Glycated hemoglobin (GHb)
An estimate of blood glucose indicates blood glucose at a particular time. By itself, it does not reflect the long-term control of diabetes. Serial records of urine and blood glucose tests give some idea of the long-term control of diabetes. Glycated hemoglobin testing gives a more accurate idea of the long-term control of diabetes.
Hemoglobin (Hb), a protein, the colouring matter of red blood cells consists of many fractions. Al (HbAl) accounts for less than 10 per cent of total Hb. This HbAl in turn, is composed of subfractions, Ala, Alb, Alc, out of which Alc is the largest.
The red blood cells are bathed in plasma which contains glucose. Glucose gets attached to hemoglobin in an irreversible fashion. This combination of hemoglobin (Hb) Al and glucose (G) is called glycated hemoglobin (GHb or GHbAl),
The higher the average blood glucose level, the higher is the fraction of hemoglobin that is combined with glucose. The life span of red blood ceils is about four months. Therefore the value of HbAl gives an overall idea of the blood glucose level for the preceding three to four months. Sometimes only HbAlc is measured. The normal value of HbAlc would naturally be lower than that of HbAl.
With slight variations HbAlc is usually 4 to 6 per cent of the total hemoglobin in a normal person. The HbAlc is an useful indicator of overall glycaemic control for the preceding three to four months.
HbAlc is a "weighted measurement". Recent glycaemia contributes more to HbAlc than the earlier one. About 50 per cent of HbAlc value reflects blood glucose levels over the preceding one month, 40 per cent of the HbAlc value reflects blood glucose levels of the previous third to second month and only 10 per cent reflect blood glucose value from the previous fourth to third month.
Further, it is independent of the blood glucose level at the time of testing, time of day, diet and meal status, exercise and anti-diabetes treatment. Hence it can be done at any time of the day. Manoeuvers such as following diet and exercise strictly for a few days prior to the test can give misleadingly normal blood glucose levels. Such manipulations cannot alter the HbAlc value. Before a woman with diabetes decides to conceive, it is essential that her glycaemic control over the preceding three months is good. The HbAlc test can thus help to make a decision regarding conception.
The HbAlc test is not without its pitfalls. There is a wide variation of normal values between different laboratories. Hence it is advisable to get the test done from one particular reliable laboratory. The HbAlc test should not be used to diagnose diabetes. A normal HbAlc value does not exclude the diagnosis of diabetes.
It is sustained hyperglycaemia rather than short-term changes in blood glucose levels that gets reflected in HbAlc.
HbAl c does not give any idea of the shape of glycaemic profile. For instance, it would not tell whether hypoglycaemia had occurred in the previous week. The HbAlc value may be normal or marginally elevated if episodes of hypoglycaemia are counteracted by episodes of hyperglycaemia.
The HbAlc test cannot be used to make day-to-day decisions of management.
The HbAlc value can be misleading in certain situations. Different individuals have different rates of glycation. Vitamin E and drugs like aspirin may reduce rate of glycation and give falsely low HbAlc values.
Factors which reduce red blood cell life span like pregnancy, blood loss, excessive breakdown of red blood cells, for example, certain anaemias and kidney disorders give falsely low values. Falsely low values of HbAlc are obtained if the kidney threshold for glucose is low.
The HbAlc test may be done once in three to four months except during pregnancy when it should be done more frequently. The result should be interpreted in the light of the points already made.
Glycation can occur with proteins other than hemoglobin. Fructosamine is glycated plasma protein. An estimation of fructosamine gives an idea of control of glycaemia over the preceding two weeks. The fructosumine level is reduced when serum albumin level is reduced in liver or kidney disease and obesity. This test has not been adopted widely.