Issue Date: January 2, 2017
Blood tests at your fingertips
Nowadays a blood test usually requires a doctor’s visit, a needle poke in the arm, and a day-long wait for results to come back from a lab. So when Silicon Valley start-up Theranos promised a device that could perform more than a hundred of the most commonly ordered blood tests from just a finger stick, doctors, patients, and investors were quick to get behind the company. Valued at $9 billion in 2015, the company opened 40 blood testing centers in Walgreen’s stores in Arizona and, in a first step toward more widespread blood testing at patients’ discretion, successfully lobbied for Arizona legislation allowing patients to obtain blood tests without a doctor’s order.
But the secretive company came under intense scrutiny after the Wall Street Journal reported in October 2015 that the capability and accuracy of its device fell short of company claims. Regulators uncovered standards violations and dangerous lab practices at one of two Theranos laboratories and banned founder and CEO Elizabeth Holmes from operating a blood-testing lab for two years. Theranos compounded its technical setbacks with its lack of transparency—citing trade secrets—and not publishing in peer-reviewed journals. The company is now facing a string of lawsuits, including some from its own investors. “It was science fiction. It is still science fiction,” says James H. Nichols, medical director of clinical chemistry at the Vanderbilt University School of Medicine, who says Holmes sold her idea without proof.
But the Theranos boom and bust highlights how hard it is to actually do the testing the company promised. “When I was in training,” says Brad Karon of the Mayo Clinic, “everybody was saying, ‘Ten years from now, central lab testing won’t exist.’ That was 20 years ago.” Karon codirects the Mayo Clinic’s point-of-care program and has watched the plodding growth of bedside and finger-stick blood testing. Those with diabetes have been testing their own blood glucose for decades, so why can’t patients yet do finger-stick tests to track a wide spectrum of ailments that have known bloodborne tracers?
Cost is part of the reason finger-prick testing hasn’t taken off. It’s hard to compete with highly efficient, automated, central laboratory tests. But stubborn challenges also lie in sampling and analysis. Small drops collected from finger sticks have more variable contents than the larger quantities drawn from veins. And, despite the development of new devices that analyze a handful of different blood markers from microliter volumes, quantifying hundreds of markers at once from such tiny amounts of blood will require new strategies. While Theranos pushed the vision of finger-stick blood testing further than it had ever gone, other innovators in both blood sampling and multianalyte testing are stepping in to try and realize it.
Central laboratories can test for about 200 to 300 routine analytes and up to 1,000 not-so-routine ones in blood. These tests fall into some general categories. For example, blood chemistry tests measure electrolytes, such as sodium and potassium; kidney function tests look at blood urea nitrogen and creatinine; complete blood counts measure red blood cells, white blood cells, and platelets; lipoprotein panels tally cholesterol and triglycerides to assess heart disease risk; and liver function panels measure liver proteins and bilirubin. A variety of hormone tests, enzyme tests, clotting tests, and tests for infectious diseases are also available.
Central laboratories primarily rely on blood drawn from veins. A venous draw collects 2 to 6 mL of blood into one or more sample tubes, which are sent through the laboratories’ robotic, automated testing systems. However, central laboratories can also test capillary blood when they can’t get a venous sample: They regularly squeeze results out of the drops from babies’ heel sticks. And they will also test arterial blood to determine some blood gases such as oxygen and carbon dioxide—though such draws are rare because they are more painful and difficult to perform than venous draws. Most laboratory tests actually take only minutes to perform. Wait times come from transporting samples to the laboratory and putting them in the queue for the automated, large-scale testing that central laboratories employ. And most laboratory tests also require only microliters of blood, leaving plenty of volume for multiple tests and for filling the standard tubes used in automated testing.
Finger stick versions of many of these blood tests are feasible and available. Although blood glucose meters—most often used by diabetic patients—handle the greatest volume of finger-stick tests by far, commercial finger-stick tests exist to measure blood clotting, anemia, lipids, basic chemistry panels, and more. Most emergency rooms now keep on hand a number of soda-bottle-sized meters for rapid tests of blood gases and blood clotting times, including tests that reveal heart muscle damage in those who come in with chest pain. One of these handy meters, the iStat made by Abbott, can perform as many as 13 tests simultaneously on one finger stick.
Yet despite decades of development, finger-stick blood testing at home or the hospital bedside has yet to gain the whole-hearted trust of doctors. “The point-of-care space has been known for a while for having less accurate tests” than venous blood draws analyzed in the lab, says Cary Gunn, CEO of the company Genalyte. Many doctors want patients to repeat point-of-care tests with central lab-based tests, he says.
Part of the problem lies in the pricking. While developing a small meter for measuring hemoglobin, Meaghan Bond and Rebecca Richards-Kortum from Rice University were surprised that their measurements of capillary finger pricks were all over the map compared with the same measurements on venous blood. “Whenever we moved into finger-prick samples, our accuracy plummeted,” Bond says. Probing further, they found that the variability wasn’t caused by the meter; it was inherent to finger-prick samples. The hemoglobin in such samples varied three times as much as that in venous blood, and white blood cell count varied six times as much. “It’s not that you could make a perfect device and get around this problem,” Bond says. They calculated that they needed to average the results from six to 10 small drops of capillary blood—about 80 μL—to reduce the error below the variability of the instrument (Am. J. Clin. Pathol. 2015, DOI: 10.1309/ajcp1l7dkmpchpeh).
Bond and Richards-Kortum’s study didn’t delve into why capillary blood showed such inconsistency, but the results don’t surprise Karon of the Mayo Clinic. Because a finger stick collects right at the puncture site, he says, more than just blood gets into a finger-stick sample. Ruptured cells release their innards into the collected liquid. And a varying amount of interstitial fluid—the liquid that bathes all cells in the body—also gets in. “Each time you do a finger stick,” Karon says, “you get slightly different amounts of each of those components.” This mixture of fluids coupled with the small sample volume makes the composition of a finger-prick sample much more variable than a sample taken from within a vein.
The capillary sample variability affects some test results much more than others, Karon adds. Blood glucose is less affected because glucose is both actively and passively transported into and out of the cell, meaning that interstitial glucose and blood glucose are roughly equal in a person who’s fasting. Hemoglobin, on the other hand, is packaged inside red blood cells, which are only present in vessels, so the extra fluids dilute the hemoglobin measurement.
Any small variations get magnified in small samples, says Vanderbilt’s Nichols. The large volume of blood that mixes in the tube from a venous draw slashes variability, even if the test itself only uses microliters of blood from that tube. Compounding the problem, even small differences in how one person does the finger prick compared with another—whether leaving a bit of antiseptic on the finger or squeezing the finger, which releases additional interstitial fluid—affect a finger-stick result. “We try to train people as best we can in the proper techniques to get a good capillary sample,” Nichols says, “but beyond that there is not much more you can do.”
Part of Theranos’s approach included addressing “the challenges associated with collecting quality capillary samples,” Holmes said, when speaking at the American Association of Clinical Chemistry conference in August 2016. “We’ve spent many years developing solutions to help mitigate these challenges.” This included a finger-prick collection device called the Nanotainer, which was approved by the Food & Drug Administration for use with one Theranos test before the company got into regulatory trouble.
Meanwhile Howard J. Weisman, CEO of Seventh Sense Biosystems, hopes to sidestep finger sticks entirely with a new method for removing blood from capillaries. The company’s golf-ball-sized Tap device pricks the skin, usually on the upper arm, with an array of microneedles and then vacuums out the equivalent of two drops of blood through the resulting micropunctures. “People say it feels like someone tugging,” Weisman says. No bandage is required, because the pores are so small, although pinprick marks do show up a bit later.
“What we end up with is a very high-quality capillary sample,” of the same quality as venous blood, Weisman says. Because it pulls the blood out from capillaries just under the skin, he says, there is less tissue damage and no milking of interstitial fluid. The company expects FDA approval soon and plans to commercialize the devices in 2017. The company is talking with instrument-development companies to possibly combine its Tap collection method with small-volume analytical devices, many of which are coming to market able to do multiple assays on just one to two drops of whole blood, Weisman says. “The idea of testing in a pharmacy is not going away. That’s an area, we think, that is ready for significant growth.”
Rise of the machines
Many of the up-and-coming instruments that work on small blood volumes use long-established analytical methods from central blood-testing laboratories, packaged into small desktop machines for doctors’ offices, hospitals, and clinics. They have at their heart a chemical reaction for each blood analyte that causes either an optical or electrochemical change that gets read by the instrument.
After Theranos was forced to shut down its own blood testing laboratories, it shifted focus toward developing its own version of a desktop device for doctors, called miniLab. It looks like a desktop computer tower with a host of miniaturized lab components inside: a mini material-handling robot, thermal cycler, centrifuge, spectrophotometer, sonicator, fluorometer, cytometer, and isothermal detector. Nichols says that what Theranos has released about miniLab reveals that its technologies are not different from those used in central laboratories.
Yet compressed laboratories such as miniLab do depend on good miniaturization engineering, small-sample handling, and effective automation, and they have benefited greatly from advances in microfluidics—systems that manipulate very small volumes of fluid for diagnostics and other applications. One on the market, Abaxis’s Piccolo Xpress, runs up to 14 tests at a time on only 100 μL of blood and is cleared by FDA for doing lipid panels with finger-stick blood. In just 12 minutes, it spins the sample into separate compartments containing freeze-dried reagents for each test, does 14 different chemical reactions in parallel, and analyzes the resulting color changes with spectrophotometry. But scrutinizing more than 10 or 20 analytes at a time by dividing one drop of blood starts to stretch the limits of repackaged miniature labs.
Adding more tests to an instrument will necessitate new detection strategies that don’t require separating the sample into ever-tinier aliquots for each test. One promising approach came from Ryan C. Bailey, now at the University of Michigan, Ann Arbor, and Genalyte, a company he helped launch in 2007 to explore the blood-testing potential of a technology based on silicon microring resonators.
When light circulates in a cavity, the light waves that are in phase resonate, amplifying the signal. It’s the optical equivalent of rubbing a finger on the rim of a wine glass to create a resonant sound wave. Altering the volume of wine in the glass changes the pitch of the note. This concept led Bailey and his coworkers to carve picometer-wide rings into silicon chips that they stud with antibodies to blood components such as proteins, nucleic acid disease markers, or pathogens. When the antibodies bind to antigens such as these in a blood sample, the bound molecules change the refractive index of each ring’s resonating light and, thus, the color of the amplified light.
The technique is sensitive and inexpensive, but what is most promising about the technology to Bailey is that it can detect many antigens at a time. Genalyte’s chips each currently hold 128 rings, and a 250-μL sample “goes over all sensors in one shot,” says CEO Gunn. Clinical studies by the company show that capillary blood works as well as venous blood with the chips.
Their first target for commercialization is a chip including the 20 or so protein tests that a rheumatologist would commonly order to help diagnose autoimmune diseases such as lupus; the company is working on getting FDA approval for this device.
Gunn says he is committed to delivering a lab-quality test result to the patient from a finger-stick collection. “That’s how you change health care,” he says. “We are a start-up company, but you have to approach this with a level of safety and a level of rigor that you don’t in any other type of industry. If we bring products that are of lesser quality, we aren’t doing a service to either patients or doctors.”
Gunn and others in the field are aware that Theranos’s saga highlighted both the promise and the substantial hurdles of bringing finger-stick blood diagnostics to patients. They believe that continuing advances in biosensors, laboratories-on-a-chip, photonics, sampling methods, and other technologies will eventually win over the trust of doctors and patients and make finger-stick blood tests a viable alternative to a trip to the phlebotomist.
Louisa Dalton is a freelance writer. A version of this story first appeared in ACS Central Science: http://cenm.ag/fingerprick.
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