The flow of sickle blood through glass capillaries: a point of care application

We show that the characteristic time for sickled blood to traverse 100 µm-diameter glass capillaries can be used as the foundation for a useful point-of-care diagnosis for sickle cell disease. 3.2 cm long capillaries were inserted into a 5 µL drop of blood, and 1/4 µL was spontaneously drawn into the tube. The rate at which the blood traversed the capillary was significantly changed by deoxygenation only for sickle genotypes, a consequence of the increased viscosity of cells containing sickle fibers. Under these conditions, we find deoxygenation causes the time for sickle blood to traverse the capillary to increase to about 3 times its original value, in contrast to normal blood, where the times are equivalent regardless of state of oxygenation. This corresponds to readily-observed time differences of around 10 s for capillary traversal. Such changes are present for blood from homozygous sickle cell patients, as well as heterozygous patients with hemoglobin AS, SC, and Sβ + thalassemia. Moreover, we demonstrate that this test is sensitive to sickling therapies that increase the production of fetal Hb. This viscosity-based measurement thus offers the prospect of an assay that is 10 times faster than any current test of which we are aware, at costs that rival the least expensive of any current assays.