Label-Free Sickle Cell Disease Diagnosis using a Low-Cost, Handheld Platform

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 9) 1600100 wileyonlinelibrary.com 10–20 d (compared to 120 d in patients without the disease), resulting in a low RBC count and low level of hemoglobin (anemia).[4] Deoxygenation results in HbS polymerization and RBC shape change to the rheologically unfavorable sickle form that can inhibit blood flow and cause organ damage, stroke, infection, and pain. Often, if SCD is left undiagnosed, children can experience silent strokes, which have no symptoms but still cause significant long-term damage.[5] SCD is caused by a mutated form of hemoglobin, Hb S. RBCs in people with SCD have been observed to become dehydrated and increase in density, including reticulocytes, discocytes, dense discocytes, and irreversibly sickled cells.[6] This mechanism is believed to arise from a combination of Hb S polymerization accompanied by a change in the activity of cation transport systems.[7] Under deoxygenated conditions, the Hb S molecules polymerize to form long, rigid fibers which aggregate and cause RBCs to take on a characteristic sickle shape.[8] Hb S polymerization leads to an increased permeability of the cell membrane to cations such as Na+, K+, Ca2+, and Mg2+.[9] This triggers the Ca2+-activated K+ channel (also known as the Gardos channel), resulting in KCl export, acidification, and dehydration.[7,9] In RBC subpopulations of reticulocytes and young RBCs which express high numbers of the K–Cl cotransporter (KCC), the pathway becomes overactive in response to acidification.[10] The KCC pumps KCl out of the cell, resulting in further dehydration and a decrease in cell volume.[7,11] Further acidification results in a positive feedback loop of KCC-induced acidification and additional dehydration.[10] In the proposed diagnostic test (Figure 1), sodium metabisulfite (SMBS) is used as an oxygen scavenger to induce this deoxygenated T-state of hemoglobin,[8,12] initiating these pathways and resulting in RBCs dehydration and an increase in RBC density which is detected using the platform presented here (Figure 2a). SCD diagnostics are currently performed using a variety of approaches (Table 1). SCD can be detected based on a solubility test which detects aggregation of abnormal hemoglobin.[13] A hemolysis assay is based on the lysis of SCD RBCs under deoxygenated isosmotic nonelectrolyte conditions.[14] Paper chromatography can be used to distinguish SCD patients from those carrying the trait and from patients without the disease based on a characteristic staining pattern on paper.[15] Another approach is based on cell density: a portion of the RBCs from Label-Free Sickle Cell Disease Diagnosis using a Low-Cost, Handheld Platform