Diagnosing Diabetes

Those with type 2 diabetes are usually diagnosed in their 30s. However, we are seeing more and more patients diagnosed in their teenage years. There may be a genetic predisposition, similar to that of type 1 diabetes, that may be linked to the development of type 2 diabetes. However, unlike those with type 1 diabetes, many people with type 2 diabetes (60 percent) are obese. This is likely due to a combination of genetic factors and may be a result of the body's need to take in more calories to keep blood sugar levels high enough for cells to function.

The common symptoms of type 1 and type 2 diabetes are similar and are directly related to the body's response to high blood sugar levels. The classic symptoms include excessive urination and thirst. When glucose is present in high levels in the blood, the kidneys produce higher volumes of urine. Thus, a person with untreated diabetes will have to empty the full bladder often. This can cause the body to become dehydrated, triggering the thirst response, resulting in excessive drinking. The volume of fluid that is lost in the urine is often great. And if this fluid is not replaced, the person can experience symptoms of dehydration as well, such as dizziness, headache, and rapid heart rate. Other symptoms include blurred vision, infections, and weight loss despite an increase in appetite and food consumption.

A major difference between type 1 and type 2 diabetes is that those with type 1 diabetes are absolutely dependent on an external source of insulin to live. These people may be presented with life-threatening symptoms. For example, if the person with type 1 diabetes does not have insulin, he will start to metabolize other energy sources in the body (such as fat) that produce harmful substances that can lead to death. This condition is called diabetic ketoacidosis (DKA). It is rare for someone with type 2 diabetes to develop DKA unless he is under very stressful conditions, such as a major illness.

The difference in body weight between those with type 1 and type 2 diabetes is commonly related to the effects insulin has on the body. Insulin supports growth of body tissues, including fat. People with type 1 diabetes, as discussed earlier, do not produce their own insulin, so they need to balance what they eat with the amount of insulin they take in order to keep their blood glucose levels normal. (I will discuss this more in chapter 5.) People with type 2 diabetes typically produce enough insulin and sometimes even two or three times the normal amount of insulin (DeFronzo 1988). In a person with untreated type 2 diabetes, the body senses that there is a low level of glucose inside the cells (despite having high levels in the blood), and the insulin level is increased and the hunger center in the brain is activated, driving the person to eat more. This combination of events often causes the person with type 2 diabetes to overeat, which leads to obesity.

Does everyone with diabetes have symptoms? Often there are symptoms. But just as often the diagnosis is made without the presence of symptoms, during routine health examinations that include blood sugar screenings. Elevated blood sugar prompts the health care provider to seek a cause of this irregularity. The physician may run more tests if an initial blood test is abnormal; she may review family history as well. We typically screen patients for diabetes starting at age 45. However, if a patient is in a high-risk group (African American, Asian, Latino, Native American) or has risk factors for diabetes (such as obesity, high blood pressure, high blood lipid levels, or a first-degree relative with diabetes), we screen the patient for diabetes earlier than age 45.

Physicians prefer to have blood glucose tested in the morning, before the patient has eaten. This is called a fasting glucose test. We consider a normal fasting glucose test to be less than 110 mg/dl (milligrams per deciliter). If a person's fasting glucose level is between 110 and 126 mg/dl, then we consider this person to have prediabetes, or what is often referred to as glucose intolerance. Fasting glucose levels above 126 mg/dl on two separate occasions are indicative of diabetes mellitus. If you have a test after you have already eaten (or what is described as a nonfasting glucose test), and it shows your blood glucose level to be above 200 mg/dl, then this is enough to make the diagnosis of diabetes mellitus (see Table 1.1).

There are other tests that may be done as well, such as a glucose tolerance test. The glucose tolerance test involves doing a fasting glucose test and then having the patient drink 75 milligrams of glucose and remeasuring the blood glucose levels after two hours. The test is normal if the fasting glucose is less than 110 mg/dl and the two-hour glucose level is less than 140 mg/dl. If the two-hour results show a level between 140 and 200 mg/dl, then this is indicative of glucose intolerance; up to 5 percent of patients with this level of blood glucose will develop diabetes. A two-hour glucose level greater than 200 mg/dl is indicative of diabetes.

Your doctor may also choose to do another test commonly referred to as the hemoglobin A(HbA). This is a test designed to give your health care provider a rough estimate of how high your blood glucose levels have been over the last three months. This can help your physician in deciding how often to check your glucose as well as in formulating your treatment plan. For instance, if your initial test confirms the diagnosis of type 2 diabetes with two fasting glucose levels of 130 mg/dl, that is just above the diagnostic threshold of 126 mg/dl. But if your HbA1C is significantly elevated, your doctor may decide to monitor you more closely and treat your condition more aggressively. This may include starting a medication regimen earlier in addition to making changes in your diet and exercise habits. It is important to note that if you are diagnosed with type 1 diabetes, then you will start taking medication (insulin) immediately.

Table 1.1 Glucose Levels and What They Mean

Other tests can differentiate between type 1 and type 2 diabetes if it becomes difficult to do through questioning the patient about symptoms. A certain molecule called C-peptide is part of the precursor molecule to the insulin molecule. When insulin is formed in the pancreas, the C-peptide separates from the insulin portion of the molecule and can be measured in the blood of those with type 2 diabetes, whereas it is not present in those with type 1 diabetes, because they do not produce insulin.