A Multi-Instrument Evaluation of a Commercial Chromogenic Factor X Assay To Monitor Oral Anticoagulation Therapy.

Introduction: A multi-instrument evaluation of the chromogenic DiaPharma Factor X Assay kit (DFX) was undertaken in order to evaluate the utility of the kit for measuring factor X levels in patients receiving oral anticoagulation therapy (OAT).

Methods: The DFX microtiter method was compared to a clottable FX (CFX)method in Laboratory 1. All clottable assays were performed on the Diagnostica-Stago STA automated coagulation analyzer using STA Neoplastine CI+ as the thromboplastin reagent with a low ISI. All testing was performed on citrated plasmas. A normal range was established using 30 normal subjects. Clinical sensitivity was tested using 30 specimens from subjects on OAT. The samples were assayed for FX levels by DFX and CFX, and international normalized ratios (INR) were calculated. Thirty-one specimens that were positive for the presence of hemolysis, icteric color, lipemia, heparin and lupus anticoagulants (LA) were analyzed for FX by both methods to check for the influence of interfering substances. Nineteen subjects with the presence of an LA on OAT and an unstable INR with specimens taken at 8 time points were evaluated by both methods. Laboratory 2 used an STA compact and reagents to evaluate both the CFX and DFX methods. A normal range was established using 25 normal subjects on both methods. Fifty-five subjects on OAT were evaluated by both the CFX and the DFX methods. Precision and accuracy testing using different levels of FX were analyzed by all methods at both institutions.

Results: The results of both laboratories are as follows:

FX Chromogenic vs. Clotting

Precision Data: Laboratory 1 performed precision testing using times 10 replicates on 6 specimens, run on the DFX in the range of 10 – 120% FX activity. Using 5 known standards for the DFX, assay accuracy ranged from 99.2 – 100.8% recovery. Laboratory 2 performed precision testing on 3 levels of FX (n = 12).

Precision Data

Conclusions: The present studies of the DFX kit demonstrated the assays robustness, precision, accuracy and utility for monitoring patients on OAT with and without interfering substances, the presence of an LA or unstable INR. The DiaPharma Factor X Kit recently received 510(k) market clearance from the FDA. This assay should offer health care providers an option for monitoring patients receiving OAT, especially those where INR values may not be reliable when an LA is present, and screening for factor X deficiencies.