Context: Type 2 diabetes mellitus (DM-2) is an important cause of morbidity and mortality for individuals and the US population. Many people have DM-2 but have not been diagnosed. Whether screening to detect and treat DM-2 would do more good than harm is not clear.
Objective: To examine the evidence of the benefits and harms of screening and earlier treatment in reducing the complications of this disease to assist the US Preventive Services Task Force.
Data Sources: We identified English language articles on the following: yield of screening, the risk of complications, the effectiveness of treatments to reduce complications for those with clinically detected DM-2, the harms of screening and earlier treatment, the effectiveness of treatments aimed at those with impaired fasting glucose or impaired glucose tolerance (IFG/IGT), the effects of treatment on quality of life, and the costs and cost-effectiveness of screening. To identify these articles, we searched the MEDLINE database from 1966 through November 7, 2001; searched the Cochrane database of systematic reviews through 2001; examined reference lists of textbooks, monographs, and review articles; and asked experts in the field.
Study Selection: To determine the yield of screening, we examined studies of the results of population-based screening. We included studies of population screening that compared one test against another, examined the ability of a test to detect pathologic evidence of diabetes, or examined the reliability of screening tests. To determine the risks of complications, we included longitudinal studies of recently diagnosed people with DM-2 of at least 1 year's duration. For the effects of treatment on numerous intermediate and four health outcomes, we examined randomized controlled trials (RCTs) of treatments for various diabetic complications. To determine the harms, costs, and cost-effectiveness of screening, we examined all study designs concerning these outcomes. We also examined all study designs of population-based groups for the effects of lifestyle interventions or medications in reducing the incidence of DM-2 among those with IFG/IGT.
Data Extraction: We abstracted the following data from included articles that dealt most directly with our key questions: demographic details about study subjects, how study subjects were selected, inclusion and exclusion criteria, drop-out and loss-to-follow-up rates, study design and duration, how randomization was accomplished, interventions and co-interventions, measurement methods, and outcome results. We evaluated the internal validity, external validity, and coherence of results of each individual study and assessed all the evidence concerning each key question.
Data Synthesis: No large RCT of screening has been performed. Thus, the evidence for the benefits of screening is indirect. A detectable preclinical period exists, but its length is uncertain. Screening tests with adequate accuracy, reliability, and acceptability are available. The health outcomes of blindness, chronic renal failure, and lower extremity amputation occur infrequently until 20 years or longer of diabetes duration. Trials of treatment after clinical diagnosis have found it difficult to demonstrate a statistically significant health benefit. How much these outcomes would be reduced by the additional few years of treatment produced by screening is uncertain. Visual impairment less severe than blindness and cardiovascular (CVD) events are more common complications in the decade after diabetes diagnosis. Tight control of blood pressure is effective in reducing these complications among those already clinically diagnosed with DM-2 and hypertension. Little information is available about harms of screening, although several harms are potentially serious problems. The costs of diagnosis, treatment, and dealing with the complications of DM-2 are high. One study examined the cost-effectiveness of screening for DM-2 but assumed that the only effective treatment was glycemic control.
Conclusion: The evidence for screening for DM-2 is indirect and mixed. The strongest case for screening comes from earlier detection and treatment of CVD risk factors, especially hypertension. An RCT of screening is needed to answer the many remaining questions.
Keywords: diabetes mellitus, impaired fasting glucose, impaired glucose tolerance, visual impairment, retinopathy, blindness, chronic renal failure, lower extremity amputation, cardiovascular disease, hypertension, dyslipidemia, glycemic control, laser photocoagulation