Caffeine does not really affect glucose levels or blood glucose. It may affect the action of glucose with its interaction with cells especially with specific medications. However, if you are administering caffeine through coffee for instance the affect of adding sugar naturally would and too the creamer.
An example is being type II I take glucose readings when first awakening referred to as a fast measurement based on not eating since dinner usually. That is normally around 110 to 120. If all I do is have 2 - 3 12 oz cups of coffee with a couple of glugs of creamer the glucose level rises.
That rise is based on the consumption of carbohydrates. Plus the coffee drinking is usually at a pace of some kind. The net result is a rise will occur bringing a subsequent glucose check say over a one hour period to a level reaching as high as the 140 - 160's. Ponder a Latte, Mocha, or Cappuccino.
That is the wake up time. Next, you eat, (and maybe you never did a fasting glucose read), you check 2 hrs later and you are over 200. A rule of thumb is 10g carbs raises glucose about 45 at consumption. The glycemic load index will offer guidance at how fast it will rise and how long that rise will last over time.