In the early 1980s, Henrik Mortensen and colleagues at Glostrup
University Hospital, in Denmark, measured the reaction rates in vitro.
Their results showed the assumption of irreversibility to be untrue. In
fact the reverse reaction (HbA1c to HbA and glucose) proceeds at about
1/8 the rate of the forward reaction, which is very far from
irreversible. Mortensen et alia also built a biokinetic model based on
the measurements, and validated the model by comparing its predictions
to actual patients. See references 3-5.

Among other things, Mortensen's work shows that after a change in
average bG level, the HbA1c level restabilizes after about 4 weeks.
This has several consequences. Clinically, the most important are
these:

First, the HbA1c is an exponentially weighted average of blood glucose
levels from the preceding 4 weeks, with the most recent 2 weeks being
by far the most important.

Second, measuring HbA1c less often than monthly results in unmonitored
gaps between measurements. To use HbA1c as a continuous monitoring
tool, you need to check it at least once a month.

Third, it is worthwhile checking the HbA1c of newly diagnosed patients
as often as once a week to determine the effectiveness of the newly
imposed treatment.

Reference 1: American Diabetes Association, Tests of Glycemia in
Diabetes, Diabetes Care 23:S80-S82, January 2000 Supplement 1.
This specific issue is no longer available online, but the most recent
version is available at http://diabetes.org/cpr/.

Reference 2: Kilpatrick ES, Maylor PW, Keevil BG:  Biological Variation
of Glycated Hemoglobin. Diabetes Care 21:261-264, February 1998.
Abstract available on the web at
http://care.diabetesjournals.org/cgi/content/abstract/21/2/261.

Reference 3: Mortensen HB, Christophersen C: Glucosylation of human
haemoglobin a in red blood cells studied in vitro. Kinetics of the
formation and dissociation of haemoglobin A1c. Clinica Chimica Acta
134:317-326, 15 November 1983.

Reference 4: Mortensen HB, Volund A, Christophersen C: Glucosylation of
human haemoglobin A. Dynamic variation in HbA1c described by a
biokinetic model. Clinica Chimica Acta 136:75-81, 16 January 1984.

Reference 5: Mortensen HB, Volund A: Application of a biokinetic model
for prediction and assessment of glycated haemoglobins in diabetic
patients. Scandinavian Journal of Clinical and Laboratory Investigation
48:595-602, October 1988.