Screening for Type 2 Diabetes Mellitus in Adults: Recommendations and Rationale

Summary of the Recommendations

The U.S. Preventive Services Task Force (USPSTF) concludes that the evidence is insufficient to recommend for or against routinely screening asymptomatic adults for type 2 diabetes, impaired glucose tolerance, or impaired fasting glucose. This is a grade I recommendation. (See Appendix Table 1 for a description of the USPSTF classification of recommendations.)

The USPSTF found good evidence that available screening tests can accurately detect type 2 diabetes during an early, asymptomatic phase. (See Appendix Table 2 for a description of the USPSTF classification of levels of evidence.) The USPSTF also found good evidence that intensive glycemic control in patients with clinically detected (not screening detected) diabetes can reduce the progression of microvascular disease. However, the benefits of tight glycemic control on microvascular clinical outcomes take years to become apparent. It has not been demonstrated that beginning diabetes control early as a result of screening provides an incremental benefit compared with initiating treatment after clinical diagnosis. Existing studies have not shown that tight glycemic control significantly reduces macrovascular complications, including myocardial infarction and stroke. The USPSTF found poor evidence to assess possible harms of screening. As a result, the USPSTF could not determine the balance of benefits and harms of routine screening for type 2 diabetes.

The USPSTF recommends screening for type 2 diabetes in adults with hypertension or hyperlipidemia. This is a grade B recommendation.

The USPSTF found good evidence that, in adults who have hypertension and clinically detected diabetes, lowering blood pressure below conventional target blood pressure values reduces the incidence of cardiovascular events and cardiovascular mortality; this evidence is considered fair when extrapolated to cases of diabetes detected by screening. Among patients with hyperlipidemia, there is good evidence that detecting diabetes substantially improves estimates of individual risk for coronary heart disease, which is an integral part of decisions about lipid-lowering therapy.

Clinical Considerations

In the absence of evidence of direct benefits of routine screening for type 2 diabetes, the decision to screen individual patients is a matter of clinical judgment. Patients at increased risk for cardiovascular disease may benefit most from screening for type 2 diabetes, since management of cardiovascular risk factors leads to reductions in major cardiovascular events. Clinicians should assist patients in making that choice. In addition, clinicians should be alert to symptoms suggestive of diabetes (for example, polydipsia and polyuria) and test anyone with these symptoms.

Screening for diabetes in patients with hypertension or hyperlipidemia should be part of an integrated approach to reduce cardiovascular risk. Lower targets for blood pressure (that is, diastolic blood pressure 80 mm Hg) are beneficial for patients with diabetes and high blood pressure. The third report of the Adult Treatment Panel III of the National Cholesterol Education Program recommends lower targets for low-density lipoprotein cholesterol for patients with diabetes. Attention to other risk factors, such as physical inactivity, diet, and overweight, is also important, both to decrease risk for heart disease and to improve glucose control.

Three tests have been used to screen for diabetes: fasting plasma glucose (FPG), 2-hour postload plasma glucose, and hemoglobin A_1c. The American Diabetes Association (ADA) has recommended the FPG test for screening (values 6.99 mmol/L [ 126 mg/dL] indicate diabetes) because it is easier and faster to perform, more convenient and acceptable to patients, and less expensive than other screening tests. The FPG test is more reproducible than the 2-hour plasma glucose test, has less intraindividual variation, and has similar predictive value for development of microvascular complications of diabetes. Compared with the FPG test, the 2-hour plasma glucose test may lead to more individuals being diagnosed as diabetic. Hemoglobin A_1c is more closely related to FPG than to 2-hour plasma glucose, but at the usual cut-points it is less sensitive in detecting lower levels of hyperglycemia. The random capillary blood glucose test has been shown to have reasonable sensitivity (75% at a cut-point of 6.66 mmol/L [ 120 mg/dL]) in detecting persons who have an FPG level greater than or equal to 6.99 mmol/L ( 126 mg/dL) or a 2-hour plasma glucose level greater than or equal to 11.1 mmol/L ( 200 mg/dL), if results are interpreted according to age and time since last meal; however, the random blood glucose test is less well standardized for screening for diabetes.

The ADA recommends confirmation of a diagnosis of diabetes with a repeated FPG test on a separate day, especially for patients with borderline FPG results and patients with normal FPG levels for whom suspicion of diabetes is high. The optimal screening interval is not known. The ADA, on the basis of expert opinion, recommends an interval of every 3 years but shorter intervals in high-risk persons.

Regardless of whether the clinician and patient decide to screen for diabetes, patients should be encouraged to exercise, eat a healthy diet, and maintain a healthy weight, choices that may prevent or forestall the development of type 2 diabetes. More aggressive interventions to establish and maintain these behaviors should be considered for patients at increased risk for diabetes, such as those who are overweight, have a family history of diabetes, or have a racial or ethnic background associated with an increased risk (for example, Native American persons). Intensive programs of lifestyle modification (diet, exercise, and behavior) should also be considered for patients who have impaired fasting glucose or impaired glucose tolerance, since several large trials have demonstrated that these programs can significantly reduce the incidence of diabetes in these patients. Evidence and recommendations regarding counseling about diet, physical activity, and obesity are provided in the USPSTF evidence summaries Counseling to Promote a Healthy Diet, Counseling to Promote Physical Activity, and Screening and Treatment for Obesity in Adults, available on the Agency for Healthcare Research and Quality Web site at www.preventiveservices.ahrq.gov.

The brief review of the evidence that is normally included in USPSTF recommendation statements is available in the complete recommendation and rationale statement on the USPSTF Web site (www.preventiveservices.ahrq.gov).

Recommendations of Others

The ADA acknowledged that data from prospective studies were insufficient to determine the benefits of diabetes screening and thus concluded that the decision to test for diabetes should be based on clinical judgment and patient preference (1). On the basis of expert consensus, the ADA recommends clinicians consider screening for diabetes with the FPG test beginning at age 45 years and at a younger age for individuals with such risk factors as family history, overweight, and hypertension, among others. The American College of Obstetricians and Gynecologists endorses the ADA recommendations (2). The American Heart Association recommends measuring fasting blood glucose in persons 20 years of age and older according to risk for diabetes, as part of overall risk assessment for cardiovascular disease (3). The Canadian Task Force on Preventive Health Care is currently updating its recommendations on diabetes screening.

Appendix

Members of the U.S. Preventive Services Task Force are Alfred O. Berg, MD, MPH, Chair (University of Washington, Seattle, Washington); Janet D. Allan, PhD, RN, Vice-Chair (University of Maryland Baltimore, Baltimore, Maryland); Paul Frame, MD (Tri-County Family Medicine, Cohocton, and University of Rochester, Rochester, New York); Charles J. Homer, MD, MPH (National Initiative for Children's Healthcare Quality, Boston, Massachusetts); Mark S. Johnson, MD, MPH (University of Medicine and Dentistry of New JerseyNew Jersey Medical School, Newark, New Jersey); Jonathan D. Klein, MD, MPH (University of Rochester School of Medicine, Rochester, New York); Tracy A. Lieu, MD, MPH (Harvard Pilgrim Health Care and Harvard Medical School, Boston, Massachusetts); C. Tracy Orleans, PhD (The Robert Wood Johnson Foundation, Princeton, New Jersey); Jeffrey F. Peipert, MD, MPH (Women and Infants' Hospital, Providence, Rhode Island); Nola J. Pender, PhD, RN (University of Michigan, Ann Arbor, Michigan); Albert L. Siu, MD, MSPH (Mount Sinai School of Medicine, New York, New York); Steven M. Teutsch, MD, MPH (Merck & Co., Inc., West Point, Pennsylvania); Carolyn Westhoff, MD, MSc (Columbia University, New York, New York); and Steven H. Woolf, MD, MPH (Virginia Commonwealth University, Fairfax, Virginia).

Members of the Task Force at the time these recommendations were finalized.

Appendix Table 1.U.S. Preventive Services Task Force Grades and Recommendations

Appendix Table 2. U.S. Preventive Services Task Force Grades for Strength of Overall Evidence