The number of U.S. teens who have diabetes has grown, and many of them are unaware that they have the condition, a new study found.

Researchers found that one-third of cases of adolescent diabetes are undiagnosed, and that an alarming number had prediabetes, which is a condition characterized by higher than normal blood sugar but not high enough to be considered diabetes.

"These findings are important because diabetes in youth is associated with early onset of risk factors and complications," Andy Menke of the Social & Scientific Systems in Silver Spring, Maryland and lead researcher said in a report by HealthDay.

In the study, which was published in the journal JAMA, the researchers analyzed data from a national health survey conducted by the Centers for Disease Control and Prevention (CDC). The survey contained information about the 2,600 adolescent participants aged 12 to 19 who were tested for diabetes between 2005 and 2014.

The participants underwent three blood sugar tests, and were considered to have diabetes if at least one of the three tests showed positive.

The adolescent participants were also asked if they had been diagnosed with diabetes before.

The results of the study indicated that about 0.8 percent of the teens had diabetes, and about 29 percent did not know that they had the condition. Hispanic and black teens were also found to be more likely to have diabetes than white teens, and around 40 to 50 percent are not aware that they have the condition.

Moreover, researchers found that about 18 percent of the teenaged participants had prediabetes, which increases a person's risk of eventually developing diabetes.

However, investigators of the study said that subjects were only tested once, and a confirmed diagnosis of diabetes requires a repeat of the blood glucose test. This indicates that the researchers might have overestimated the results, Live Science reports.

Moreover, the researchers said that the study could not determine whether the participants had type 1 or type 2 diabetes. Previous studies showed that about 87 percent of diabetes cases among teens are type 1.