By Richard S. Beaser, M.D., and Richard Jackson, MD
If an oral glucose tolerance test (OGTT) is done and a diagnosis of diabetes is confirmed, do you need to perform another test? Do you know the procedures to perform an OGTT?….
The first challenge in treating diabetes is making the diagnosis. This may sound like an obvious statement until you realize that about forty percent of all people who actually have diabetes have not yet been diagnosed. Most of these, of course, have type 2 diabetes, which can develop insidiously and have such subtle onset of symptoms that they are hardly noticed or are blamed on something else. Yet, by the time of diagnosis, the disease has had a head start in causing the chronic complications of diabetes.
Follow-up studies of the first two large clinical trials showing the positive effects of lower A1C values on reducing complications in both type 1 (DCCT) and type 2 (UKPDS) diabetes show strikingly similar results. The benefits of attaining good glycemic control early in the natural history of these conditions last for years, and if early glycemic control wasn’t optimal, it may take decades before a person can ‘catch up’ in lowering his or her risk for developing diabetes complications. These findings, discussed in more detail later, serve as a driving force behind the need to screen at-risk people to identify undiagnosed diabetes. Because of a number of practical difficulties, including the populations targeted, and follow-up, screening programs in the community have been unsuccessful. At this time, screening is best done in the healthcare setting, at a provider’s office, in a clinic, or at a hospital. As there is a need to make early diagnoses, it is important to screen people at-risk for undiagnosed diabetes everyone to three years. And the number of your patients who are at risk is high:
- Anyone over 45, or
- Anyone overweight (BMI >25), and has one of the following characteristics:
- is non-Causasian
- has pre-diabetes (impaired fasting glucose, or impaired glucose tolerance, see definitions in table 1)
- has had gestational diabetes or an infant over 9 pounds
- has other components of the metabolic syndrome
- Blood pressure _ 140/90
- HDL cholesterol < 35
- Triglycerides > 250
- has conditions associated with insulin resistance
- acanthosis nigricans
- polycystic ovary disease
The current diagnostic criteria for diabetes are summarized in Table 2-1. The current recommended method for diagnosing diabetes is to use the venous fasting glucose level. Based on this measurement, diabetes is defined as a glucose level 126 mg/dl, confirmed on repeat determination. A “casual” (new terminology replacing “random”) glucose level >200 mg/dl in the presence of classic symptoms would also make the diagnosis. The traditional oral glucose tolerance test (OGTT) is also still valid; however, as it is cumbersome to perform, it is not needed if a patient meets other criteria. Usually, an OGTT would be indicated if diabetes is strongly suspected and other test results are equivocal. If either the fasting glucose or the 2-hour OGTT result is used for the diagnosis, a confirmatory test (fasting glucose, or OGTT, whichever was used initially) needs to done on another day to secure the diagnosis. If the diagnosis is made on the basis of a high blood glucose and classic symptoms, a confirmatory test is not needed. Guidelines for screening pregnant women for gestational diabetes are different, and are covered in Chapter 20. These guidelines eliminate the need to evaluate women at low risk for gestational diabetes.
Fingerstick glucose measurement should not be used to diagnose diabetes. Also, urine testing should not be used to either diagnose or screen for diabetes. Blood glucose levels could be high enough to represent diabetes, but glycosuria may not be present due to a high renal threshold. Conversely, the presence of glucose in the urine without elevated blood glucose is not diagnostic for diabetes. Renal glycosuria, with the presence of urinary glucose but normal blood glucose levels, can be common in children. If glucose in the urine is detected, blood testing should be done.
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