How to prepare for the Glycated Hemoglobin (HbA1c) test: rules for taking it

Glycated hemoglobin, or HbA1c, reflects the average blood glucose level over the previous 2-3 months. It is one of the few tests for which it barely matters what you had for breakfast. However, it has its own pitfalls tied to the state of the red blood cells. The editors explain how preparing for HbA1c differs from preparing for a glucose test and when the result can be misleading.

What HbA1c is and why it 'remembers' months

Glucose in the blood attaches to the hemoglobin of red blood cells without the involvement of enzymes. This process is called glycation. The higher the glucose level and the longer a red blood cell lives, the greater the share of its hemoglobin that becomes glycated. HbA1c is the percentage of hemoglobin A to which glucose has attached.

A red blood cell lives on average about 120 days. That is why HbA1c reflects the integrated glucose level over the last 2-3 months. However, the contribution of different periods is not equal: the last weeks carry the most weight, because red blood cells of different ages are present in the blood at once, and 'young' cells predominate.

This leads to a practical rule: after changing your diet, training regimen or treatment, there is no point in checking HbA1c as early as two weeks later. Noticeable changes appear roughly after 6-8 weeks, and a completely new level is reflected after about 3 months.

The ADAG study showed that HbA1c correlates closely with the mean glucose calculated from continuous monitoring. For example, an HbA1c of 7% corresponds to a mean glucose level of about 8.6 mmol/L. Such a calculation helps patients better understand what the percentage on the form means.

Is preparation needed

Unlike fasting glucose, HbA1c does not require fasting: a single meal or a single sweet drink does not noticeably change the proportion of glycated hemoglobin. That is precisely why the American Diabetes Association recognizes it as a convenient diagnostic test that can be taken at any time of day.

However, if HbA1c is taken together with glucose, a lipid panel or other tests that require fasting, it is those that set the preparation logic. In that case, come in the morning on an empty stomach, after 8-12 hours without food.

Neither coffee, nor training the day before, nor mild stress significantly affect HbA1c. But this does not mean that no preparation is needed at all: it is important to tell your doctor about conditions that alter the lifespan of red blood cells - anemia, recent blood loss, blood transfusion, blood donation, kidney or liver disease, pregnancy.

The analysis method also matters. Modern laboratories use methods certified by the NGSP program and standardized against the IFCC reference system. If you are tracking trends, it is better to take the test each time at the same laboratory.

When HbA1c lies: blood conditions

The main weakness of HbA1c is its dependence on red blood cells. Any condition that changes their lifespan shifts the result regardless of the glucose level. Systematic reviews show that this applies primarily to anemias and changes in iron metabolism.

In all these situations, the doctor may prefer other markers - fasting glucose, a glucose tolerance test, fructosamine or continuous glucose monitoring.

If your HbA1c differs substantially from your glucose level based on home measurements, that is a reason to check a complete blood count, ferritin and hemolysis markers.

Also pay attention to regular blood donation. After donating blood, the body actively produces new red blood cells, and HbA1c may temporarily decrease without any real improvement in glucose metabolism.

Considerations for athletes

For athletes, iron metabolism is an important factor. Endurance loads, especially running, are associated with increased iron loss and mechanical destruction of red blood cells. Iron deficiency can falsely elevate HbA1c, while increased red blood cell turnover, on the contrary, lowers it. The result depends on which process predominates.

Training at altitude and hypoxic protocols stimulate erythropoiesis, that is, the formation of new red blood cells. Young cells contain less glycated hemoglobin, so against the background of such training blocks the values may be atypically low.

The use of hormonal drugs can also affect the indicator indirectly: androgens stimulate erythropoiesis, changing the age composition of red blood cells, while somatotropin and glucocorticoids worsen insulin sensitivity. Therefore the doctor must know about all medications in order to interpret the result correctly.

The editors' practical tip: take HbA1c together with a complete blood count and, if possible, ferritin. Such a combination lets you see right away whether anemia or increased erythropoiesis is distorting the result.

Reference values, units and monitoring frequency

HbA1c is expressed as a percentage (the NGSP system) or in mmol/mol (the IFCC system). By the ADA criteria:

• below 5.7% (less than 39 mmol/mol) - normal;
• 5.7-6.4% (39-47 mmol/mol) - prediabetes;
• 6.5% and above (48 mmol/mol and above) - a criterion for diabetes that requires confirmation.

In different countries the thresholds for prediabetes may differ slightly, so the final assessment is made by the doctor. It is also important that HbA1c is acceptable as a diagnostic criterion only when the method is standardized and there are no conditions that distort the result.

For people without diabetes and without risk factors, the test is usually done during routine check-ups. With prediabetes the doctor may recommend monitoring about once a year, and with diabetes - every 3-6 months depending on stability.

For an athlete undergoing regular medical monitoring, HbA1c is a useful indicator of metabolic health, especially during periods of using drugs that affect insulin sensitivity, or during a bulking phase with a large amount of carbohydrates.

Editors' conclusions

HbA1c does not require fasting and reflects average glycemia over 2-3 months, so it does not 'break' because of yesterday's dinner.

The main sources of error are anemia, blood loss, blood donation, blood transfusion, increased erythropoiesis and hemoglobin variants.

To interpret the result correctly, take HbA1c together with a complete blood count, and assess changes no sooner than after 2-3 months.

We also recommend reading our materials on preparing for the fasting glucose test, on ferritin and on cortisol.

References

1. American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2024. Diabetes Care. 2024;47(Suppl 1):S20–S42.
2. International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes. Diabetes Care. 2009;32(7):1327–1334.
3. Nathan DM, Kuenen J, Borg R, et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473–1478.
4. English E, Idris I, Smith G, et al. The effect of anaemia and abnormalities of erythrocyte indices on HbA1c analysis: a systematic review. Diabetologia. 2015;58(7):1409–1421.
5. Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1–e47.
6. World Health Organization. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus. Geneva: WHO; 2011.