Why The HbA1C Test Is A Poor Measure of Blood Sugar Control

HBA1C (Glycated or glycosylated hemoglobin) is a blood test that measures your average blood glucose levels for the last three months.

The blood test measures the amount of blood glucose that is attached to hemoglobin. The fact that it is connected to hemoglobin has a very significant impact on what the number means.

In the diabetes world, people talk about your HbA1C number like it is a report card of some kind! 

"Your HbA1C is very high." Or "Your HbA1C is not in range".

Or "Our intervention reduced HbA1C by 2.5%."

If the number is in range, it implies your blood glucose is well-controlled. But is that true?

Factors affecting HbA1C levels:

Factors affecting HbA1C levels. Some of these conditions raise HbA1C levels, some lower them.

1. Genetics. South Asian ancestry
2. Anemia. 
3. Iron Deficiency (This is not the same as anemia!)
4. Vitamin B12 deficiency
5. Hemoglobinopathies. Different types of hemoglobin variants
6. Pregnancy
7. Splenectomy (removal of the spleen)
8. Chronic liver disease
9. Taking Vitamin C, E
10. Taking large doses of Aspirin
11. Alcoholism
12. Chronic Renal Failure
13. Use of erythropoietin
14. Rheumatoid Arthritis
15. Use of Antiretroviral drugs

In the list above, you can see that factors 1-4 (in bold) are extremely common in India.

HbA1C provides a screenshot, while continuous glucose monitoring gives you a video recording of your glucose levels. Moreover, HbA1C doesn't provide you with information about the very highs and the very lows. Both are bad. 

If your blood glucose levels vary between 50 to 250 mg/dl, the HbA1C test will not capture the very low number. Hypoglycemia is not a simple problem, even when you are not aware. It is related to a higher risk of heart disease and stroke. 

If there are so many factors affecting HbA1C levels, why is it used for estimating average blood glucose?

Convenience: Fasting and post-meal blood glucose tests mean blood has to be collected twice or going to the lab twice. You can get an HbA1C test at any time of the day without bothering with meal times.

Any benefits to testing HbA1C?

Yes! HbA1C is a measure of glycation, basically "rusting" of your tissue. High levels of glycation or Advanced Glycation End-products (AGEs) are related to accelerated aging. 

Higher levels of HbA1C are also related to higher levels of abnormal blood lipids, which are also risk factors for heart disease and stroke.

Bottom Line:

HbA1C test is not the best test to measure your blood glucose levels, but helpful in measuring other things like glycation.

References

1. Yedla N, Kuchay MS, Mithal A. Hemoglobin E disease and glycosylated hemoglobin. Indian J Endocrinol Metab. 2015;19(5):683-685. doi:10.4103/2230-8210.163211
2. Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus: Abbreviated Report of a WHO Consultation. Geneva: World Health Organization; 2011. Annex 1, Some of the factors that influence HbA1c and its measurement. Available from: https://www.ncbi.nlm.nih.gov/books/NBK304266/
3. Mohan, Viswanathan et al. "A1C cut points to define various glucose intolerance groups in Asian Indians." Diabetes care vol. 33,3 (2010): 515-9. doi:10.2337/dc09-1694