BAKERSFIELD, Calif. -- Twenty-four million Americans have diabetes, and a quarter of them don't even know it.

It's vital for at-risk patients to be screened, because if left untreated, blood sugar problems cause heart disease, nerve damage and premature death.

But new research published in the New England Journal of Medicine finds that the gold standard screening test for diabetes may not be the best option.

The fasting glucose test -- in which patients refrain from eating for at least eight hours before doctor measures their blood-sugar levels -- is less effective than the A1C test, which can be done at any time and gives a long-term picture of patients' blood-sugar levels.

Doctors studying 11,000 adults found that participants' A1C test results were closely tied to their future odds of heart disease and diabetes.

Those with the highest A1C readings had nearly double the risk for heart problems and 16 times the risk for diabetes -- but the fasting glucose test was unrelated to heart and diabetes risk. Researchers said this is convincing evidence that the A1C test is superior for diagnosing diabetes.