A patient on the operating table starts bleeding profusely. Medical staff use sponge after sponge to soak up the blood, unsure of exactly how much has been lost. Should the attending physician order a transfusion? The patient may still have enough red blood cells for the rest of the surgery, but a lab test to confirm the count can take 15 to 45 minutes.

Confronted with this scenario many physicians opt for a transfusion—to the tune of 1.9 million given in the U.S. during surgery annually. It’s unclear, however, if all these are necessary. Now, a new technology could clear that up and staunch the flow. Researchers have developed a noninvasive device that takes real-time measurements of a patient’s red blood cells (specifically, hemoglobin levels), giving physicians continuous information about when a patient actually needs a transfusion.

In a small clinical trial of elective orthopedic surgery procedures (such as knee and hip replacements), researchers found that standard protocol, which includes lab work and observation, resulted in about 4.5 percent of patients receiving blood transfusions during surgery, whereas for those relying on the new device, only 0.6 percent required extra blood. The device also seemed to reduce the amount of blood used on average. Jesse Ehrenfeld, director of the Center for Evidence Based Anesthesia at Vanderbilt University Medical Center, will present his team’s results Monday afternoon at the American Society of Anesthesiologists annual meeting in San Diego.

“It’s got the potential to be a technology that will be really helpful in the operating room,” Ehrenfeld says. In his hospital alone, he notes, some 33,000 to 38,000 units of blood were given to patients, and chances are good that “some of that blood we probably didn’t need to give.”

The device does not puncture the skin. Rather, a sticker (which, Ehrenfeld says, “looks like a Band–Aid”) is placed on a patient’s finger. A clip-on device employs near-infrared light to measure hemoglobin levels—similar to blood oxygen level-monitoring technology. Blood absorbs light differently based on the concentrations of its relative components, including hemoglobin; the device relies on several different wavelengths to assess the relative quantity of red blood cells circulating in the body, explains Dana Banks, a spokesperson for Masimo Corp., the company that makes the sensor.