EVIDENCE-BASED ANSWER
No randomized clinical trials or prospective studies have demonstrated adequate evidence to screen individuals for diabetes mellitus. A recently published meta-analysis for the United States Preventative Services Task Force (USPSTF) stated that “until we have better evidence about its benefits, harms, and costs, the role of screening as a strategy to reduce the burden of suffering of diabetes will remain uncertain” (strength of recommendation [SOR]: B, based on inconclusive studies).
The group of patients most likely to benefit from diabetes screening are patients with hypertension (SOR: B), or those whose risk for coronary heart disease is such that a diagnosis of diabetes would mandate addition of aspirin or lipid-lowering agents (SOR: C).
Evidence summary
It is estimated that by the year 2010 approximately 216 million individuals worldwide will be affected with diabetes; 90% of these people will have type 2.1 In addition, it is well documented that diabetes significantly increases the risk of morbidity and mortality, especially due to retinopathy, nephropathy, neuropathy, and coronary artery disease.2
For screening to be effective, the disease of interest must have an easily detectable asymptomatic state, and a treatment that improves outcomes by intervening before symptoms develop. Diabetes does have an asymptomatic state, which is of uncertain duration (likely years), and is detectable with simple, inexpensive tests: specifically, either a fasting blood glucose or a 2-hour post-glucose-load blood glucose. In order to be useful, a screening program must also lead to an intervention that reduces morbidity or mortality. The data are much less clear whether any interventions during the presymptomatic period improve patient outcomes.
No randomized trials have tested whether screening provides any benefits.3 In a thorough systematic review using USPSTF methodologies, several potential postscreening interventions were evaluated.3 While tight glycemic control reduces progression of albuminuria and retinopathy, it is unclear how large the long-term clinical benefit would be, or at what cost. Reasonable evidence supports more aggressive control of blood pressure for patients with diabetes to reduce adverse cardiovascular outcomes. It is important to note that the data for these interventions (aggressive blood sugar and blood pressure control) were derived in studies of patients with established diabetes; no studies have tested these interventions for patients who had early diagnosis by screening.
Since undiagnosed diabetes doubles the risk of coronary artery disease, there is the potential that intervention with prophylactic aspirin and lipid-lowering agents could reduce coronary artery disease, although this has not been tested. There is no evidence that the diagnosis of diabetes per se alters individual patients’ behavior in response to lifestyle counseling, particularly about smoking cessation, diet, and exercise. It is unlikely that screening for foot ulcers would provide any benefit in those with an early diagnosis of diabetes.
There is reasonable evidence that aggressive counseling and behavioral interventions can postpone the diagnosis of diabetes for patients with glucose intolerance. The studies were too small and short to detect any meaningful difference in morbidity or mortality. In addition, it is unknown if this postponing of the onset of diabetes is cost-effective.
The risks of screening include false-positive diagnosis, labeling effect, and subjecting patients to potentially harmful medications. There is little data to estimate the size of these effects.
Using a best-case scenario, the number needed to screen (NNS) is 500 to prevent cardiovascular outcomes by aggressive hypertension therapy. This assumes a baseline rate of 6% undetected diabetes, with a 5-year lead-time benefit to screening, and 50% increase in the rate of aggressive hypertension control. Assuming the baseline rate is 3% and the lead time is 2.5 years, the NNS is 3600.
The NNS for preventing monocular blindness is higher, even using best-case assumptions. The calculations for blindness rely on greater extrapolations of the data; the other potential interventions described above had inadequate data even to make such calculations.
Recommendations from others
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. The USPSTF recommends screening for type 2 diabetes in adults with hypertension or hyperlipidemia. They report that it is likely that more aggressive treatment of hypertension, hyperlipidemia, and other cardiovascular risk factors could reduce cardiovascular morbidity and mortality.4