If it's called "Fasting BSL" then you won't be required to drink the glucose drink and stay for further testing. These are GTT and GCT that require that. Fasting BSL can be a good initial indicator of how your pancreas is functioning, eating carbs before bed can help keep a diabetic's fasting BSL in a good range (was essential when I had GDM). So you may be required to avoid them so it doesn't hide that you actually have a problem.