Nurses at Tufts Medical Center moved closer to a strike yesterday, notifying administrators they will hit the picket lines May 6 if they don't reach an agreement on their patient ratio demands. Tufts Medical Center boss Tufts Medical Centersaid she has replacements lined up. "We have no concerns about being able to properly staff the medical center," said Zane, who said she has arranged for about 200 out-of-state registered nurses to care for patients. "We would prefer our own nurses, but these nurses come highly recommended." The Massachusetts Nurses Association, representing 1,100 Tufts nurses, handed its formal 10-day strike notice to Tufts management at the end of yesterday's negotiating session. The union said the one-day strike will begin at 6 a.m. on May 6 and end the next day at 7:00 a.m. The notice doesn't guarantee a strike, and talks will continue until May 6. But last night, neither side showed signs of backing down.