Without a way to screen potential donors, blood banks in areas experiencing outbreaks would have been forced to shut down local collections of whole blood and red blood cells. Many had already begun planning to import as much as they could from states farther north that were free of the Zika virus.

“It is logistically difficult, but it can be done,” said Dr. Jeffrey McCullough, professor of laboratory medicine and pathology at the University of Minnesota Medical School. But unexpected surges in demand in these areas would have been tough to accommodate.

Blood Systems, which has banks in 24 states, had hoped to import blood from Rocky Mountain states, or those in the Northern Plains, if donations had to be suspended in Texas or Louisiana because of local Zika transmission, said Dr. Ralph R. Vassallo, chief medical and scientific officer.

OneBlood, which collects more than one million blood products annually, including in Florida, had planned to shift collections from affected areas to ones that were not to maintain the safety of the blood supply with no interruption of their services, said Dr. Rita Reik, the chief medical officer.

The Centers for Disease Control and Prevention, which tracks the yellow-fever mosquito, works with the F.D.A. to determine which areas have active transmission of the Zika virus.

“If public health officials can pinpoint where local transmission is occurring and it’s in a tiny area, we’re likely to temporarily suspend collections there,” Dr. Vassallo said. “If a larger area is affected, we’ll be able to continue collections by using this test.”