Glycated Hemoglobin
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p406-408
2025-11-08
30
The red blood cells of a healthy adult subject contain mainly hemoglobin A (HbA) (97%), hemoglobin A2 (HbA2) (2–3%), and traces of fetal hemoglobin (HbF) (0.5–1%). Chromatographic analysis of HbA led to the identification of several minor fractions: HbA1a, HbA1b, and HbA1c, collectively referred to as HbA1. HbA1c is a significant fraction and constitutes about 80% of HbA1. HbA1c is the product of the nonenzymatic condensation reaction between the aldehyde group of glucose and the amino residues of some amino acids, mainly the terminal valines, of the hemoglobin β-chain.
Because red blood cells are permeable to glucose, the extent of HbA1c formation is directly proportional to the con centration of glucose to which red blood cells are exposed in the circulation and the relative exposure time.
The measurement of HbA1c in the blood is the gold standard for assessing glycometabolic control in diabetic sub jects. From the results of the DCCT study, elevated HbA1c levels increase the risk of micro- and macrovascular complications; consequently, this parameter is used as a target for possible therapeutic choices. Moreover, HbA1c measurement is also used to confirm the diagnosis of diabetes: values ≥48 mmol/mol (6.5%), if confirmed on at least two subsequent occasions, would allow the diagnosis of diabetes to be made. In any case, it is not advisable to use HbA1c for diagnosis in children and adolescents, in women within 2 months after childbirth, in subjects treated with glucocorticoids, immediately after surgery, or in the presence of acute pancreatitis.
The frequency of HbA1c measurements is theoretically related to the average life of red blood cells (4 months) and, therefore, should be three times per year. The ADA recommends measuring HbA1c twice a year in patients in stable metabolic control who have reached the therapeutic targets, with a greater number of determinations for patients in unstable control. Finally, in patients with gestational diabetes and in particular cases, measurements can also be made at shorter time intervals. In any case, the measurement carried out less than 2 months later is considered inappropriate for evaluating glycemic compensation.
Preanalytical Aspects
Table 1 shows the main interferences in the measurement of HbA1c, specifically listed regarding the risk of false positives or negatives using this parameter for the diagnosis of diabetes. There is no doubt that there are differences between males and females in HbA1c values in the absence of diabetes, with females showing slightly lower levels than males. Age appears controversial, but some authors reported an increase in HbA1c of about 1 mmol/mol (0.1%) from age 30 to 70 for each decade of age starting at age 30.
Table1. Limitations in the use of glycated hemoglobin for diagnosing diabetes
Regarding ethnicity, it has now been ascertained that black subjects have, on average, higher HbA1c values than Caucasians, with the same glycemic control, although the risk of developing microvascular complications in the long term does not seem to be affected. In the population, there are also subjects with a “fast” or “slow” glycation phenotype, depending on whether their HbA1c values are higher or lower than average, respectively, with the same average daily glycemia. Other factors, such as lifestyle and diet between summer and winter, are related to possible seasonal variations in HbA1c concentrations. Such variations can cause changes of 5–7% from the mean annual value.
The intake of vitamin C or E seems to reduce HbA1c due to an inhibition of the glycation process. Hypertriglyceridemia, hyperbilirubinemia, chronic salicylate intake, and opioid dependence may cause interference with the method, resulting in increased HbA1c values. Alcohol consumption also influences HbA1c levels since it has been shown that abstinence from alcohol consumption in alcoholics in rehabilitation is associated with a decrease in HbA1c of about 4 mmol/mol (0.4%). Finally, the presence of hemoglobin variants can cause positive and negative interference, depending on the method in use.
For sample collection, venous or capillary blood samples can be taken using a lancing device. The anticoagulant generally used is EDTA. The stability of the whole blood sample is 3 days at 20 °C, at least 5 days at 4 °C, and at least 6 months at −80 °C. If samples are stored at −80 °C, they should be tested within a short period of time after thawing. Some diagnostic kit manufacturers have introduced capillary blood collection systems that provide stability of approximately 1–2 weeks at room temperature. However, these systems are method dependent and cannot be adapted to other analytical systems.
Analytical Aspects
To date, more than 100 different methods have been used to measure HbA1c. These methods can be grouped as follows: chromatographic methods based on the isoelectric point difference between HbA1c and HbA (ion exchange, high- performance liquid chromatography [HPLC]) or the presence of glucose covalently linked to hemoglobin (affinity chromatography); immunochemical and enzymatic methods (detection of ketoamine). Usually, the results obtained by different methods are very well correlated, and there is no evidence that data obtained by one method are superior to data obtained by another.
It is important to remember that the HbA1c result should always be contextualized; if it is in contrast with the patient’s clinical picture, the test should be repeated, using a different method (if possible) or assessing glycemic control using other analytes (e.g., glycated albumin [GA]).
As with other tests, the measurements must be standardized for the HbA1c figure to be usable. To this end, the IFCC established a working group in 1995 to standardize HbA1c results globally. Subsequently, in 2007, another working group consisting of members of the ADA, the European Association for the Study of Diabetes, and the International Diabetes Federation (IDF), based on the IFCC’s achievements, produced a consensus document on HbA1c standardization, which dictated the strategy at the global level.
In the National Glycohemoglobin Standardization Program certified system, HbA1c is reported as a percentage of total Hb. In contrast, the IFCC has recommended that HbA1c be expressed in mmol/mol (HbA1c/Hb). The comparison between the two systems has created an equation allowing conversion between the two units. In order to ensure a standardized result, it is considered appropriate that the inaccuracy be contained within 2% and that the bias for the target value (verifiable by participating in EQ programs with switchable materials and titer assigned by the IFCC reference method) be minimal, with a total error target within 6%.
Postanalytical Aspects
As previously reported, the measurement of HbA1c is also used to confirm the diagnosis of diabetes: values ≥48 mmol/mol (6.5%), if confirmed on at least two subsequent occasions, would allow the diagnosis of diabetes to be made. HbA1c values between 43 and 47 mmol/mol (between 6.1 and 6.4%) are associated with a high risk of diabetes.
The DCCT study found that every 10 mmol/mol (1%) increase in HbA1c is associated with a worsening of mean blood glucose of approximately 35 mg/dL, and current ADA- recommended treatment targets indicate that the primary goal of therapy should result in an HbA1c value of no more than 53 mmol/mol (7%). If HbA1c is consistently above 64 mmol/mol (8%), therapeutic treatment should be promptly reassessed. Other organizations, such as the IDF, recommend achieving and maintaining HbA1c values below 48 mmol/mol (6.5%) to minimize the risk of complications. Another typical use of HbA1c is to measure the quality of care provided to patients with diabetes. The ADA criteria indicate that the percentage of patients with HbA1c values greater than 75 mmol/mol (9%) should not exceed 20% of the total adult patients; the share of diabetics with values less than 53 mmol/mol (7%) should be at least 40% of the total; 84% of pediatric patients should have HbA1c values less than 86 mmol/mol (10.0%) and 34% should have values less than 64 mmol/ mol (8.0%).
الاكثر قراءة في التحليلات المرضية
اخر الاخبار
اخبار العتبة العباسية المقدسة
