Relevance of HbA1C

How helpful is Haemoglobin A1c  as a diagnostic test for diabetes mellitus . ?? Does this index alone speak of  development of micro vascular  complications ?? Ans; Diabetes  mellitus is a metabolic and vascular  disease  with hyperglycemia  and specific micro vascular complications in those who are poorly controlled over  a long term  period is bound to suffer from micro vascular complications which is a typical complication after say 10 yrs of appearance of DM.  . However  there is no well defined  threshold  level of blood  glucose  beyond which  micro vascular  complications develop and below which  there is a  complete immunity  from complications. Thus fasting venous plasma glucose   value of 126 mg % and two hours post 75g oral glucose   load   value of 200 mg%  are somewhat arbitrary diagnostic  values for diabetes  mellitus. At present   for the want   of better diagnostic test blood glucose values are used as the only criteria for the diagnosis of diabetes however these have certain limitations.

 Among the micro vascular  complications of diabetes diabetic retinopathy is the most extensively studied complication as regards  its co relationship  with  fasting and post  glucose   load blood glucose   values. The criteria for diagnosis  of diabetes based on fasting blood glucose were lowered  from 140 to 126  mg.  Even now there is no clear cut threshold  blood glucose value for onset of retinopathy!!!

Ans:--Till 1997 fasting venous plasma  glucose and two hours post 75 g oral glucose load cut off point  for diagnosis of diabetes were 140 mg% and 200

mg% respectively  . Why such figures were selected?? The answer is that these points were based on symptoms of diabetes and not on risk for development of micro vascular  complications.   Even though there is no clear cut threshold  blood glucose value for retinopathy some people with fasting  blood glucose values  between  126-140 mg% have  evidence  of early non proliferative diabetic retinopathy however retinopathy  is very rare in those having fasting venous  plasma glucose value below 126 mg% . Thus in 1997 criteria for diagnosis  of diabtes based on fasting blood glucose were lowered  from 140 to 126  mg.

Do you know that  a false  lower values of blood glucose is quite possible  if the blood sample is not analyzed with  in 1 hour  due to glycolysis!!   Moreover  fasting value of 126 mg% has better correlation with post glucose  load  value of 200 mg%  as regards  micro vascular   complications.   However, a lowering  of diagnostic  fasting blood  glucose  value i.e. 126 mg %  was seen  as a definite  improvement  using blood glucose  values for diagnosis  of diabetes but  still have some limitations such as 1)  poor reproducibility due to analytical variance 2) need to remain in fasting state  for 8hours 3) false  lower values if the blood sample is not analyzed with  in 1 hour  due to glycolysis