06/09/2019
Glycated hemoglobin (A1C, hemoglobin A1c, HbA1c.
It is measured primarily to determine the three-month average blood sugar level and can be used as a diagnostic test for diabetes mellitus and as an assessment test for glycemic control in people with diabetes.
The test is limited to a three-month average because the average lifespan of a red blood cell is four months. Since individual red blood cells have varying lifespans, the test is used 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. In diabetes, higher amounts of glycated hemoglobin, indicating poorer control of blood glucose levels, have been associated with :
1.cardiovascular disease,
2. nephropathy,
3.neuropathy,
4.retinopathy.
Glycated hemoglobin testing is recommended for both checking the blood sugar control in people who might be prediabetic 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.
Persistent elevations in blood sugar (and, therefore, HbA1c) 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, and 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.
The American Diabetes Association guidelines are similar to others in advising that the glycated hemoglobin test be performed at least twice a year in patients with diabetes who are meeting treatment goals (and who have stable glycemic control) and quarterly in patients with diabetes whose therapy has changed or who are not meeting glycemic goals.[37
INTERPRETATION OF HbA1C RESULTS:
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 who have donated blood recently, are not suitable for this test.
Concentrations of hemoglobin A1 (HbA1) are increased, both in diabetic patients and in patients with renal failure, when measured by ion-exchange chromatography. The thiobarbituric acid method (a chemical method specific for the detection of glycation) shows that patients with renal failure have values for glycated hemoglobin similar to those observed in normal subjects, suggesting that the high values in these patients are a result of binding of something other than glucose to hemoglobin.
In autoimmune hemolytic anemia, concentrations of HbA1 is undetectable. Administration of prednisolone will allow the HbA1 to be detected.The alternative fructosamine test may be used in these circumstances and it also reflects an average of blood glucose levels over the preceding 2 to 3 weeks.
All the major institutions such as the International Expert Committee Report, drawn from the International Diabetes Federation, the European Association for the Study of Diabetes, and the American Diabetes Association, suggest the HbA1c level of 48 mmol/mol (6.5 DCCT %) as a diagnostic level.The Committee Report further states that, when HbA1c testing cannot be done, the fasting and glucose-tolerance tests be done.
Diagnosis of diabetes during pregnancy continues to require fasting and glucose-tolerance measurements for gestational diabetes, and not the glycated hemoglobin
RECENT ADVANCES:
Higher levels of HbA1c are found in people with persistently elevated blood sugar, as in diabetes mellitus. While diabetic patient treatment goals vary, many include a target range of HbA1c values. A diabetic person with good glucose control has a HbA1c level that is close to or within the reference range.
The International Diabetes Federation and the American College of Endocrinology recommend HbA1c values below 48 mmol/mol (6.5 DCCT %), while the American Diabetes Association recommends HbA1c be below 53 mmol/mol (7.0 DCCT %) for most patients.
Recent results from large trials suggest that a target below 53 mmol/mol (7 DCCT %) for older adults with type 2 diabetes may be excessive: Below 53 mmol/mol, the health benefits of reduced A1c become smaller, and the intensive glycemic control required to reach this level leads to an increased rate of dangerous hypoglycemic episodes.
A retrospective study of 47,970 old, type 2 diabetes patients found that patients with an HbA1c more than 48 mmol/mol (6.5 DCCT %) had an increased mortality rate,but a later international study contradicted these findings.
A review of the UKPDS, Action to Control Cardiovascular Risk in Diabetes (ACCORD), ADVANCE and Veterans Affairs Diabetes Trials (VADT) estimated that the risks of the main complications of diabetes (diabetic retinopathy, diabetic nephropathy, diabetic neuropathy, and macrovascular disease) decreased by about 3% for every 1 mmol/mol decrease in HbA1c.
However, a trial by ACCORD designed specifically to determine whether reducing HbA1c below 6.0% using increased amounts of medication would reduce the rate of cardiovascular events found higher mortality with this intensive therapy, so much so that the trial was terminated 17 months early.
Practitioners must consider patients' health, their risk of hypoglycemia, and their specific health risks when setting a target HbA1c level. Because patients are responsible for averting or responding to their own hypoglycemic episodes, their input and the doctors' assessments of the patients' self-care skills are also important.
ABNORMAL HbA1C RESULTS:
Lower-than-expected levels of HbA1c can be seen in people with shortened red blood cell lifespans, such as glucose-6-phosphate dehydrogenase deficiency, sickle-cell disease, or any other condition causing premature red blood cell death.
Blood donation will result in 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 HbA1c to underestimate the actual average levels. There may also be distortions resulting from blood donation, which occurred as long as two months before due to an abnormal synchronization of the age of the RBCs, resulting in an older than normal average blood cell life (resulting in an overestimate of actual average blood glucose levels).
Higher-than-expected levels can be seen in people with a longer red blood cell lifespan, such as with vitamin B12 or folate deficiency.
Results can be unreliable in many circumstances:
1. blood loss,
2.after surgery,
3.blood transfusions,
4.anemia, or
5.high erythrocyte turnover;
6.in the presence of chronic renal or liver disease;
7.After administration of high-dose vitamin C; or erythropoetin treatment.
8.Glycated hemoglobin measurement is not appropriate where a change in diet or treatment has been made 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 who have donated blood recently, are not suitable for this test.
