Screening for Type 2 Diabetes in Adults: Recommendations and Rationale

U.S. PREVENTIVE SERVICES TASK FORCE

This statement summarizes the current U.S. Preventive Services Task Force (USPSTF) recommendations on screening for type 2 diabetes in adults and the supporting scientific evidence. They update the 1996 recommendations contained in the Guide to Clinical Preventive Services, second edition.¹ Explanations of the ratings and of the strength of overall evidence are given in Tables 1 and 2, respectively. The complete information on which this statement is based, including evidence tables and references, is available in the summary of the evidence² and the systematic evidence review³ on this topic, which is available on the USPSTF Web site (www.preventiveservices.ahrq.gov) and through the National Guideline Clearinghouse (www.guideline.gov). The summary of the evidence and the recommendation statement are also available in print through the Agency for Healthcare Research and Quality Publications Clearinghouse (800-358-9295; e-mail: ahrqpubs@ahrq.gov).

Summary of Recommendations

• The 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. I recommendation.

The USPSTF found good evidence that available screening tests can accurately detect type 2 diabetes during an early, asymptomatic phase. 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. 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.

TABLE 1 USPSTF Recommendations and Ratings The USPSTF grades its recommendations according to one of five classifications (A, B, C, D, or I) reflecting the strength of evidence and magnitude of net benefit (benefits minus harms). A. The USPSTF strongly recommends that clinicians routinely provide [the service] to eligible patients. The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms. B. The USPSTF recommends that clinicians routinely provide [the service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms. C. The USPSTF makes no recommendation for or against routine provision of [the service]. The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation. D. The USPSTF recommends against routinely providing [the service] to asymptomatic patients. The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits. I. The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service]. Evidence that [the service] is effective is lacking, of poor quality, or conflicting, and the balance of benefits and harms cannot be determined. USPSTF = U.S. Preventive Services Task Force.

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 (i.e., polydipsia, 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 (i.e., diastolic blood pressure ¾80 mm Hg) are beneficial for patients with diabetes and high blood pressure. The 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 to decrease risk for heart disease and to improve glucose control.

• Three tests have been used to screen for diabetes: fasting plasma glucose (FPG), two-hour post-load plasma glucose (2-hr PG), and glycosylated hemoglobin A_1c (HbA_1c). The American Diabetes Association (ADA) has recommended the FPG test (>=126 mg per dL [7 mmol per L]) for screening 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-hr PG test, has less intraindividual variation, and has similar predictive value for development of microvascular complications of diabetes. Compared with the FPG test, the 2-hr PG test may lead to more individuals being diagnosed as diabetic. HbA_1c is more closely related to FPG than to 2-hr PG, but at the usual cut-points it is less sensitive in detecting lower levels of hyperglycemia. The random capillary blood glucose (CBG) test has been shown to have reasonable sensitivity (75 percent at a cut-point of >=120 mg per dL) in detecting persons who have either an FPG level >=126 mg per dL or a 2-hr PG level >=200 mg per 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 three years but shorter intervals in high-risk persons.

TABLE 2 USPSTF Strength of Overall Evidence The USPSTF grades the quality of the overall evidence for a service on a three-point scale (good, fair, or poor). Good: Evidence includes consistent results from well-designed, well-conducted studies in representative populations that directly assess effects on health outcomes. Fair: Evidence is sufficient to determine effects on health outcomes, but the strength of the evidence is limited by the number, quality, or consistency of the individual studies; generalizability to routine practice; or indirect nature of the evidence on health outcomes. Poor: Evidence is insufficient to assess the effects on health outcomes because of limited number or power of studies, important flaws in their design or conduct, gaps in the chain of evidence, or lack of information on important health outcomes. USPSTF = U.S. Preventive Services Task Force.

• 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 developing 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 (e.g., American Indians). Intensive programs of lifestyle modification (i.e., diet, exercise, behavior) also should 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.

Scientific Evidence

The brief review of the evidence that is normally included in the USPSTF recommendation statements is available in the complete Recommendation and Rationale statement at 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.⁴ On the basis of expert consensus, the ADA recommends that clinicians consider screening for diabetes with the FPG test beginning at age 45 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.⁵ The American Heart Association recommends measuring fasting blood glucose in persons 20 years of age and older according to patients' risk for diabetes, as part of overall risk assessment for cardiovascular disease.⁶ The Canadian Task Force on Preventive Health Care is currently updating its recommendations on diabetes screening.

Address correspondence to Alfred O. Berg, M.D., M.P.H., Chairman, U.S. Preventive Services Task Force, Center for Practice and Technology Assessment, Agency for Healthcare Research and Quality, 6010 Executive Blvd., Suite 300, Rockville, MD 20852 (telephone: 301-594-4016; fax: 301-594-4027; e-mail: uspstf@ahrq.gov).

The U.S. Preventive Services Task Force recommendations are independent of the U.S. government. They do not represent the views of the Agency for Healthcare Research and Quality, the U.S. Department of Health and Human Services, or the U.S. Public Health Service.

REFERENCES

1. U.S. Preventive Services Task Force. Guide to clinical preventive services. 2d ed. Washington, D.C.: U.S. Department of Health and Human Services, 1996.
2. Harris R, Donahue K, Rathore SS, Frame P, Woolf SH, Lohr KN. Screening adults for type 2 diabetes: a review of the evidence for the U.S. Preventive Services Task Force. Ann Intern Med 2003;138: 215-29.
3. Harris RP, Lux LJ, Bunton AJ, Sutton SF, Lohr KN, Donahue KP, et al. Screening for type 2 diabetes mellitus. Systematic Evidence Review No. 19 (prepared by the Research Triangle Institute-University of North Carolina Evidence-based Practice Center under Contract No. 290-97-0017, Rockville, Md: Agency for Healthcare Research and Quality, 2002.
4. Screening for diabetes. Diabetes Care 2002;25 (suppl 1):S21-4.
5. American College of Obstetricians and Gynecologists. Guidelines for women's health care. 2d ed. Washington, D.C.: American College of Obstetricians and Gynecologists, 2002.
6. Pearson TA, Blair SN, Daniels SR, Eckel RH, Fair JM, Fortmann SP, et al. AHA guidelines for primary prevention of cardiovascular disease and stroke: 2002 update: consensus panel guide to comprehensive risk reduction for adult patients without coronary or other atherosclerotic vascular diseases. American Heart Association Science Advisory and Coordinating Committee. Circulation 2002;106: 388-91.