A couple of ppl (sorry, can't remember their names) have posted about conducting their own oral glucose tolerance tests to check how they are doing after a period of weight loss (e.g. Newcastle Diet). If anyone wants to do it, the criteria for an OGTT are quite strict, so perhaps read up on the instructions first (they are available through Google).
The new NICE guidelines suggest that any discrepency between HbA1c and Fasting Glucose test should be investigated - but don't say what that investigation should be.
There are also a couple of other things to bear in mind, when following a low carb diet and taking an OGTT
Firstly, when you eat Low Carb your body gets used to it, and it 'downgrades its enzyme production'. This means that it isn't prepared to cope with a large dose of carbs, so blood glucose may shoot up very high if you did an OGTT without some prep. The standard suggestion is that you eat 'normal' carbs for 3 days before the test, giving your body time to 'upgrade'. All well and good. Except that I trialled this for a week when on hol a couple of years ago, and was vastly amused to discover that MY body didn't start 'upgrading its enzyme production' until day 8. And I can't believe I am the only person whose pancreas is so lazy! So I would take The 3 Day Rule as an absolute minimum, and assume that you need to eat normal carbs for at least a week to 10 days - and if I eat 'normal carbs' for that period of time I generally put on about 7-10 pounds of weight, so it isn't something I would choose to do lightly.
Secondly, I did not know this during my last OGTT. As a result, I question the result of the test and have refused to take an OGTT since then despite pressure from both doc and nurse.
I had done as instructed - and eaten 'normal carbs' for the 3 days, and thought that was OK. However, I felt truly dreadful during the test. Thumping headache, general aches and nausea and lethargy and awfulness. Then, following the test I had all-body aching for 2 days, a vicious headache that lasted 48 hours and over a day of blurred vision. Had to take a day and a half off work because I couldn't see the computer screen or drive.
So, I am not suggesting that everyone will have the issues I did. Most people don't. But they are something to consider when self testing, or pushing for tests with the doc.