CHICAGO (Reuters) - About 6.3 million Americans have diabetes without knowing it, and complications from their undiagnosed disease account for an estimated $18 billion in U.S. healthcare costs each year, U.S. researchers said on Wednesday.

Prior studies have shown that diabetes cost the U.S. economy about $174 billion in 2007 in medical expenses and lost productivity.

But many people often begin having complications from diabetes years before they get a diagnosis, and these costs are often overlooked, according to the study by Yiduo Zhang of the healthcare research company the Lewin Group and published in the journal Population Health Management.

Zhang and colleagues studied the healthcare use patterns of nearly 30,000 people for the 2-year period leading up to a diagnosis of diabetes and compared them with medical claims of people never diagnosed with diabetes.

They estimated that undiagnosed diabetes in 2007 cost an additional $2,864 per person, or a total of $18 billion per year.

Complications often present in people with newly diagnosed diabetes include retina damage, nerve pain and heart disease. They said programs aimed at earlier detection and treatment could help reduce these costs.

(Reporting by Julie Steenhuysen; Editing by Xavier Briand)

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