Testing for Diabetes

Diabetes mellitus has a vast array of ocular complications, and you may note these in any aspect of the examination. Refractive changes, ocular motility and pupillary dysfunction, retinal hemorrhages and infarcts may all signal a problem with glucose metabolism. Specific ocular signs that warrant testing for diabetes are outlined in the table below. It's critically important that you understand and know how to order the appropriate tests.

You may perform testing for diabetes in many ways. Urinalysis is an inexpensive and rapid method of testing grossly for diabetes. Laboratories examine the urine specimen primarily for two components: glucose and ketones. The presence of glucose in the urine indicates insulin dysfunction--the body is not adequately converting the blood glucose into energy or storage materials, and hence dumps it out via the renal system.

What diabetes mellitus does to the eyes: at left, proliferative retinopathy in advanced diabetic retinopathy; and, right, diabetic macular edema. (National Eye Institute, National Institutes of Health)

Ketones represent end products of fatty-acid metabolism. In most patients, ketones are formed in the liver and are completely metabolized so that only negligible amounts are present in the urine. When the body is unable to use carbohydrates as an energy source, however, fat becomes the predominant body fuel, and ketones are then formed. The presence of ketones in the urine represents a significant problem and advanced disease.

Ocular Signs That May Warrant Testing for Diabetes • Fluctuating vision • Changes in refractive error >1.00D in an adult • Recurrent corneal erosion • Unresponsive or poorly responsive pupils • Premature cataracts (especially posterior subcapsular) • Sudden onset cranial neuropathies (III, VI or IV) • Neovascularization of the iris • Unexplained retinal or vitreous hemorrhages • Unexplained macular edema or retinal exudates • Unexplained retinal infarcts (cotton-wool spots) • Retinal venous occlusions • Neovascularization of the retina or optic nerve head • Unilateral disc edema with hemorrhage (NAAION)

Most often, blood testing is the definitive method of diagnosing diabetes and the preferred way to monitor therapy. The most common screening method is the fasting plasma glucose (FPG) test; many physicians still refer to this by its older, more antiquated term, fasting blood sugar (FBS). This test measures the level of glucose in the blood at a minimum of eight hours after food intake. Normal values for the FPG are 110mg/dL or below. Those with values between 110-126 are said to have impaired fasting glucose, and should be suspect for diabetes. Those with an FPG greater than 126 on two or more separate occasions may be diagnosed with diabetes mellitus.

Physicians may also measure the serum glucose levels at other times when patients have not fasted. This test is called the casual plasma glucose or random plasma glucose (RPG). RPG values of 200mg/dL or greater measured on two or more separate occasions when symptoms of diabetes are present indicate a diagnosis of the disease.

The oral glucose tolerance test (OGTT) was the quintessential diagnostic test given to diabetes suspects. This elaborate test begins with the patient fasting for eight hours, and then drinking a syrupy beverage with a glucose load equivalent to 75g. Two hours after ingesting the liquid, serum glucose is measured. Normal values for the OGTT are 140mg/dL or less. Findings between 140 and 200mg/dL indicate impaired glucose tolerance, and those greater than 200 are consistent with diabetes.

The newest test in the diabetic arsenal is the glycosylated hemoglobin (HbA1c) test. Glycosylated hemoglobin refers to the end product of glucose binding with hemoglobin within red blood cells; this is a natural occurrence in all individuals. However, in uncontrolled diabetic patients there are much higher levels of ambient glucose in the blood and hence, higher levels of glycosylated hemoglobin. Unlike urine testing or FPG, which are essentially spot checks, glycosylated hemoglobin yields an assessment of blood sugar control over a 1-3 month period. It's very valuable for monitoring patient compliance and effectiveness of therapy.

At this time, the HbA1c is not useful as a diagnostic test because laboratories have not standardized the test. This has led the American Diabetes Association (ADA) to dissuade practitioners from utilizing the HbA1c for diagnosis. However, there is a movement under way to standardize hemoglobin measurement in the United States.