Chemistry, toxicology & urinalysis

General chemistry

Metabolism

Hemoglobin A1C


Resident / Fellow Advisory Board: Frido Bruehl, M.D.
Deputy Editor-in-Chief: Patricia Tsang, M.D., M.B.A.
Archana Shetty, M.B.B.S., M.D.

Last author update: 30 November 2022
Last staff update: 30 November 2022

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PubMed Search: Hemoglobin A1C

Archana Shetty, M.B.B.S., M.D.
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Cite this page: Shetty A. Hemoglobin A1C. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/chemistryhemogloblinA1c.html. Accessed December 2nd, 2022.
Definition / general
  • Gold standard laboratory test used to evaluate long term glucose control
Essential features
  • Indicator of an average of body blood sugar levels over the preceding 90 days
  • A standard of care (SOC) test for diagnosis and management of diabetes
  • Elevated levels of HbA1c have been identified as a significant risk factor for cardiovascular diseases and stroke in subjects who may have diabetes
  • HbA1c exhibits positive correlations with cholesterol, triglycerides and low density lipoprotein cholesterol, as well as an inverse correlation with high density lipoprotein cholesterol
History
  • First isolated by Huisman et al. in 1958
  • Characterized as a glycoprotein by Bookchin and Gallop in 1968
  • Elevated levels of HbA1c were first reported in diabetic patients by Rahbar et al. in 1969 (Indian J Endocrinol Metab 2012;16:528)
  • Chemical pathway leading to the formation of HbA1c was identified by Bunn et al. in 1975
  • First proposed to be used as a biomarker for monitoring levels of glucose among diabetics by Koenig et al. in 1967 (Biomark Insights 2016;11:95)
Terminology
ICD coding
  • ICD-10: R73.09 - other abnormal glucose
Chemical aspects
  • HbA1c is a minor portion of adult hemoglobin (HbA)
  • Formed by the nonenzymatic condensation of the carbonyl group of glucose and the amino group at the N terminus of the beta chain of hemoglobin A in red blood cells (RBCs), resulting in a labile aldimine or Schiff base (reversible reaction)
  • As RBCs circulate, some of the aldimine undergoes a slow, irreversible conversion (Amadori rearrangement) to a stable ketoamine and forms HbA1c (Sensors (Basel) 2017;17:1798)
  • Increase in glycated hemoglobin is proportional to both the level of blood glucose and lifespan of the red cell (Biomark Insights 2016;11:95)
  • This specific characteristic of the hemoglobin biomarker is utilized for estimating the average blood glucose levels over the previous 2 - 3 months
Indications for testing
  • Diagnostic criterion of diabetes: A1c ≥ 6.5% (American Diabetic Association) (J Gen Intern Med 2014;29:388)
  • Monitor long term glycemic control
  • Adjust therapy in diabetic patients
  • Assess quality of diabetic care
  • Predict risk for development of complications related to diabetes (Biomark Insights 2016;11:95)
Laboratory
  • Specimen for testing:
    • Preferred dipotassium ethylenediaminetetraacetic acid (K2 EDTA); venous whole blood specimen collected in ethylenediaminetetraacetic acid (EDTA) is required
    • Tubes containing heparin, potassium oxalate or sodium fluoride are acceptable
    • Studies have reported no difference when collected in tripotassium ethylenediaminetetraacetic acid (K3 EDTA), sodium citrate, sodium heparin or sodium fluoride / ethylenediaminetetracetic acid disodium salt (Na2 EDTA) blood tubes (J Lab Physicians 2013;5:143)
    • HemaSpot devices:
      • Can be used as an alternative to current blood collection methods if calibrated as per standards
      • Use dry blood spots, which are tested within 3 days of preparation (Diabet Med 2020;37:1463)
      • Advantages:
        • Provide convenient sample collection service for those living in remote or rural locations
        • Provide sample collection service for those who are in urban areas but might be working or housebound
  • Specimen integrity:
    • Whole blood specimens in K2 or K3 EDTA are stable for up to 7 days stored at 2 - 8 °C, 3 days at room temperature (15 - 30 °C) or 12 months at -70 °C
  • Timing of test:
    • Convenience; not a timed testing or bound by fasting state
    • Can be collected at any time
Laboratory analysis
  • Methods based on charge difference:
    • Ion exchange chromatography
    • Capillary electrophoresis
  • Methods based on structural difference:
  • Point of care testing:
Frequency of testing
  • At least 2 times a year in patients who are meeting treatment goals and have stable glycemic control
  • At least 4 times a year to assess glycemic status and as needed in patients whose therapy has recently changed or who are not meeting glycemic goals (Clin Diabetes 2022;40:10)
Reference values
Glycosylated hemoglobin (HbA1c) mmol/L Status
< 5.7% 39 Normal
5.7 - 6.4% 39 - 47 Prediabetes
6.5% or higher 48 Diabetes

National Glycohemoglobin Standardization Program (NGSP) HbA1c (%) International Federation of Clinical Chemistry Working Group (IFCC) HbA1c (mmol/mol) Estimated average glucose (eAG) (mg/dL) Estimated average glucose (eAG) (mmol/L)
5.0 31 97 5.4
6.0 42 126 7.0
7.0 53 154 8.6
8.0 64 183 10.2
9.0 75 212 11.8
10.0 86 240 13.3
11.0 97 269 14.9
12.0 108 298 16.6

  • Officially, there is worldwide consensus that HbA1c should be reported in both NGSP (%) and IFCC (mmol/mol) units along with eAG (in either mmol/L or mg/dL) (World J Methodol 2016;6:133)
  • The formula (NGSP = [0.09148 * IFCC] + 2.152) describes the relationship between NGSP (column 1) and IFCC (column 2) (World J Methodol 2016;6:133)
Variations
  • Any condition that prolongs the life of RBCs or is associated with decreased red cell turnover exposes the cell to glucose for a longer period of time and results in falsely elevated A1C levels (J Gen Intern Med 2014;29:388):
    • Asplenia
    • Iron deficiency anemia
    • Vitamin B12 or folate deficiency
    • Severe hypertriglyceridemia (concentrations > 1,750 mg/dL)
    • Severe hyperbilirubinemia (concentrations > 20 mg/dL)
    • Uremia
    • Chronic alcohol consumption
    • Chronic salicylate ingestion
  • Conditions that shorten the life of RBCs or are associated with increased red cell turnover shortens the exposure of the cell to glucose, resulting in falsely low A1C levels:
    • Anemia from acute or chronic blood loss (hemolytic anemia, such as sickle cell anemia, spherocytosis)
    • Splenomegaly
    • End stage renal disease
  • Pregnancy: can cause falsely low A1c through second trimester and high results in third trimester; hence, not useful for diagnosing gestational diabetes (J Gen Intern Med 2014;29:388)
  • A1C results in a recently transfused patient should be considered uninterpretable
  • Vitamin C supplementation can either increase or decrease the HbA1c level depending on the method used for its measurement (World Health Organization: Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus [Accessed 29 September 2022])
  • Not a suitable index for glycemic control for patients with short term fluctuations of plasma glucose
  • When accurate HbA1c measurement is not possible, as in the above conditions, alternative measures of chronic glycemia such as fructosamine or glycated serum albumin should be used
Board review style question #1
HbA1c is usually a reliable indicator of diabetic control in

  1. Adjusting therapy in diabetic patients
  2. Known cases of hemoglobinopathies
  3. Monitoring immediate term glycemic control
  4. Predicting risk for development of neural complications in diabetics
Board review style answer #1
A. Adjusting therapy in diabetic patients

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Reference: Hemoglobin A1C
Board review style question #2
Falsely low HbA1c laboratory values are seen in which one of the following conditions?

  1. B12 deficiency anemia
  2. Chronic alcoholism
  3. Hyperbilirubinemia
  4. Sickle cell anemia
Board review style answer #2
D. Sickle cell anemia. Increased RBC turnover shortens the exposure of the cells to glucose, causing the A1c level to be falsely reduced.

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Reference: Hemoglobin A1C
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