February 7, 2001
A1c (Glycohemoglobin, HgbA1c)
Question from Lyndhurst, New Jersey, USA:
Lately, my son’s A1cs have been low, but his fructosamines have been high. Can you tell me what this means? These are the results:
A1c 6.0%; Fructosamine 290 (reference range: 0-285)
A1c 6.5%; Fructosamine 357
A1c 6.9%; Fructosamine 408
A1c 7.3%; Fructosamine 349.
Does this mean my son does not have as good control as I thought he did?
I think that both sets of figures are in fact telling you that there has been a slight diminution in your son’s level of control since last year. You are by no means the first person to be confused when comparing Hemoglobin A1c results to fructosamine. The difficulty, which was first demonstrated by a group in New Zealand about ten years ago, stems from the fact that both tests are rather imprecise. Specifically, this means that the 95% confidence limits can be around +/- as much as 20%. This means that the tests really have to be interpreted over a period of time and in relation to the blood sugar pattern. Very briefly, if you took the means of the blood glucose values during the period represented by the A1c and fructosamine levels and plotted them on graph paper against time for each test, it would show a line that sloped a little upward over the year (by about 14% for the A1c and by about 17% for fructosamine). My suggestion then would be to judge control on the general trend of the quarterly A1c test and never to compare single A1c and fructosamine tests.
You need to remember too, that both tests reflect the mean values over about one month for fructosamine and three months for the A1c so that you get the same results when you have a mixture of blood glucoses which are spread widely between too high and too low as you do when all the levels are closely grouped together.
I hope this hasn’t confused you. The important thing is that both tests are really telling you the same thing