It would seem (from this study) that, contary to the normally state the 90-120 day average, the HbA1c test 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'.
Interesting and worrying. Who's going to tell this lot.
Diabetes UK:- The HbA1c test measures diabetes management over two to three months.
Diabetes.co.uk: - Because red blood cells in the human body survive for 8-12 weeks before renewal, measuring glycated haemoglobin (or HbA1c) can be used to reflect average blood glucose levels over that duration, providing a useful longer-term gauge of blood glucose control.
NHS:- The HbA1c test gives your average blood glucose levels over the previous two to three months.
Wikipedia:- This serves as a marker for average blood glucose levels over the previous 3 months prior to the measurement as this is the half life of red blood cells.
Abbot Diabetes Care:- This test is usually done on a sample of blood taken from a vein in your arm and the result shows your overall control of glucose levels over the last 2-3 months.
I try to get more than one opinion on anything that the net spews up, I wouldn't always trust Wikipedia, but enough people have the opportunity to edit it, so I've included them.
I've also noticed that Mortensen's findings were based on blood "in vitro", I wonder if it's the same in a person.
Also this work was done in the 1980s, I'm surprised that it's not better known.
The statement "To use HbA1c as a continuous monitoring tool, you need to check it at least once a month". is probably true if there is no other monitoring going on.
Innovations in Participatory Medicine: The Advent of Do-It-Yourself Blood Glucose Monitoring by Jeanna Frost PhD suggests that self monitoring was only really taken up in the 80's, although machines had been around for 20 years or so.
Maybe this bit of research is just a bit too out of date to worry about.