Blood banks are reporting dangerously low supplies of “universal donor” O-negative blood, a new study warns.

O-negative blood can be given to patients of any blood type, making it essential in medical emergencies where a person’s blood type is unknown, researchers said.

But O-negative supplies have dropped to critically low levels every year since 2019, researchers reported in the journal Anesthesiology Open.

Shortages have led the American Red Cross to offer $15 Amazon gift cards to anyone who donates blood through the end of March, researchers noted.

"O-negative blood is critical when patients need immediate transfusions before their blood type can be confirmed, such as during trauma care or other life-threatening emergencies," lead researcher Dr. Brent Lee said in a news release. Lee is director of clinical excellence and performance improvement with North American Partners in Anesthesia.

The problem is that demand for O-negative blood has grown while supply remains limited, researchers said.

About 13% of transfusions performed in 2023 involved O-negative blood, but only 7% of the entire U.S. population has O-negative blood, researchers reported.

Doctors are contributing to the problem by using O-negative blood in situations where other blood types could be used, the study found.

"Because anesthesiologists administer up to 60% of all blood transfusions in the United States, they play a key role in preserving and promoting responsible management of this scarce resource," Lee said.

The study argues that doctors should instead use O-positive blood, which is more readily available, for emergency transfusions in all male patients and for female patients who aren’t likely to become pregnant.

O-positive blood is called that because the blood cells are “positive” for the Rhesus factor D (RhD) antigen, which can trigger an immune response against the donated blood.

However, research has found that the risk of an immune response to an O-positive blood transfusion is “generally low” — around 3% to 6% of all cases — given that most people are RhD-positive, the study says. 

The exception is women with childbearing potential, as an RhD mismatch could be harmful to babies in future pregnancies, researchers said.

Because of that, O-negative blood should be saved for these women, as well as for people with O-negative blood, the study said.

The report also calls for performing blood typing as quickly as possible in medical emergencies so that patients can be switched off O-negative blood as soon as possible.

"In collaboration with other specialties and national organizations, anesthesiologists play a key role in alleviating chronic blood shortages and preserving the availability of O-negative blood for those who need it most," Lee said. "It is imperative that we maximize the benefits of blood transfusion, while strengthening the security of the nation’s blood supply."