The sensitivity of two new APTT reagents to factors VIII, IX, and XI

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A. Bowyer, S. Kitchen and R. Maclean, Department of Coagulation, Sheffield Haemophilia and Thrombosis Centre, UK

Introduction

The APTT is used as a screening test for deficiencies of factors VIII, IX and XI. It is recommended that a prolongation to the APTT should be observed when the clotting factor is below 30 IU/dL [ 1 ].
APTT reagents may demonstrate different sensitivities to these clotting factors because of their composition.

Aim

To compare the APTT sensitivity to clotting factors VIII, IX and XI of two new APTT reagents, Yumizen G APTT 4 and Yumizen G APTT Liq 4, (HORIBA Medical, Montpellier, France), with 6 commonly used APTT reagents.

Method

FVIII deficient plasma (Siemens, Marburg, Germany), FIX and FXI deficient plasma (Precision Biologic, Halifax, Canada) was spiked with normal plasma (CCNRP, Precision Biologic) to obtain plasmas with factor concentrations of 0 90 IU/dL.
APTT were performed on 5 different days with the 8 APTT reagents.
Local reference ranges were generated for each reagent using 30 normal donors.
The intersection of the upper limit of the APTT reference range and concentration of clotting factor determined the reagent sensitivity.
All testing was performed on Sysmex CS5100i instrumentation (Kobe, Japan).

Results

Sensitivity to FVIII ranged from 31.5-67.3 IU/dL (Synthasil, Actin FS), median of 43.8 IU/dL.
Sensitivity to FIX ranged from 15.9-36.1 IU/dL (APTT SP, Synthasil), median 27.2 IU/dL. Sensitivity to FIX was greater than 30 IU/dL with Synthasil only.
Sensitivity to FXI from 28.3-55.2 IU/dL (Actin FSL, Synthafax), median 46.1 IU/dL.
Sensitivity to FVIII and FXI was greater than 30 IU/dL with all reagents, with the exception of FXI using Actin FSL (28.3 IU/dL).
The sensitivity of Yumizen G APTT 4 ranged from 26.8-43.3 IU/dL.
The sensitivity of Yumizen G APTT Liq 4 ranged from 29.7-48.9 IU/dL.
Inter assay CV was less than 5.5% for all reagents and clotting factors measured.
Average inter assay CV for Yumizen G APTT 4 was 3.7%, 3.9%, 2.6% and Yumizen G APTT Liq4 was 0.9%, 1.3%, 0.9% for FVIII, FIX, FXI respectively.

Figure 1: Factor concentration (APTT Liq4)

Figure 1: The sensitivity of APTT4 and APTT Liq4 to FVIII:C (●-red-), FIX:C (●-orange-) and FXI (●-blue) in IU/dL as determined by the intersection between the lower limit of normal (dotted line) and the individual curves. Mean APTT and standard deviation of each point are shown.