PB1126 - The use of Slope Ratio to identify the presence of inhibitors in one-stage intrinsic clotting factor assays on Sysmex CN-series analysers

Background: In the presence of inhibitors of coagulation, clotting times in one-stage factor assays (OSFA) can be prolonged, leading to a factor levels that may be lower than the true value. Multi-dilution analysis is recommended by national and international guidelines to identify the presence of such inhibitors.

Software on the new Sysmex CN-series analysers enables reporting of Slope Ratio (SR), where the line of best fit is compared mathematically to the standard curve (when using the line-of-best-fit).

Aims: The study aimed to identify whether SR can be used to identify the presence of inhibitors in OSFA for factor (F)VIII, FIX, FXI and FXII assays using Siemens Actin FS (AFS), Actin FSL (AFSL) or Pathromtin SL (PSL) on a Sysmex CN-series analyser.

Methods: Standard assays for FVIII, FIX, FXI and FXII using AFS, AFSL and PSL were performed on a Sysmex CN-6500 analyser using Siemens factor deficient plasmas. Samples were selected from patients with normal PT and APTT and lupus anticoagulant (LA) negative ("Normal"), and compared to the following patient group: short APTT ("Activated"); single factor deficiency but no inhibitor ("Factor Deficiency); triple-positive APLS ("LA+"); fast-acting anti-FVIII inhibitors ("Inhibitor"); and anticoagulants (Apixaban, Edoxaban, Rivaroxaban, Dabigatran and Argatroban).

Results: The use of SR was able to identify inhibitors in all the groups where an inhibitor was expected (see table 1 and figure 1), except for LA+ patients, which were not identified when tested with AFS or PSL.

Conclusion(s): The use of SR was able to identify the presence of all types of inhibitor analysed. Multi-dilution analysis is necessary for the accurate analysis of factor activity levels in OSFA, even in those patients whose single-point analysis may indicate normal levels, so that further assays can be performed to correctly quantify factor activity when SR is reduced below that seen in normal patients.