What is a dangerously high blood sugar reading?

According to the University of Washington's Women's Health Department, blood sugar levels above 300 are dangerously high and require immediate medical attention. It also says that anything above 160 is abnormal and requires action to reduce it.

ABC News explains that the effects of having blood sugar levels of above 180 to 200 can cause glucose to be excreted in urine rather than absorbed for use by the body while levels of 400 to 500 can cause abnormalities in mental functioning. For a diabetic person, a reading of more than 240 indicates that the diabetes is out of control, according to the University of Washington.