and a sickled cell
Credit: Betty Pace
Two new tests can diagnose sickle cell disease (SCD) quickly, simply, and cheaply, researchers have reported in PNAS.
The tests separate red blood cells based on density and can provide an SCD diagnosis in less than 12 minutes for as little as 50 cents.
When run against clinical samples, both tests showed high sensitivity and specificity for SCD. And one of the tests could distinguish between the two main subclasses of SCD.
Ashok A. Kumar, PhD, of Harvard University in Cambridge, Massachusetts, and his colleagues developed the tests by connecting two ideas scientists have understood for decades.
The first is the notion that blood cells affected by SCD are denser than normal cells, and the second is that many polymers, when mixed in water, automatically separate into layers ordered by density.
Dr Kumar and his colleagues discovered the layers could be used to separate red blood cells by density.
So the researchers designed 2 tests to separate red blood cells into multiple bins of density. The presence or absence of cells in the bins distinguished individuals with the most prevalent forms of SCD (Hb SS or Hb SC) from individuals with either normal hemoglobin (Hb AA) or sickle-cell trait (Hb AS).
“We wanted to make the test[s] as simple as possible,” Dr Kumar noted. “The idea was to make [them] something you could run from just a finger prick. Because these gradients assemble on their own, that meant we could make them in whatever volume we wanted, even a small capillary tube.”
The design the team settled on is barely larger than a toothpick. Running the tests is as simple as uncapping a tube, pricking a patient’s finger, and allowing the blood to wick into the tube.
The researchers evaluated the tests in 59 subjects, 33 who were negative for SCD and 26 who were positive for SCD. Both tests identified SCD-positive samples with a sensitivity greater than 90% and a specificity greater than 88%.
The simpler test, called SCD-AMPS-2, involves 2 phases. It identified Hb SS and Hb SC with a sensitivity of 90% (73%–98%) and a specificity of 97% (86%–100%).
The higher-resolution test, called SCD-AMPS-3, involves 3 phases. It identified the 2 types of SCD with a sensitivity of 91% (78%–98%) and a specificity of 88% (74%–98%). This test could also distinguish between Hb SS and Hb SC.
Despite these promising results, Dr Kumar said additional testing will be needed to determine whether the tests are truly accurate enough to use in the field.
