High blood levels of glucose can cause several problems, including frequent urination, excessive thirst, hunger, fatigue, weight loss, and blurry vision. However, because type 2 diabetes develops slowly, some people with high blood sugar experience no symptoms at all.

Symptoms of type 1 diabetes:

• Increased thirst
• Increased urination
• Weight loss in spite of increased appetite
• Fatigue
• Nausea
• Vomiting

Symptoms of type 2 diabetes:

• Increased thirst
• Increased urination
• Increased appetite
• Fatigue
• Blurred vision
• Slow-healing infections
• Impotence in men

• Fasting blood glucose level -- diabetes is diagnosed if higher than 126 mg/dL on two occasions.
• Random (non-fasting) blood glucose level -- diabetes is suspected if higher than 200 mg/dL and accompanied by the classic symptoms of increased thirst, urination, and fatigue. (This test must be confirmed with a fasting blood glucose test.)
• Oral glucose tolerance test -- diabetes is diagnosed if glucose level is higher than 200 mg/dL after 2 hours (This test is used more for type 2 diabetes.)

Patients with type 1 diabetes usually develop symptoms over a short period of time, and the condition is often diagnosed in an emergency setting. In addition to having high glucose levels, acutely ill type 1 diabetics have high levels of ketones.

Ketones are produced by the breakdown of fat and muscle, and they are toxic at high levels. Ketones in the blood cause a condition called "acidosis" (low blood pH). Urine testing detects both glucose and ketones in the urine. Blood glucose levels are also high.