On September 4, 2015, 17-year old Kenney Bui's coach removed him from his Seattle high-school team’s football game with a mild concussion. Thirteen days later, a doctor cleared him to play. At a game a few weeks after that, on October 2, Bui took another hit, walked off the field showing signs of confusion, and while athletic trainers asked him questions from the school district's standardized concussion screening protocol, he closed his eyes. He never opened them again.

Bui was one of six high schoolers to die on the football field in 2015 from a traumatic brain injury, but every year millions of people suffer milder concussions while playing sports. Recognizing these more subtle concussions is crucial for preventing deaths like Bui’s, referred to as second impact syndrome. But the tools available to do that are crude and often subjective. Doctors and trainers gauge how badly you’ve been banged up by instructing you to count backward from ten to one or follow a finger with your eyes. But with high-powered protein assays and microfluidic technologies, soon there will be a better way. And it will all start with the prick of a finger.

When your head takes a hit (from an airbag, or a fall, or a 300-pound defensive tackle), your brain is subjected to shear forces that can actually tear it apart from the inside—without any of the structural damage you can see on a CT scan or an MRI. Deep in the brain’s white matter, tissues of different densities pull and strain against each other as they accelerate and decelerate at different speeds. Axons, the long, stretched-out arms of neurons that allow them to talk to each other, get frayed and severed. This is why you might have trouble remembering things or thinking clearly if you get concussed—and why a doctor might ask you to tell them what year it is or who’s the president.

But there’s another thing that happens when these axons get damaged: They release a number of proteins into the cerebral spinal fluid. About one in a thousand of those proteins crosses the blood-brain barrier to enter the bloodstream. The more damage, the higher the blood protein concentration. Scientists have known this for a long time, but until very recently it’s been irrelevant to all but the most severe traumatic brain injuries. So few of the proteins end up in the blood that no one's been able to detect them.

“It’s like trying to find grains of sand in a thousand Olympic-size swimming pools,” says Jessica Gill, a researcher at the National Institutes of Health who studies brain trauma and PTSD in soldiers. “That’s why we hadn’t been looking for it, even though we always thought it was there.” But now, scientists like Gill are finding those proteins in the blood of athletes within hours of receiving a blow to the head—with a little help from a super-sensitive, digitized biomarker assay machine called Simoa.

Cambridge, Massachusetts-based biotech firm Quanterix makes the refrigerator-sized blue and white box that looks like a space-age photo booth. Inside the machine is a fully automated, digital form of a popular biochemistry assay used to detect a wide range of substances—from bits of DNA to more complicated molecules like proteins and hormones. Tau is one such complicated protein. Made by neurons, it gets tangled into snarled plaques in the brains of people with Alzheimer’s and chronic traumatic encephalopathy, a neurodegenerative disease linked to the premature death of NFL players and professional boxers. And it’s a known biomarker for concussions.

To find it in a blood sample, Simoa’s robotic arms first mix the blood with magnetic beads coated in a tau-binding antibody. Any tau present sticks to the beads. A few rounds of mixing and washing with luminescent molecules later and all the tau-studded beads now glow in the dark. This is where the digitization part comes in. Another robotic arm pours the sample onto a CD-shaped disk laser-etched with 24 rectangles. Each of these grids contains 216,000 wells that hold 10-15 liters each—enough space for exactly one magnetic bead. Once all the beads settle into their wells, a computer takes a snapshot of the disc and analyzes how many wells light up. A little bit of complicated computation later, and you know exactly how much tau protein is in your blood sample.

In a paper published earlier this month in Neurology, Gill and her colleagues used this method to track tau levels in over 600 collegiate athletes over the course of two sports seasons. Every time a coach or trainer witnessed an on-field concussion, they pulled that player and sent them to have blood taken within six hours of the incident. For each injured player, an age, gender, and sport-matched athlete also gave blood to be tested. Gill's team found that concussed players had elevated levels of tau protein, over both other athletes, and over a non-athlete control group, often for days after the injury. And as they monitored different athletes' recoveries, they saw that tau levels closely predicted not only the severity of the injury, but the amount of time needed before the players could return to activity. What they’d discovered was a blood test for concussions, more sensitive and less biased than any human evaluator.

“Right now, coaches and players are making decisions based on the subjective self-reporting of symptoms,” says Gill. “Having something objective like this can make these safety decisions much more informative.” The goal is to get the test sensitive enough to have a tau readout within 15 minutes of the injury. Quanterix thinks they’ll be ready to begin trials with the National Football League sometime in the next year. And in the meantime, they’re also working on shrinking Simoa down to something that would fit in a trainer's cart on a sideline.

Maybe someday, they'll be cheap enough to sit on high school sidelines, too—so kids like Kenney Bui don't have to leave them in an ambulance.