Hi purple moon,
If the test is a fasting blood test then having a very low level of carbs in the 24 hours before the test will certainly lower your BS when they test it. But what does that prove? - that if you gave up eating bread and pasta you could have permanently good BS levels!
If it is a glucose tolerance test then what your BS level is at the start of the test is simply a guideline. What they are looking at is how your body reacts to the glucose and whether your BS drops back to its starting point after 2 hours.
There are some types of test where you can frig the results, but the only person that is cheating is you!