Six Long Island hospitals scored an A in patient safety in a national ranking by a medical quality organization.

The Leapfrog Group, founded 14 years ago by large employers to improve health care quality and safety, last week issued its fifth hospital safety score of more than 2,600 hospitals nationwide. Reviewing data on 28 measures from national surveys from 2010 to 2013, the nonprofit gave each hospital a score of A to F on how well it prevented errors, infections, injuries and drug mix-ups. The scores, first released in 2012, come out every six months.

For the fifth time, John T. Mather Memorial Hospital in Port Jefferson and St. Francis Hospital in Roslyn, which is part of Catholic Health Services, scored A's, while Winthrop-University Hospital in Mineola scored an A for the second time in a row. Nassau University Medical Center in East Meadow, St. Charles Hospital in Port Jefferson and Mercy Medical Center in Rockville Centre -- both also part of CHS -- scored A's as well.

Of the 137 hospitals ranked in the state, 27 received the top score. Four Long Island hospitals received B's, and 12 received C's.

Mercy's executive vice president, Dr. Aaron Glatt, said Mercy was "proud to be among the six Long Island hospitals that received an A."

Dr. Victor Politi, chief executive of NuHealth, which includes NUMC, called the hospital's top rating "wonderful news."

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The high ranking "requires a tremendous amount of dedicated work from the entire hospital staff and a real commitment to safety," he said.

Mather president Kenneth Roberts said, "Quality and safety . . . are core pillars of the culture at Mather Hospital. These pillars underscore all we do."