Low molecular weight heparin (LMWH) remains the most commonly prescribed pediatric anticoagulant. There is debate whether LMWH anti-Xa assays with or without exogenous antithrombin (AT) best reflect anticoagulation effect, and how much discrepancy exists between assay types.We assessed the effect of variable AT activity on LMWH anti-Xa levels in plasma samples from anticoagulated pediatric and young adult acute lymphoblastic leukemia and lymphoma (ALL/L) patients, using two instruments and their commercial kits with and without exogenous AT (ie, four platforms).We analyzed LMWH anti-Xa levels on 60 plasma samples with known AT activity from 12 enoxaparin-treated ALL/L patients, using four commercial kits from Siemens and Stago containing AT or not, on Siemens BCS and Stago STA R Max, respectively.Of 236/240 samples with interpretable results, mean AT activity was 80% (46-138%). Correlation was acceptable for published kit ranges of LMWH anti-Xa levels when comparing kits containing AT (r=0.82, P<.0001), or not (r=0.93, P<.0001), and within a manufacturer (Berichrom to Innovance, r=0.92, P<.0001; Stachrom to STA-Liquid Anti-Xa r=0.98, P<.0001). LMWH anti-Xa levels were lower in platforms without added AT (P<.0001). For Stago kits, this effect increased when AT<70% (P=.001, n=19, mean 56%). Assay variability, measured as mean percent difference, was less pronounced with Stago kits (14.7%, n=49) than Siemens (41.9%, n=50).Although LMWH levels from anti-Xa assays with added AT trend higher than in those without, correlation was fairly good between platforms in pediatric ALL/L plasmas, even when AT activity was <70%. 2020 Wiley Periodicals LLC.

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Impact of exogenous antithrombin on low molecular weight heparin anti-Xa activity assays in a pediatric and young adult leukemia and lymphoma cohort...