Assessment of Bleeding Phenotype in Hemophilia Α By a Novel Point-of-Care Global Assay

Introduction: Patients with Hemophilia A have considerable phenotypic heterogeneity with respect to clinical severity based on their baseline factor levels. As clinical bleeding risk is helpful to individualize factor replacement therapy in hemophilia patients, previous studies have utilized direct and indirect methods of thrombin generation to classify individual bleeding phenotypes, however, with variable results. An easy to use, point-of-care, global assay to assess bleed phenotype, can be a useful tool in the clinical setting to determine intensity of prophylaxis therapy for patients with hemophilia. We have previously introduced a novel, point-of-care (POC), dielectric microsensor, ClotChip, and demonstrated its sensitivity to factor replacement in patients with severe hemophilia A. We aim to further test the ability of ClotChip in assessment of a bleeding phenotype, as described by a bleeding score, in patients with hemophilia A.

Methods: After IRB approval, 28 patients with hemophilia A of varying severity and well-characterized bleeding history, were enrolled in this study at the time of trough factor levels. The bleeding history was extracted from patient charts and included number of bleeds (joint and soft-tissue), annual factor usage in terms of units/kg, and number of target joints. These parameters were used to generate a bleeding score (range: 0 - 24), and patients were divided in to 2 categories with scores between 0 - 12 (n=14) and > 12 (n=14). Healthy volunteers (n=17) were accrued as controls. Whole blood samples were obtained by venipuncture into collection tubes containing 3.2% sodium citrate. Samples were then tested with the ClotChip within 2 hours of collection. ClotChip is based on the electrical technique of dielectric spectroscopy (DS) and features a low-cost (material cost < $1), small- sized (26mm × 9mm × 3mm), and disposable microfluidic biochip with miniscule sample volume (< 10 µL). The ClotChip readout was taken as the temporal variation in the real part of blood dielectric permittivity at 1 MHz. Our previous studies have shown that the ClotChip readout is sensitive to the global coagulation process and the time to reach a peak in permittivity (T_peak) is a sensitive parameter to assess coagulation factor defects. Thrombin generation assay (TGA) using low tissue factor concentration was also performed on blood samples according to the manufacturer's direction. TGA was not available for 4 hemophilia and 2 control samples. Endogenous thrombin potential (ETP) parameter of TGA was used in this study to assess thrombin generation. Data are reported as mean ± standard deviation (SD). Analysis of variance (ANOVA) was used to test for statistical significance between groups with P < 0.05. Spearman's correlation test was used to derive correlation statistics.

Results: ClotChip exhibited a mean T_peak of 2186s ± 1560s for hemophilia patients in the group with higher bleeding scores (i.e. score >12), a mean T_peak of 931s ± 496s for the group with lower bleeding scores (i.e. score <12) and a mean T_peak of 441s ± 74s for the healthy group (Figure 1A). A significant difference in T_peak was found between the group with higher bleeding scores compared to the group with lower bleeding scores (P = 0.002) as well as between higher bleeding scores and the healthy group (P < 0.0001). However, no significant difference in the TGA ETP parameter was detected between the groups with higher bleeding scores (mean ETP: 470 ± 814) and lower bleeding scores (mean ETP: 471 ± 897) (Figure 1B). ETP exhibited a statistical difference between the healthy group (mean ETP: 3462 ± 575) and both hemophilia groups (P < 0.0001). We also carried out studies to investigate the correlative power of the ClotChip T_peak parameter to the TGA ETP parameter when including additional blood samples that were collected at various times during a hemophilia patient's prophylaxis regimen. The ClotChip T_peak parameter exhibited strong negative correlation to the TGA ETP parameter (Spearman's r_s= -0.73, P < 0.0001).

Conclusions: Our studies suggest that a novel dielectric microsensor (ClotChip) could be useful in assessing bleeding phenotype in hemophilia A patients, allowing rapid assessment of hemostasis using a miniscule amount of whole blood (<10 µL) at the POC. Further studies are needed to determine if ClotChip data can be used to individualize prophylactic factor replacement regimens in hemophilia A patients.

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