Preanalytical stability of aPTT and anti-Xa levels for therapeutic unfractionated heparin monitoring: a systematic review and critical appraisal using CRESS criteria

INTRODUCTION: The preanalytical stability of activated partial thromboplastin time (aPTT) and anti-factor Xa (anti-Xa) levels is critical for reliable monitoring of therapeutic unfractionated heparin (UFH). However, current recommendations regarding acceptable preanalytical delays are based on limited and heterogeneous evidence. We systematically reviewed the literature on the preanalytical stability of aPTT and anti-Xa levels in patients receiving therapeutic UFH and assessed study quality using the Checklist for Reporting Stability Studies (CRESS), with studies graded from A (best quality) to D (fail). CONTENT: A systematic review was conducted according to the PRISMA 2020 guidelines. Studies evaluating the preanalytical stability of aPTT and/or anti-Xa levels in patients receiving therapeutic UFH were identified through a systematic literature search. Study quality and reporting were assessed using the 20-item CRESS checklist. SUMMARY: Nine studies published between 1983 and 2025 met the inclusion criteria. Marked heterogeneity was observed in study design, preanalytical conditions, analytical methods, stability criteria, and statistical analyses. According to the CRESS, seven studies were graded C or D, while only two achieved grade B. Reported stability limits varied widely according to sample type, collection tube (citrate vs. CTAD), analytical reagent, storage conditions, and study methodology. Delayed centrifugation and delayed analysis were consistently associated with decreases in aPTT and anti-Xa levels. CTAD tubes and dextran sulphate-containing anti-Xa assays did not demonstrate a consistent stability advantage. Importantly, studies evaluating patient-level outcomes reported clinically relevant patient reclassification despite apparently acceptable analytical stability. OUTLOOK: Evidence regarding the preanalytical stability of aPTT and anti-Xa levels remains limited by methodological heterogeneity, inconsistent stability criteria, and incomplete reporting. Based on the available evidence, centrifugation within 1 hour after blood collection followed by prompt analysis should remain the reference approach for therapeutic UFH monitoring. Future stability studies should comply with CRESS recommendations, use standardized acceptability criteria, and incorporate clinically relevant outcomes, including patient reclassification, to establish evidence-based recommendations for UFH monitoring.