A1c and BGs

[Guest guest]

Hi All

There is some confusion over HbA1c, partly because some authorities

still persist in calling it an " average " when it isn't, and publish

tables of direct equivalence to BGs - which are misleading.

To be quite clear, it is generally (with some exceptions) an excellent

indicator of your diabetic health - but it is not an average. Nor can

it be simply related to BG readings using current methods; that may be

possible when continuous glucometers become a practical reality, but

not at the moment.

I'm not a chemist, so I'll copy the best explanation I've seen from

the mhd FAQs:

http://www.faqs.org/faqs/diabetes/faq/part2/

" Hb = hemoglobin, the compound in the red blood cells that transports

oxygen. Hemoglobin occurs in several variants; the one which composes

about 90% of the total is known as hemoglobin A. A1c is a specific

subtype of hemoglobin A. The 1 is actually a subscript to the A, and

the c is a subscript to the 1. " Hemoglobin " is also spelled

" haemoglobin " , depending on your geographic allegiance.

Glucose binds slowly to hemoglobin A, forming the A1c subtype. The

reverse reaction, or decomposition, proceeds relatively slowly, so any

buildup persists for roughly 4 weeks. Because of the reverse reaction,

the actual HbA1c level is strongly weighted toward the present. Some

of the HbA1c is also removed when erythrocytes (red blood cells) are

recycled after their normal lifetime of about 90-120 days. These

factors combine so that the HbA1c level represents the average bG

level of approximately the past 4 weeks, strongly weighted toward the

most recent 2 weeks. It is almost entirely insensitive to bG levels

more than 4 weeks previous. "

So it isn't an average of BGs - but it is directly affected by your

recent blood glucose levels and acts as a representation or indicator

of those past levels, heavily weighted towards your most recent BGs.

As to whether there is a direct relationship, such as those published

in tables (even by reputable authorities), it varies depending on a

number of factors - particularly the level of the HbA1c itself. A

number of studies have shown that at higher A1c levels, fasting BGs

contribute more, but at lower levels the post-prandial BGs contribute

more. For more detailed reading, see:

http://care.diabetesjournals.org/cgi/content/full/

26/3/881?ijkey=eaae2599d3c12bbf0cd0423b76e1e6280eb9c8c3

or http://tinyurl.com/cygkb

Contributions of Fasting and Postprandial Plasma Glucose Increments to

the Overall Diurnal Hyperglycemia of Type 2 Diabetic Patients

" RESULTS—The relative contribution of postprandial glucose

decreased

progressively from the lowest (69.7%) to the highest quintile of HbA1c

(30.5%, P < 0.001), whereas the relative contribution of fasting

glucose increased gradually with increasing levels of HbA1c: 30.3% in

the lowest vs. 69.5% in the highest quintile (P < 0.001).

CONCLUSIONS—The relative contribution of postprandial glucose

excursions is predominant in fairly controlled patients, whereas the

contribution of fasting hyperglycemia increases gradually with

diabetes worsening. These results could therefore provide a unifying

explanation for the discrepancies as observed in previous studies. "

To me, such studies make the various formulae and tables published in

the past misleading; they are rough guides at best.

Having said all that, my personal philosophy is simple - do my best to

keep my BGs down and the A1c will follow. But I don't get stressed

about tables and equations.

Cheers, Alan, T2 d & e, Australia