Comparison of Low Molecular Weight Heparin Activity Assays with and without Addition of Exogenous Antithrombin in a Pediatric and Young Adult Leukemia Cohort
Authors: Schaefer Beverly , Hausfeld Adriane , Steele Paul , Martin Jan , Brannon Sandra , Lane Adam , Luchtman-Jones Lori 
Published: November 2019

Introduction: Enoxaparin (low molecular weight heparin, LMWH) is the most commonly prescribed anticoagulant for pediatric patients with venous thromboembolism (VTE) (Goldenberg et al. 2015). The gold-standard LMWH activity (anti-Xa activity) assays differ in whether they add exogenous antithrombin (AT) or dextran sulfate (Ignjatovic et al. 2007). Adding AT would "standardize" results in patients with low antithrombin, such as infants and asparaginase-treated patients (Mitchell et al 2010); however, questions remain about which assay best reflects the patient's anticoagulation effect and the degree of discrepancy between assays. We assessed LMWH activity in residual plasma samples from a cohort of anticoagulated pediatric acute lymphoblastic leukemia and lymphoma (ALL) patients, with history of VTE and variable AT levels, on four platforms (two instruments and their kits +/- exogenous AT).
Methods: We analyzed 60 de-identified residual plasma samples from 12 anticoagulated ALL patients (2-19 years. Mean 13.75 yr) who had AT levels obtained for clinical care. All consented to the IRB-approved Oncology Tissue Repository. LMWH activity was assessed on Siemens and Stago instruments using their recommended kits that did or did not contain exogenous AT (Table 1), according to manufacturer recommendations, by experienced laboratorians.
Results: Results were interpretable on 236/240 with 4 rejected for lipemia. Mean AT activity was 80 (42-138 ng/ml, lab normal >81%). Correlation was acceptable for the published kit ranges of LMWH activity when comparing kits +AT (Berichrom® to Stachrom®, r=0.82, p<0.0001), -AT (Innovance® to STA®-Liquid Anti-Xa: r=0.93, p<0.0001), and within the same manufacturer (Berichrom® to Innovance®, r=0.92, p<0.0001, Stachrom® to STA®-Liquid Anti-Xa r=0.98, p<0.0001) (Table 2, Figure 1, Figure 2). Comparing +AT or not by manufacturer, there was a nonsignificant trend to higher LMWH activity result with +AT kits. When AT levels were <70 ng/ml (n=19, mean 56 ng/dl) there was a trend to underestimate LMWH activity when AT was not added; this reached significance when the Stago methods were compared (Table 2.)
Conclusions: There is acceptable correlation of LMWH activity using kits with or without exogenous AT in anticoagulated pediatric plasma samples; however, in low AT samples, LMWH activity trends lower in platforms without exogenous AT. Use of residual samples precluded conclusions about the clinical impact of this difference; however, there were a few instances where subtherapeutic activity in the -AT assay was in the therapeutic range (0.5-1 activity units) in the +AT assay. Clinicians should know whether exogenous AT is used in their own laboratory's assay and understand that added AT could overestimate the LMWH effect in patients with low AT levels.

Disclosures
Schaefer: Stago: Research Funding; Siemens: Research Funding.