Measuring Blood with Light
Some components of blood can be measured without a needle stick. Now an innovation in light-based methods could make even more of them visible. For about 40 years, physicians have been able to measure some qualities of blood using pulses of light.
A recent breakthrough in the field may also let them noninvasively measure white blood cells—a key indicator in the treatment of some cancer patients, but a tricky holdout for light measurement. As early as the 1930s, researchers realized that passing light through blood could yield important data. Blood cells loaded up with oxygen, for instance, absorb infrared wavelengths but let red light pass through, a discovery that paved the way for the pulse oximeter, which beams light through an earlobe or (more commonly) a fingertip to measure how much oxygen is circulating in the bloodstream. Refinements of this method and other light-based techniques can now monitor many qualities of a patient’s blood without a needle stick.
Yet these devices focused, of necessity, on red blood cells. White blood cells, which play a key role in fighting infection, are far fewer in number and don’t absorb light in a way that can be easily measured. But a research team from MIT and Massachusetts General Hospital led by biomedical engineer Carlos Castro-Gonzalez, a postdoctoral associate, has come up with a light-based method to estimate the state of a patient’s circulating neutrophils—the most common type of white blood cell.
The team’s device floods the nailfold—the base of the fingernail, where capillaries are close to the surface—with deep blue light. Red blood cells absorb this light, and appear black, while white blood cells don’t show up at all, remaining transparent.
As the neutrophils squeeze through the capillaries in single file, their passage creates visual gaps in the flow of blood. The device takes minute-long videos of this flow, which the team then scrutinizes for these telling blank spaces. That lets them extrapolate the number of gaps into a neutrophil assessment for the patient.
In a recent proof-of-concept study, the researchers used their prototype to analyze the blood of 11 chemotherapy patients at the Massachusetts General Hospital and La Paz University Hospital in Madrid. The system indicated whether a patient had severe neutropenia—a neutrophil count of less than 500 per microliter of blood—with 95% accuracy. The findings were reported this March in Scientific Reports.
“This innovation could quickly give us a handle on a patient’s white blood cell count, and its noninvasiveness makes it especially useful for pediatric patients,” says Carlo Brugnara, professor of pathology at Harvard Medical School and Boston Children’s Hospital, who was not involved in the research. He finds the results promising, and thinks the device could also play a critical role in cancer treatment.
Each dose of chemotherapy destroys white blood cells, and it can take weeks for the body to replenish their numbers. Doctors have to track white blood counts between treatment cycles to avoid infections. But blood tests involve a visit to the hospital, and that limits the extent to which a patient’s progress can be monitored. As a consequence, one in six U.S. chemotherapy patients ends up hospitalized with life-threatening infections.
Castro-Gonzalez says that the immediate goal of his team is not to replace the gold standard of testing —blood tests—but to create a portable blood monitor that chemotherapy patients could use at home to check their neutrophil count daily. If levels dip dangerously low, the patient can call their doctor.
The next steps in making such a device are to automate the counting of neutrophils and to home in on a more exact count. This noninvasive tool could also be used in the emergency room to detect quickly whether feverish patients are severely immunosuppressed and lacking white blood cells, says Castro-Gonzalez—shining a new light on patients in dire need.
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PUBLISHED ON MAY 3, 2018
TECHNOLOGY