What is HbA1c Test

HbA1c, (hemoglobin A1c, A1C, A1c, HbA_1c, Hb1c, HgbA1c, Glycated hemoglobin or less commonly hemoglobin, etc) is a form of hemoglobin that is covalently bound to glucose. It is formed in a non-enzymatic glycation pathway by hemoglobin’s exposure to plasma glucose. It is measured primarily to identify the three-month average plasma glucose concentration and thus can be used as a diagnostic test for diabetes and an assessment test for glycemic control in people with diabetes. Siddha Spirituality of Swami Hardas Life System has taken note of the need of HbA1c test, and hence wish our readers to know about in detail.

The HbA1c test is limited to a three-month average because the lifespan of a red blood cell is four months (120 days). However, since red blood cells do not all undergo lysis at the same time, HbA1C is taken as a limited measure of three months. Normal levels of glucose produce a normal amount of glycated hemoglobin. As the average amount of plasma glucose increases, the fraction of glycated hemoglobin increases in a predictable way. This serves as an indicator that blood sugar is increasing and that action should be taken.

Association of blood glucose levels

In diabetes mellitus, higher amounts of glycated hemoglobin, indicating poorer control of blood glucose levels, have been associated with:

• Cardiovascular disease
• Nephropathy
• Neuropathy
• Retinopathy

A trial on a group of patients with Type 1 diabetes found that monitoring by caregivers of HbA_1c led to changes in diabetes treatment and improvement of metabolic control compared to monitoring only of blood or urine glucose. However, a trial designed specifically to determine whether reducing HbA_1c below the normal 6%, using primarily insulin and sulfonylureas, would reduce the rate of cardiovascular events in Type 2 diabetes found higher mortality, and the trial was terminated.

Damage mechanisms

HbA1c causes an increase of highly reactive free radicals inside blood cells. Radicals alter blood cell membrane properties. This leads to blood cell aggregation and increased blood viscosity which results in impaired blood flow.

Another way glycated Hb causes damage is via inflammation which results in atherosclerotic plaque formation. 

HbA1c: A principle in medical diagnostics

Glycation of proteins is a frequent occurrence, but in the case of hemoglobin, a nonenzymatic condensation reaction occurs between glucose and the N-end of the beta chain. This reaction produces a Schiff base (R-N=CHR’, R = beta chain, CHR’= glucose-derived), which is itself converted to 1-deoxyfructose. This second conversion is an example of an Amadori rearrangement.

When blood glucose levels are high, glucose molecules attach to the hemoglobin in red blood cells. The longer hyperglycemia occurs in blood, the more glucose binds to hemoglobin in the red blood cells and the higher the glycated hemoglobin.

Once a hemoglobin molecule is glycated, it remains that way. A buildup of glycated hemoglobin within the red cell, therefore, reflects the average level of glucose to which the cell has been exposed during its life-cycle. Measuring glycated hemoglobin assesses the effectiveness of therapy by monitoring long-term serum glucose regulation.

A1c is a weighted average of blood glucose levels during the life of the red blood cells (117 days for men and 106 days in women). Therefore, glucose levels on days nearer to the test contribute substantially more to the level of A1c than the levels in days further from the test.

This is also supported by data from clinical practice showing that HbA1c levels improved significantly after 20 days from start or intensification of glucose-lowering treatment.

HbA1c Measurement

A number of techniques are used to measure hemoglobin A1c. Laboratories use high-performance liquid chromatography (the HbA_1c result is calculated as a ratio to total hemoglobin using a chromatogram); immunoassay; enzymatic assay; capillary electrophoresis; or boronate affinity chromatography. Point of care (e.g., doctor’s office) devices use immunoassay ororonate affinity chromatography.

In the United States, HbA_1c testing laboratories are certified by the National Glycohemoglobin Standardization Program (NGSP) to standardize them against the results of the 1993 Diabetes Control and Complications Trial (DCCT). An additional percentage scale, Mono S has previously been in use by Sweden and KO500 is in use in Japan.

Switch to IFCC units

The American Diabetes Association (ADA), European Association for the Study of Diabetes (EASD) and International Diabetes Federation (IDF) have agreed that, in the future, HbA_1c is to be reported in the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) units. IFCC reporting was introduced in Europe except for the UK in 2003; the UK carried out dual reporting from 1 June 2009  until 1 October 2011.

Interpretation of results

Laboratory results may differ depending on the analytical technique, the age of the subject, and biological variation among individuals.

Higher levels of HbA_1c are found in people with persistently elevated blood sugar, as in diabetes mellitus. While diabetic patient treatment goals vary, may include a target range of HbA_1c values. A diabetic person with good glucose control has an HbA_1c level that is close to or within the reference range.

A retrospective study of 47,970 old type 2 diabetes patients found that patients with an HbA_1c more than 48 mmol/mol (6.5 DCCT %) had an increased mortality rate, but a later international study contradicted these findings.

Practitioners must consider an individual patient’s health, their risk of hypoglycemia, and their specific health risks when setting a target HbA_1c level. Because patients are responsible for averting or responding to their own hypoglycemic episodes, the patient’s input and the doctor’s assessment of the patient’s self-care skills are also important.

Persistent elevations in blood sugar (and, therefore, HbA_1c) increase the risk of long-term vascular complications of diabetes such as:

• Coronary disease
• Heart attack
• Stroke
• Heart failure
• Kidney failure
• Blindness
• Erectile dysfunction
• Neuropathy (loss of sensation, especially in the feet)
• Gangrene
• Gastroparesis (slowed emptying of the stomach)

Poor blood glucose control also increases the risk of short-term complications of surgery such as poor wound healing.

Lower-than-expected levels of HbA_1c can be seen in people with shortened red blood cell lifespan, such as with glucose-6-phosphate dehydrogenase deficiency, sickle-cell disease, or any other condition causing premature red blood cell death.

Blood donation will result in the rapid replacement of lost RBCs with newly formed red blood cells. Since these new RBCs will have only existed for a short period of time, their presence will lead HbA_1c to underestimate the actual average levels. 

Results can be unreliable in many circumstances, for example:

• After blood loss
• After surgery
• Blood transfusions
• Anemia
• High erythrocyte turnover
• In the presence of chronic renal or liver disease
• After administration of high-dose vitamin C
• Erythropoetin treatment

HbA1c calculation

The approximate mapping between HbA_1c values given in DCCT percentage (%) and eAG (estimated average glucose) measurements is given by the following equation:

eAG(mg/dl) = 28.7 × A1C − 46.7
eAG(mmol/l) = 1.59 × A1C − 2.59
Data in parentheses are 95% confidence intervals

HbA1c		eAG
%	mmol/mol	mmol/L	mg/dL
5	31	5.4 (4.2–6.7)	97 (76–120)
6	42	7.0 (5.5–8.5)	126 (100–152)
7	53	8.6 (6.8–10.3)	154 (123–185)
8	64	10.2 (8.1–12.1)	183 (147–217)
9	75	11.8 (9.4–13.9)	212 (170–249)
10	86	13.4 (10.7–15.7)	240 (193–282)
11	97	14.9 (12.0–17.5)	269 (217–314)
12	108	16.5 (13.3–19.3)	298 (240–347)
13	119	18.1 (15–21)	326 (260–380)
14	130	19.7 (16–23)	355 (290–410)
15	140	21.3 (17–25)	384 (310–440)
16	151	22.9 (19–26)	413 (330–480)
17	162	24.5 (20–28)	441 (460–510)
18	173	26.1 (21–30)	470 (380–540)
19	184	27.7 (23–32)	499 (410–570)

HbA1c Normal, prediabetic, and diabetic ranges

The 2010 American Diabetes Association Standards of Medical Care in Diabetes added the =HbA_1c ≥ 48 mmol/mol (≥6.5 DCCT %) as another criterion for the diagnosis of diabetes.

Diagnostic Standard for HbA1C in Diabetes

HbA1C	Diagnosis
<5.7%	Normal
5.7-6.4%	Prediabetes
>6.5%	Diabetes

HbA1c recommended testing

Glycated hemoglobin testing (HbA1c) is recommended for both checking the blood sugar control in people who might be pre-diabetic and monitoring blood sugar control in patients with more elevated levels, termed diabetes mellitus.

For a single blood sample, it provides far more revealing information on glycemic behavior than a fasting blood sugar value. However, fasting blood sugar tests are crucial in making treatment decisions. 

HbA1c measurement is not appropriate where there has been a change in diet or treatment within 6 weeks. Likewise, the test assumes a normal red blood cell aging process and mix of hemoglobin subtypes (predominantly HbA in normal adults).

Hence, people with recent blood loss, hemolytic anemia, or genetic differences in the hemoglobin molecule (hemoglobinopathy) such as sickle-cell disease and other conditions, as well as those that have donated blood recently, are not suitable for this test.

Due to glycated hemoglobin’s variability (as shown in the table above), additional measures should be checked in patients at or near recommended goals. People with HbA_1c values at 64 mmol/mol or less should be provided additional testing to determine whether the HbA_1c values are due to averaging out high blood glucose (hyperglycemia) with low blood glucose (hypoglycemia) or the HbA_1c is more reflective of elevated blood glucose that does not vary much throughout the day.

Devices such as continuous blood glucose monitoring allow people with diabetes to determine their blood glucose levels on a continuous basis, testing every few minutes. Continuous use of blood glucose monitors is becoming more common, and the devices are covered by many health insurance plans but not by Medicare in the United States.

The supplies tend to be expensive since the sensors must be changed at least weekly. Another test that is useful in determining if HbA_1c values are due to wide variations of blood glucose throughout the day is 1,5-anhydroglucitol, also known as GlycoMark. GlycoMark reflects only the times that the person experiences hyperglycemia above 180 mg/dL over a two-week period.

Concentrations of hemoglobin A1 (HbA1) are increased, both in diabetic patients and in patients with renal failure, when measured by ion-exchange chromatography. 

In autoimmune hemolytic anemia, concentrations of hemoglobin A1 (HbA1) is undetectable. Administration of prednisolone (PSL) will allow the HbA1 to be detected.

Diagnosis of diabetes during pregnancy continues to require fasting and glucose tolerance measurements for gestational diabetes and not the glycated hemoglobin.

Diet modification

Meta-analysis has shown probiotics to cause a statistically significant reduction in HbA1c in type 2 diabetics. Trials with multiple strains of probiotics had statistically significant reductions in HbA1c, whereas trials with single strains did not.

Modification by exercise training

A meta-analysis of research done to identify the effect of two different kinds of training programs on the HbA1c levels of individuals with type 2 diabetes mellitus found that the effect of combining resistance exercise with aerobic exercise improved the glucose control more than just the aerobics alone, and it should be noted that BMI of the resistance plus aerobic exercise group decreased more than the aerobic only group.

Standardization and traceability

HbA1c is now standardized and traceable to IFCC methods HPLC-CE and HPLC-MS.

A new unit (mmol/mol) is used as part of this standardization. The standardized test does not test for iodine levels in the blood. Hypothyroidism or iodine supplementation are known sources that artificially raise the A1c number.

HbA1c History

Hemoglobin A1c was first separated from other forms of hemoglobin by Huisman and Meyering in 1958 using a chromatographic column. It was first characterized as a glycoprotein by Bookchin and Gallop in 1968. Its increase in diabetes was first described in 1969 by Samuel Rahbar et al. The reactions leading to its formation were characterized by Bunn and his coworkers in 1975.

The use of hemoglobin A1c for monitoring the degree of control of glucose metabolism in diabetic patients was proposed in 1976 by Anthony Cerami, Ronald Koenig, and coworkers.

Veterinary medicine

HbA1c is not generally used in the treatment of cats and dogs with diabetes; fructosamine is favored instead. HbA1c has not been found useful in these animals.

Reference: https://en.wikipedia.org/wiki/Glycated_hemoglobin