NEWARK – Prosecutors in the corruption trial of Sen. Bob Menendez sought to impeach the credibility of a former staffer for the senator, who testified for the defense today and said she was not working on behalf of Menendez’s co-defendant Dr. Salomon Melgen, but rather on fixing confusing Medicare billing policy.

Carissa Willhite, who was Menendez’s deputy chief of staff when she was told to work on the issues surrounding Lucentis, an eye medicine Melgen had multi-dosed and overbilled Medicare for $8.9 million.

“The confusion was based on how you prescribe this drug – it’s in a vial and you prescribe it – and how you get reimbursed for that,” she testified.

Willhite and others in the office were tasked with getting answers, and simply used Melgen’s case as an example of the larger problem.

“Well one, this example showed it wasn’t just this case, that this case was an example of a bigger issue…was there a clear response on how you do this and how you reimburse and how you get paid?” Willhite said.

During cross-examination this afternoon, prosecution attorney Peter Koski tried to show close coordination between Melgen, his lawyers, Menendez’s Senate office and the senator himself on the doctor’s case against Medicare, in contrast to Willhite’s earlier testimony.

Like a chemistry professor taking a class through a complicated but understandable formula, Koski stood at the overhead projector wielding a yellow highlighter, which he wielded on paper copies of emails under the projection lamp and emphasized key words. Koski showed multiple references to Melgen – one email’s subject line was “Melgen case” – and one July 2009 email said the staff spoke to Melgen himself “every few days.”

“He’s already made a decision as of June 12, 2009 that he’s going to use the resources of his office to help Dr. Melgen on this issue,” Koski said of Menendez.

“No,” Willhite said.

“That is not something that we were looking at…the question we were working on was the confusing policy from (the Centers for Medicare and Medicaid,” Willhite said.

In contrast, the defense through Willhite tried to show jurors that the senator’s office was trying to “seek clarity and consistency” with Medicare and the Health Department, rather than being at the beckon call of co-defendant Melgen.

Both the defense and prosecution used many of the same exhibits, emphasizing different portions they hoped would prove one point or another to the jurors.

Willhite testified Melgen’s name was well-known around the Senate office as one of Menendez’s friends. She said Menendez’s friends often contacted the senator for help.

“It was common,” she said. “I mean, we did that fairly regularly.”