Blood clots form naturally to stop bleeding when a person is injured. But blood clots in patients with medical conditions, such as mechanical heart valves or other heart problems, can lead to stroke or heart attack. That’s why millions of Americans take blood thinners, like warfarin, which make it harder for blood to clot.
However, warfarin is not perfect and requires patients to be tested frequently to ensure their blood is in the correct range – blood that clots too easily can still lead to a stroke or heart attack while the blood that does not clot can lead to prolonged bleeding after an injury. To be tested, patients must either go to a clinical laboratory or use an expensive home testing system.
Researchers at the University of Washington have developed a new blood clotting test that uses just a single drop of blood and a vibration motor and smartphone camera. The system includes a plastic attachment that holds a small cup under the phone’s camera.
A person adds a drop of blood to the cup, which contains a small particle of copper and a chemical that triggers the blood clotting process. Then the phone’s vibration motor shakes the cup while the camera monitors the movement of the particle, which slows and then stops as the clot forms. The researchers showed that this method was within the accuracy range of standard instruments in the field.
The team published these results on February 11 in Nature Communication.
“Back then, doctors used to manually rock blood tubes back and forth to monitor how long it took for a clot to form. However, this requires a lot of blood, making it impossible its use at home,” said the senior. author Shyam Gollakota, UW professor at the Paul G. Allen School of Computer Science & Engineering. “The creative leap we’re making here is that we’re showing that by using the vibration motor on a smartphone, our algorithms can do the same thing, except with a single drop of blood. And we get similar accuracy to best available techniques. in trade.”
Doctors can classify blood clotting ability using two numbers:
- the time it takes for the clot to form, called the “prothrombin time” or PT
- a ratio calculated from the PT that makes it easier for doctors to compare results between different tests or laboratories, called the “international normalized ratio” or INR
“Most people who take this drug take it for life. But it’s not a set and forget thing – in the US most people are only in what we call the ‘desirable range’ PT/INR levels about 64% of the time,” said co-author Dr. Kelly Michaelsen, assistant professor of anesthesiology and pain medicine at the UW School of Medicine. “That number is even higher. low – only about 40% of the time – in countries like India or Uganda where testing is less frequent. How can we improve this? We need to make it easier for people to test more frequently and take ownership of their healthcare.”
Patients who can monitor their PT/INR levels at home would only need to see a clinician if the test suggested they were outside this desirable range, Michaelsen said.
The researchers wanted an inexpensive device that could work similarly to home blood glucose monitors for people with diabetes: a person can prick their finger and test a drop of blood.
“We started by vibrating a single drop of blood and trying to monitor the waves on the surface,” said lead author Justin Chan, a UW doctoral candidate at the Allen School. “But it was really difficult with such a small amount of blood.”
The team added a small copper particle because its movement was so much more reliable to track.
“As the blood clots, it forms a web that tightens. And in the process, the particle goes from bouncing happily to not moving,” Michaelsen said.
To calculate PT and INR, the phone collects two timestamps: the first when the user inserts the blood and the second when the particle stops moving.
“For the first time, we are looking for the moment when the user inserts a capillary tube containing the sample into the frame,” Chan said. “For the end of the measurement, we look directly inside the cup so that the only movement inside these frames is the copper particle. The particle suddenly stops moving because the blood coagulates very quickly, and you You can observe this difference between the frames. From there, we can calculate the PT, and that can be mapped to the INR.”
The researchers tested this method on three different types of blood samples. As a proof of concept, the team started with plasma, a component of blood that is transparent and therefore easier to test. The researchers tested plasma from 140 anonymized patients at the University of Washington Medical Center. The team also examined plasma from 79 patients with known blood clotting problems. For both of these conditions, the test gave similar results to commercially available tests.
To mimic what a patient at home would experience, the team then tested whole blood from 80 anonymized patients at Harborview and University of Washington Medical Centers. This test also gave results that were within the accuracy range of commercial tests.
This device is still at the proof of concept stage. Researchers have made the code public and are exploring commercialization opportunities along with further testing. For example, currently, all these tests have been carried out in the laboratory. The next step is to work with patients to test this system at home. The researchers also want to see how the system performs in regions and countries with more limited resources.
“Almost every smartphone of the last decade has a vibration motor and a camera. That means almost anyone who has a phone can use it. All you need is a simple plastic attachment, no hardware. ‘additional electronics of any kind,’ Gollakota said. “It’s the best of all worlds – it’s basically the holy grail of PT/INR testing. It makes them frugal and accessible to millions of people, even when resources are very limited.”
Additional co-authors of this article are Joanne Estergreen, clinical laboratory supervisor in the Department of Laboratory Medicine and Pathology at the UW School of Medicine, and Dr. Daniel Sabath, professor of Laboratory Medicine and Pathology at the U.W. School of Medicine. This research was funded by the Moore Foundation Fellowship.