Diabetes Mellitus Clinical Presentation, Laboratory findings, and Diagnosis

The onset of type 1 diabetes is usually abrupt whereas type 2 diabetes is often present for years without overt signs or symptoms. Patients with undiagnosed diabetes may present with one or more signs and symptoms. The diagnosis of diabetes is based on the presence of clinical signs and symptoms, along with specific laboratory findings. The most recent diagnostic guidelines were established by the American Diabetes Association in 1997. These guidelines provide for the use of fasting glucose and casual (nonfasting) glucose levels for diagnosis and restrict routine use of the oral glucose tolerance test. The diagnosis of diabetes is not made until the patient has exceeded threshold glucose levels on two separate occasions. Urinary glucose analysis is no longer used in establishing the diagnosis of diabetes.

Both the fasting and casual plasma glucose tests provide a determination of glucose levels at a single moment in time, namely, at the time the blood sample is collected. It is often useful to assess the long-term control of glycemia, especially in known diabetic patients.

The glycated (or glycosylated) hemoglobin assay (also called the glycohemoglobin test) allows the determination of blood glucose status over the 30 to 90 days prior to collection of the blood sample. As glucose circulates in the bloodstream, it becomes attached to a portion of the hemoglobin molecule on red blood cells. The higher the plasma glucose levels are over time, the greater is the percentage of hemoglobin that becomes glycated.

There are two different glycated hemoglobin assays: the hemoglobin A₁ (HbA₁) test and the hemoglobin A_1c (HbA_1c) test. Because these tests measure two different portions of the hemoglobin molecule, the normal ranges for the test results differ. The normal HbA1 value is less than approximately 8% whereas the normal HbA_1c is less than 6.0 to 6.5%. These tests are not currently standardized across all laboratories; therefore, glycated hemoglobin values must be interpreted in the context of normal ranges for the specific laboratory performing the test. The American Diabetes Association recommends that individuals with diabetes attempt to achieve a target HbA_1c value of less than 7% whereas an HbA_1c value of more than 8% suggests that a change in patient management may be needed to improve glycemic control. The glycated hemoglobin assay is not currently recommended as a screening tool or as an initial test for the diagnosis of diabetes. It is used to monitor glycemic control in patients with previously diagnosed diabetes.

{embed="db/static-menu-oral"} Another assay that can be used to determine long-term glucose control is the fructosamine test. This test is not used as widely as the glycated hemoglobin assay but is often helpful in managing women with gestational diabetes. The fructosamine assay assesses glycemic control over the 2 to 4 weeks preceding the test. The normal range for fructosamine is 2.0 to 2.8 mmol/L. This test may become more widely used in the future, since at-home testing is now available.