).

## Sources

1. Khaw KT, Wareham N, Bingham S, Luben R, Welch A, Day N. Association of hemoglobin A1c with cardiovascular disease and mortality in adults: the European Prospective Investigation into Cancer in Norfolk. Ann Intern Med 2004;141:413–20. [doi:10.7326/0003-4819-141-6-200409210-00006](https://doi.org/10.7326/0003-4819-141-6-200409210-00006)
2. American Diabetes Association. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes 2024. Diabetes Care 2024;47(Suppl 1):S20–42. [doi:10.2337/dc24-S002](https://doi.org/10.2337/dc24-S002)
3. Bergenstal RM, et al. Diabetes Care 2018;41:2275–80. [doi:10.2337/dc18-1581](https://doi.org/10.2337/dc18-1581)
4. Vitality Tracker, Recommended baseline lab panel v0.1.

## Related

- [Glucose management indicator](https://vitalitytracker.app/knowledge/glucose-management-indicator)
- [Estimated A1c](https://vitalitytracker.app/knowledge/estimated-a1c)
- [Fasting glucose](https://vitalitytracker.app/knowledge/fasting-glucose)
- [Mean glucose](https://vitalitytracker.app/knowledge/mean-glucose)
- [Vitality Score](https://vitalitytracker.app/knowledge/vitality-score)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/hba1c