Making the Diagnosis
Diabetes is diagnosed through a lab test that measures the level of glucose in your blood. Because a rise in blood sugar levels might be attributable to some other factor, such as illness or stress, a second blood test is usually performed the following day to establish the diagnosis.
There are three different types of diagnostic blood tests available for diabetes: the fasting plasma glucose test, the casual plasma glucose, and the oral glucose tolerance test. In the absence of overt symptoms or a hyperglycemic (i.e., high blood sugar) crisis, two tests taken on different days are recommended to confirm the diagnosis.
The normal range for the fasting plasma glucose test (FPG) is considered to be less than 100 mg/dl (5.6 mmol/l). Individuals with FPG results between 100 mg/dl (5.6 mmol/l) and 125 mg/dl (6.9 mmol/l) are said to have impaired fasting glucose (IFG), also known as prediabetes.
Fasting Plasma Glucose Test (FPG)
The FPG is a carbohydrate metabolism test that measures plasma (blood) glucose levels after a fast of at least eight hours. Fasting stimulates the release of the hormone glucagon, which in turn raises plasma glucose levels by triggering the breakdown of glycogen (stored glucose) in the liver. In people without diabetes, the body will produce and process insulin to counteract this rise in glucose levels. With diabetes, this does not happen, and the tested glucose levels will remain high.
Can the OGTT be used to diagnose prediabetes?
Yes, both the FPG and the oral glucose tolerance test (OGTT) can be used to diagnose prediabetes. Individuals with two-hour plasma glucose OGTT results between 140 mg/dl (7.8 mmol/l) and 199 mg/dl (11.0 mmol/l) are said to have impaired glucose tolerance (IGT).
The fasting plasma glucose test should be administered in the morning because, aside from the fact that it is easiest to fast during sleeping hours, blood glucose tests given in the afternoon tend to provide lower readings and could miss some cases of prediabetes and diabetes.
According to ADA clinical practice guidelines, a fasting reading of 126 mg/dl (7.0 mmol/l) or higher indicates diabetes, and a second follow-up oral glucose tolerance test on a subsequent day should be performed to confirm the diagnosis if symptoms of diabetes are not present. The OGTT is considered a more sensitive (albeit more time-consuming and expensive) test than the FPG.
Oral Glucose Tolerance Test (OGTT)
The OGTT is a test that measures blood glucose at hour intervals over a two-hour period. The patient is given a 75-gram drink of glucose solution (Glucola), which should cause glucose levels to rise in the first hour, and then fall back to normal within two hours as the body produces more insulin to normalize glucose levels.
Blood drawn two hours after drinking the glucose solution (also called two-hour postload blood draw) that has glucose levels of less than 140 mg/dl (7.8 mmol/l) is considered normal. Two-hour postload levels of 140 mg/dl (7.8 mmol/l) or higher but less than 200 mg/dl (11.1 mmol/l) are an indication of impaired glucose tolerance. Blood glucose levels of 200 mg/dl (11.1 mmol/l) or higher two-hour postload point to diabetes, and should be confirmed by a second OGTT or FPG test on a different day if symptoms of diabetes are not present.
If you are considering going on an insulin pump, a c-peptide test may be ordered. As of 2008, Medicare regulations required a fasting c-peptide level of “less than or equal to 110 percent of the lower limit of normal of the laboratory's measurement method” for coverage of insulin pumps and related supplies.
Random or Casual Plasma Glucose Test
The random plasma glucose test, also called a casual plasma glucose test, can be given at any time of the day, regardless of whether the patient has eaten or not. Casual plasma glucose levels of 200 mg/dl (11.1 mmol/l) or higher, along with symptoms of hyperglycemia, are considered diagnostic of diabetes. A second FPG or OGTT test on another day is recommended to confirm the diagnosis if numbers are high but symptoms are not present.
After a diagnosis is made, further lab tests may be ordered to determine the progression of your disease and the possibility of coexisting conditions that are common in diabetes.
ADA Guidelines for Diagnosing Diabetes
*Diagnosis should be confirmed with an FPG or OGTT on a subsequent day.
**Two-hour postload readings
If you have, or your doctor suspects you have, type 1 diabetes, a c-peptide test may be ordered. C-peptide is one component of the molecule proinsulin (the other component is insulin itself). The levels of c-peptide in your bloodstream can help establish how much insulin your pancreas is still able to produce.
C-peptide can also diagnose hyperinsulinemia (high levels of circulating insulin) in type 2 diabetes. The normal range for c-peptide levels varies, as there are several different laboratory methods of performing the test; ask your physician for assistance in interpreting your specific results.
Conditions such as pregnancy or renal failure can affect c-peptide levels, as can certain medications and alcohol intake. Also, the test should not be performed immediately following a glucose tolerance test (GTT), as a GTT can elevate c-peptide levels.
If you are a new patient, your physician will take a detailed medical history at your initial visit. A thorough, head-to-toes physical examination to check for the presence of possible complications of diabetes will also be undertaken. This will include an evaluation of your cardiac (heart) function, blood pressure, and a neurological examination of your reflexes and ability to recognize certain types of stimulation.
A monofilament test, which involves touching the bottom of your foot with a piece of fiber resembling a thick strand of fishing line, is an easy and inexpensive way to establish if you have lost sensation in your feet due to nerve damage — a condition called peripheral neuropathy.
Your feet will also be examined carefully for infection, ulceration, and circulatory problems. A weak pedal pulse (pulse taken on the foot) may be an early sign of peripheral vascular disease, or PVD. PVD is a condition where the blood vessels in the extremities become narrowed or blocked and, as a result, blood flow is reduced to the surrounding tissues — usually the feet. For more information on monofilament testing, PVD, neuropathy, and other complications of diabetes, see Chapter 15.
Other tests that may be performed at or shortly following your initial diagnosis include the following:
A urine test for microalbumin, a protein that can indicate problems with kidney function
An A1C blood test, to see what your three-month blood glucose average is
A fasting blood lipid profile, to check cholesterol and triglyceride levels
A TSH (thyroid-stimulating hormone) test, to check for thyroid dysfunction, a common risk for people with type 1 diabetes
Electrocardiogram (ECG or EKG), to assess cardiac function
Additional tests and procedures may also be indicated, depending on your specific medical history. Chapter 6 has extensive information on laboratory tests for diabetes and diabetes-related complications.