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Originally Posted by Alice My endo told me that the test from blood that is drawn will always vary from the finger tip tests. She said both numbers are correct, just different blood sources. (Vein vs capillary?) |
There are plenty of websites, books etc. that will quote tables correlating A1c and Mean Plasma Glucose (MPG). However these need to be interpreted with a bit of caution and it helps to understand the original published research on which these numbers are based.
This involved measuring A1c's for a large number of T1's and then measuring their MPG's. The first is relatively easy (although even here there are uncertainties, see below). To do the second ACCURATELY, you would need to hook all these T1's up to continuous monitoring over the 3 month period, or at least have them test 8+ times per day. In fact most T1D's won't do this, so instead, individuals were extensively monitored over 1 or 2 days out of a 3 month period. As any T1 knows, individual days can be far from typical!
Perhaps as a consequence of this, The data points (one for each individual T1) show a huge amount of scatter. To illustrate this I have pasted the actual figure from the key paper from which came the oft-quoted "fact" is that an A1c of 6.0 corresponds to a MPG of 7.6 (137). However, the 95% confidence interval is 3.8 mM. This says that you would expect 95% of individuals with an A1c of 6.0 to have a MPG of between 5.7 and 9.5 (or in US$ about 104 and 170). That's a huge difference.
A second difficulty is that the fingerstick and blood draw methods give different A1c numbers. The fingerstick values are on average 0.2 higher, so your numbers will appear to be WORSE using this method. To complicate things further the difference gets bigger the lower the numbers, as much as 0.5 for really good A1c values (in the 5's).
The A1c is a valuable indicator of control, and lower MPGs means less health problems. However, we should perhaps be careful when playing Liar's Poker with our A1c numbers!
My 2c
Joel