United Blood Services has issued a plea for donors to step forward, saying Arizona's supply of blood type O-negative has slipped to extremely low levels.

As of Monday, the nonprofit said just 16 units of O-negative blood remained to fill the needs of 58 hospitals. That leaves the state more than 90 percent below necessary levels.

United Blood Services points to increased demand. It said about 13 percent of all red cell transfusions have required O-negative while only 6 percent of the population has this rare blood type.

O-negative is the blood type that trauma patients, pregnant women and babies generally receive when lifesaving transfusions are required.

The nonprofit said it has exhausted out-of-state resources to meet patient needs in Arizona. Donation centers are set up in Phoenix and the surrounding area.