Not Medical Advice: Any level over 300 mg/dL is considered as very high blood sugar level.
According to Yale School of Medicine, very high blood sugar levels mean that the body does not have enough insulin around to use sugar as energy. Even though the level of sugar in the blood stream is very high, the body cells may be sending out the alarm that they are starving. In this case, the body will switch to burning fat as an energy source. This is a problem because excess burning creates byproducts such as ketones and acids. After several hours the blood can become too acidic for the body organs to work right. This leads to a life-threatening diabetes emergency called Diabetic Ketoacidosis (DKA). Full-blown DKA must be treated in a hospital, often in the Intensive Care Unit.
When you have diabetes, your blood sugar (glucose) levels may be consistently high. Over time, this can damage your body and lead to many other problems.
You can learn more about diabetes, the complications related to the disease, and how you can prevent these complications at the following sites:
WebMD
University of Washington Medical Center
Joslin Diabetes Center
Get some tips for mainting safe blood sugar levels at EndocrineWeb.