Five Long Island hospitals scored an A and one a D in patient safety in a twice-yearly ranking by a national health quality group.

The Leapfrog Group, a nonprofit founded by large companies to improve health care quality and safety, Wednesday issued its spring safety report on more than 2,500 hospitals.

Using 2012-14 data on 28 measures from nonprofit and government surveys, the group graded hospitals from A to F on how well they prevented errors, injuries, infections and drug mix-ups.

For the seventh time in a row, both John T. Mather Memorial Hospital in Port Jefferson and St. Francis Hospital in Flower Hill, part of Catholic Health Services, scored A's. For the third time in a row, Nassau University Medical Center in East Meadow and St. Charles Hospital in Port Jefferson scored A's. And Mercy Medical Center in Rockville Centre, also part of CHS, scored an A.

NUMC chief executive Dr. Victor Politi said in a statement that the score reflects "our staff's dedication and commitment to overall safety."

Peconic Bay Medical Center in Riverhead scored a D.

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In a statement, Peconic Bay said "we have already identified the areas for improvement, have initiated changes and we would expect our efforts to reflect in an improved score in the future."

The hospital scored below average in correct use of antibiotics before and after surgery, removal of a catheter soon after surgery, and steps taken to prevent blood clots.

Peconic Bay said it scored in the top 20 percent for patient safety among state hospitals in a 2014 Consumer Reports ranking. "There are many consumer rating agencies and no grading system is comprehensive, so consumers should consult multiple sources," the hospital said.

Brookhaven Memorial Hospital Medical Center in East Patchogue, which in the last survey got a D, and before that C's, was not included because of insufficient data, according to a Leapfrog spokeswoman.

St. Joseph Hospital in Bethpage, which last fall earned a D, this time scored a B.

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Overall, New York ranked 38th nationally in patient safety. In the last survey, in November, New York ranked 25th.

Leapfrog uses national performance measures from the Leapfrog Hospital Survey, the Agency for Healthcare Research and Quality, the Centers for Disease Control and Prevention, the Centers for Medicare & Medicaid Services and the American Hospital Association's Annual Survey and Health Information Technology Supplement.

For more information, go to hospitalsafetyscore.org.