When you eat something, your body should break it down and keep your blood glucose level constant,
A random blood glucose test is when your blood glucose level at any given time, this should always be below 10mmol/l
8.7 isn't particularly high, if you had eaten in the last hour or so it would be fine. If there was a problem then the nurse would have referred you so dont worry!
It doesn't mean that you are succeptible to diabetes, especially if you are young.
Diabetes type 1 comes on very quickly and you would show symptoms rapidly. It is much more common in young people
If you are older then diabetes type II is more commom, it comes on slowly and symptoms include tiredness, thirst, weeing a lot, itching down under & slow healing of wounds.
If you have any of these symptoms and are over 40 then go the docs, tell him/her that you've had a RBG of 8.7mmol/l and he/she will test your glucose when you haven't eaten, then give you food & check it again.