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Monday, May 1st, 2004
A new Brazilian study published in the latest edition of Journal of Clinical Pathology shed new lights on the accuracy of interpreting glycated hemoglobin levels in a heterogeneous population.
The glycated hemoglobin test (often called the glycosylated hemoglobin test) or hemoglobin (A1C) is a measurement of the overall control of the diabetic for the previous two to three months. Most diabetic specialists feel it is now the single most important blood test for known diabetics. The American Diabetes Association recommends that if you are a diabetic, this test be taken every 3 months and values should be maintained below 7% to prevent and/or decrease the risk of chronic complications.
The fasting blood sugar (FBS) test, which is still the mostly commonly performed test for Diabetes, does not reflect the true picture of diabetic control over a long period of time. The FBS only measures the level of sugar in the blood at the moment it is taken from your finger or arm.
How it works
The glycated hemoglobin test was introduced as a routine test in the late 1970s and early 1980s. It measures how much glucose is attached to hemoglobin cells, the part of the blood that carries oxygen. As the hemoglobin floats around in the blood, it picks up glucose in about the same proportion as the glucose that exists in the bloodstream. If your blood glucose is generally running high, the hemoglobin will have more "glucose coating" (glycosylization). If your glucose generally runs low, it will have less. Since the red blood cells have about a two to three month life span in the body before they are recycled, we can measure the "glucose coating" of a sample of hemoglobin. This gives you an approximate average of glucose control over the last two to three month period.
Every person, whether or not he or she has diabetes, has a certain amount of glycosylization present. Because of more sugar in their blood, people with diabetes have a greater amount of glycosylization present. A low result on the glycated hemoglobin test is a good result. If your test is in the good control category, you can be satisfied that your diabetes management plan is working well. If results are in the marginal category, some fine tuning of your treatment plan may be needed. A poor result can be improved. This test gives you valuable feedback on how well you are controlling your diabetes. It can also let you know when to work on improving your diabetes management.
Glycated hemoglobin is formed in vivo by a reaction between glucose and the N-terminal region of haemoglobin (Hb) or ?chains. This irreversible non-enzymatic reaction between glucose and haemoglobin A, the main type of hemoglobin in normal adults, occurs over the life span of the erythrocyte. The resulting HbA1c (glycated haemoglobin) is a stable glycated hemoglobin containing primarily glycated N-terminal chains, and its total amount depends directly on the average glucose concentration over the two to three months before the measurement.
The clinical usefulness of the test was supported especially by two major trials: Diabetes Control and Complications Trial (DCCT) in type 1 diabetes4 and the United Kingdom Prospective Diabetes Study (UKPDS) in type 2 diabetes. These trials showed that improved glycaemic control, as assessed by HbA1c, could lead to substantial reductions in the risk of developing the microvascular complication of diabetes such as retinopathy, nephropathy, and neuropathy. The UKPDS trial findings also suggested that the risk of myocardial infarction (the main cause of premature death in diabetes) could be improved by reducing HbA1c values.
Limitations of glycated hemoglobin
Several limitations have been known to interpreting glycemic control with glycated hemoglobin. The presence of hemoglobin variants may falsely lower glycated hemoglobin values.
The prevalence of the most common hemoglobin variants (HbS, HbC, and HbD) depends on the genetic background of the population being analyzed. Although relatively rare in white individuals, these variants are common in populations with heterogeneous ethnic backgrounds. In such populations, misleadingly low glycated hemoglobin values have been identified by some methods, but not by others.
In addition, several other factors besides the presence of genetic variants or chemically modified derivatives of haemoglobin, such as drugs, anaemia, uraemia, and alcoholism, may falsely lower glycated hemoglobin results. Nevertheless, the impact of the effects of these interferences in routine clinical practice has not yet been well established.
Evidence of the usefulness of glycated hemoglobin in preventing diabetic complications stems mainly from the two studies mentioned above. This creates further limitations; namely, how applicable are findings from these studies to patients from parts of the world outside the USA and the UK where these studies were performed.
To address these limitations a study was performed by Camargo and Gross and published in the latest issue of Journal of Clinical Pathology. They investigated the causes of spuriously low HbA1c values in their Brazilian patients with diabetes. From August 1996 to December 2001, 29 657 samples from patients with diabetes attending the outpatient diabetic clinic at Hospital de Cl?icas de Porto Alegre, Brazil, were collected for glycated hemoglobin determination. The study population were classified as white, 8.4% as mulatto, 4.0% as black, and 0.9% as native, yellow, or not defined. They found that the presence of haemoglobin variants and undiagnosed anaemia accounted for most of these cases. The cosmopolitan nature of their population (in comparison to those in the DCCT and UKPDS studies) probably contributed to the prevalence of haemoglobinopathy.
Among the patients presenting glycated hemoglobin values below 4.7%, 56% (70% of whom were classified as white) of the results may be accounted for by the presence of the hemoglobin variant. Although the number of samples showing low values was small (130 patients from 29 657 samples) this is certain to be just the tip of the iceberg because they defined a very low HbA1c as being below the normal (non-diabetic) lower reference limit of just 4.7%. Thus, there may have been many other cases where the effect on the HbA1c value was much more subtle but still clinically relevant.
Even when patients have no abnormalities, the fact that HbA1c measures average glycaemia and says nothing about swings in blood glucose can result in some individuals only achieving their treatment goal at the expense of a poor quality of life, as a result of frequent, disabling, and unpredictable hypoglycaemia. The methods currently used to estimate glycated hemoglobin do not recognise hemoglobin variants and the calculation for the glycated component is only related to HbA1c, not to HbS1c, HbC1c, or to HbD1c, resulting in very low glycated hemoglobin values.
Camargo JL, Gross JL. Conditions associated with very low values of glycohemoglobin measured by an HPLC method. J Clin Pathol 2004;57:346?9.
The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med 1993;329:977?86.
UK Prospective Diabetes Study (UKPDS) Group. Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). Lancet 1998;352:837?53.
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