There are two common ways that physicians assess how well diabetes is controlled:  Frequent measurements of blood glucose, and  measurement of glycohemoglobin (A1c). Each method has its good and bad points, but combined they give a fairly accurate picture of the state of glucose control in a diabetic. Most physicians will use both methods.
When we speak about measuring blood glucose levels, it can be done 2 different ways. Blood glucose can be measured randomly from a sample taken at any time (called a "random blood sugar" or RBS).
Blood glucose can also be measured in the "fasting" state, meaning that the person has not eaten or taken in any calories in the past 8 hours (usually this is done overnight and it is referred to as an overnight fast and is called a "fasting blood sugar" or FBS).
In a person with normal insulin production and activity (a non-diabetic) blood sugar levels will return to "fasting" levels within 3 hours of eating. People with diabetes (type 1 and type 2) may not be able to get their blood glucose down this quickly after a meal or drinking a calorie-containing drink. More about this can be found on our Diagnosing Diabetes page.
Remember, the normal fasting blood glucose level is between 70 and 110 mg/dL.
Frequent Measurements of Blood Glucose. The goal in this part of diabetes management is to strive to keep fasting blood sugars under 140 mg/dL and preferably closer to the 70 to 120 mg/dL range.
Ideally, one could monitor blood sugars 4 times per day (or more) to follow how well the sugars are controlled. This information could be used to adjust your diet and medications to achieve this goal. Usually blood glucose measurements are done before each meal and at bedtime.
Occasionally a doctor may want a patient with diabetes to test their blood sugar at 2:00 am to assess what the blood sugar is doing overnight. Generally, it is desirable to have blood sugars at 2:00 am run greater than 65 mg/dL to avoid overnight hypoglycemia (low blood sugar).
It is very desirable for patients to keep a diary of these blood sugar measurements since this information will be a great help to the treating physician as he/she makes decisions regarding how best to treat the diabetes.
Bring this diary with you when you go to your doctor. He or she will be quite pleased, but don't fudge the results in an attempt to please your doctor; this will prevent the appropriate changes (if any) to be made in your management.
Periodic Measurement of Glycosylated Hemoglobin. Another method to monitor the control of blood sugar in people with diabetes is through a blood test called hemoglobin A1c or glycohemoglobin (or glycosylated hemoglobin).
This test can be done through a local laboratory or can be drawn at a doctor's office, but unlike simple measurement of blood glucose levels, this cannot be done at home. The level of glycosylated hemoglobin correlates very well with a person's recent overall blood sugar levels.
Hemoglobin A1c will tell what the diabetic's blood sugar levels have been running for the past 2 to 3 months. If the blood sugars have generally been running high during the previous few months, the level of hemoglobin A1c will be high. If blood glucose concentrations have been running close to normal during this time, the hemoglobin A1c level will be close to values seen in normal persons.
It is an important value to monitor periodically. Studies have shown that glycohemoglobin values in the "better ranges" correlate with less incidence of diabetic complications later in life. People with type 1 diabetes will typically have hemoglobin A1c levels determined every 3 to 4 months, while people with type 2 diabetes will often require measurements less often.
Values vary from lab to lab but below is a common value system for hemoglobin A1c:
Normal: Less than 6.5
Poor: Greater than 9.5
Studies have recently shown that overall good control of blood sugar in diabetes does correlate with decreased incidence of diabetic complications. So, the answer is yes, it is important to control glucose levels as best as possible. In people with type 1 diabetes who are on insulin and in some people with type 2 diabetes, efforts to have control too tight may result in too many episodes of hypoglycemia (low blood sugar). Therefore, the goal is to balance trying to have control as near normal as possible while trying to avoid hypoglycemic episodes.