My friend is experiencing a sudden onset of high blood sugar, in the 400s.  Male, 63, 206 lbs who was 216 about 3 weeks ago.  Great shape, exercises frequently.  Doctors have taken CBC and everything looks good except the blood sugar.  He drinks a bottle of water and urinates it out in 20 minutes.  Does diabetes come on this suddenly or is there a hidden cause we could investigate?
Good day. Noted. You have not mentioned his height and hence his body mass index couldn't be calculated. Sudden onset blood glucose in an otherwise healthy individual need investigations to rule out a secondary cause. However, I must say that Type 2 diabetes can have such a presentation. They may remain asymptomatic for years with a high blood sugar undetected. Please get a HAC done which will tell if he had diabetes for a long time or not. On examination, doctors may elicit signs of secondary causes if any.