Effects of apixaban on prothrombin time, activated partial thromboplastin time and anti-Xa assays: a European survey

Direct FXa inhibitor apixaban affects routine coagulation assays of PT/INR and APTT to some extent. Generally, increasing apixaban concentrations only modestly prolongs coagulation times [1]. Yet, responses are reagent-dependent and laboratories need to recognize the sensitivities of their reagents. Previously, we conducted a survey in 73 and 22 European laboratories with dabigatran and rivaroxaban, respectively, to assess the anticoagulant effects on routine coagulation assays [2]. Here, we repeated the survey using apixaban. In September 2015, an EQA organization Labquality Ltd (Finland) sent lyophilized apixaban samples (kindly donated by Pfizer Finland) to 28 European laboratories participating in an anti-Xa EQA round (Finland, Denmark, Estonia, Greece, Iceland, Ireland, Norway) spiked in different concentrations: 80 ng/mL (Diagnostica Stago), 120 ng/mL (Technoclone), 270 ng/mL (Stago), and 300 ng/mL (Technoclone). Laboratories were instructed to reconstitute the samples with distilled water and incubate for 30 min at room temperature, before analysis. They were instructed to report INR, APTT and anti-Xa (with both heparin and apixaban calibration), where available. Twenty-one laboratories reported results (16 Finland, one Estonia, one Greece, one Iceland, one Ireland, one Norway). The laboratories included 10 university centers (five in Finland), with other participants being regional laboratories. PT results were reported as activity percentage by 12 laboratories and eight reported PT in seconds (s). As the activity percentage or second (s) values could not be compared directly, and data from normal controls was not available to calculate the PT ratio, the INR results were used. The results for INR, APTT and anti-Xa using LMWH and apixaban calibrators are summarized in Table 1. INR results were reported by all. The overall correlation with apixaban concentration was poor (Pearson’s correlation R2 = 0.21). HemosIL reagent showed the strongest response to apixaban, but also those INR results remained below 1.6. APTT results were reported by 20 laboratories (Table 1). For all the reagents combined, the correlation between APTT and the apixaban concentration was poor (R2 = 0.08). For all samples CVs ranged between 13.0% and 14.8%. Laboratories using SynthasIL or STA-PTT-A reagents more often than others reported APTT values above the reference range (1.3-fold prolongation, p < 0.001). Of note, only rarely the APTT results for apixaban concentration 120 ng/mL differed from those of 270 ng/mL, probably due to the sample matrix effect. This emphasizes even more, that sensitivity data obtained with spiked samples are of limited value. Albeit only seven (33%) laboratories, all of them reported anti-Xa-based apixaban concentrations accurately with apixaban calibration, the CVs ranged between 10.8% and 16.1% (Table 1). The correlation with actual concentrations was excellent (R2 = 0.93). Fifteen laboratories reported heparin-calibrated anti-Xa results. Each laboratory used the same heparin calibration they would use for routine samples. As expected, the heparin-calibrated (U/mL) anti-Xa assay increased with increasing apixaban concentrations. However, the results varied widely among different laboratories (CV 21.1%–48.8%; Table 1). *Corresponding author: Lotta Joutsi-Korhonen, MD, PhD, Coagulation Disorders Unit, Clinical Chemistry and Hematology, HUSLAB Laboratory Services, Helsinki University Hospital, University of Helsinki, POB 720, 00029 Helsinki, Finland, Phone: + 358 50 427 2402, Fax: + 358 9 471 74016, E-mail: lotta.joutsi-korhonen@hus.fi Tuukka A. Helin: Coagulation Disorders Unit, Clinical Chemistry, HUSLAB Laboratory Services, Helsinki University Hospital, University of Helsinki, Helsinki, Finland Anja Pakkanen: Labquality Ltd, Helsinki, Finland Riitta Lassila: Coagulation Disorders Unit, Hematology and Clinical Chemistry (HUSLAB Laboratory Services), Helsinki University Hospital, University of Helsinki, Helsinki, Finland