How low do vit b12 levels need to be to be low?

I have been having repeated blood tests for a while now as i am constantly tired. My 1st blood test my b12 levels were 127, the next 152 and now 168 which the dr says is normal. When they were 152 my folate level was 1.9 and i was prescribed folic acid and a mth later it is now 7.9. I have read tho that supplementing folic acid will hide b12 problems. Could someone shed some light on this as the dr thinks my tiredness is all in my head