Diabetes affects up to 14 percent of the U.S. population - an increase from nearly 10 percent in the early 1990s - yet over a third of cases still go undiagnosed, according to a new analysis.

Screening seems to be catching more cases, accounting for the general rise over two decades, the study authors say, but mainly whites have benefited; for Hispanic and Asian people in particular, more than half of cases go undetected.

"We need to better educate people on the risk factors for diabetes - including older age, family history and obesity - and improve screening for those at high risk," lead study author Andy Menke, an epidemiologist at Social and Scientific Systems in Silver Spring, Maryland, said by email.

Globally, about one in nine adults has diagnosed diabetes, and the disease will be the seventh leading cause of death by 2030, according to the World Health Organization.

Most of these people have Type 2, or adult-onset, diabetes, which happens when the body can't properly use or make enough of the hormone insulin to convert blood sugar into energy. Left untreated, diabetes can lead to nerve damage, amputations, blindness, heart disease and strokes.

Average blood sugar levels over the course of several months can be estimated by measuring changes to the hemoglobin molecule in red blood cells. The hemoglobin A1c test measures the percentage of hemoglobin - the protein in red blood cells that

carries oxygen - that is coated with sugar, with readings of 6.5 percent or above signaling diabetes.

People with A1c levels between 5.7 percent and 6.4 percent aren't diabetic, but because this is considered elevated it is sometimes called "pre-diabetes" and considered a risk factor for going on to develop full-blown diabetes.

Menke and colleagues estimated the prevalence of diabetes and pre-diabetes using data from the National Health and Nutrition Examination Survey (NHANES) collected on 2,781 adults in 2011 to 2012 and an additional 23,634 adults from 1988 to 2010.

While the prevalence of diabetes increased over time in the overall population, gains were more pronounced among racial and ethnic minorities, the study found.

About 11 percent of white people have diabetes, the researchers calculated, compared with 22 percent of non-Hispanic black participants, 21 percent of Asians and 23 percent of Hispanics.

Among Asians, 51 percent of those with diabetes were unaware of it, and the same was true for 49 percent of Hispanic people with the condition.

An additional 38 percent of adults fell into the pre-diabetes category. Added to the prevalence of diabetes, that means more than half of the U.S. population has diabetes or is at increased risk for it, the authors point out.

The good news, however, is fewer people are undiagnosed than in the past, Dr. William Herman and Dr. Amy Rothberg of the University of Michigan in Ann Arbor note in commentary accompanying the study in JAMA.

In it, they note that the increase in diabetes prevalence between 1988 and 2012 seen in the study was due to an increase in diagnosed cases, and that overall undiagnosed cases fell from 40 percent in 1988-1994 to 31 percent in 2008-2012.

This "likely reflects increased awareness of the problem of undiagnosed diabetes and increased testing," they said by email.

The drop in undiagnosed cases, they added, may be due in part to the newer, simpler A1c test, which doesn't require fasting or any advance preparation.

It's also possible that new cases of diabetes are starting to fall for the first time in decades because more people are getting the message about lifestyle choices that can contribute to diabetes, noted Dr. David Nathan, director of the diabetes center at Massachusetts General Hospital in Boston and a professor at Harvard Medical School.

In particular, more patients now understand that being overweight or obese increases the risk for diabetes, Nathan, author of a separate report in JAMA on advances in diagnosis and treatment, said by email.

"Behavioral changes, including healthy eating and more activity can prevent, or at least ameliorate, the diabetes epidemic," Nathan said.

Prevalence of Undiagnosed Diabetes in US