Background: The validity of laboratory results is influenced by pre-analytical variables including the collection technique, transportation and storage. Laboratories often rely on a model of centralisation of specialised testing with onsite processing and subsequent transport to referral centres. Low molecular weight heparin (LMWH) offers improved predictability of anticoagulation effect but in certain clinical scenarios (e.g. extremes of age and renal dysfunction) anti-Factor Xa (aFXa)activity testing is indicated. This test is not always available in smaller laboratories and samples must be centrifuged, separated and frozen on-site. Freezing may influence test results and the impact should be assessed in line with the Clinical and Laboratory Standards Institute (CLSI) guidelines.

Aims: Assess the effect of a freeze-thaw cycle on aFXa results of patients treated with enoxaparin.

Methods: Ethics approval for the study was obtained from the research committee. 25 random samples received within 4 hours of collection in citrate tubes at Johannesburg Academic Hospital, National Health Laboratory Service (NHLS) laboratory for aFXa testing were analysed on a STAGO-R MAX^® coagulation analyser with STA-Liquid^® aFXa reagent. The samples were then frozen (-80 °C) and transported within 60 days to another NHLS laboratory at Chris Hani Baragwanath Academic Hospital and analysed on Sysmex CS-2500i^® analysers with Siemens Innovance^® reagent. The aFXa results were compared for agreement.

Results: The aFXa results on the 2 analytical platforms demonstrated acceptable agreement (Figure 1 and Table1):

Figure 1: Bland-Altman difference plot Siemens vs. STAGO anti-Factor Xa (aFXa):

[Figure 1: Bland-Altman difference plot Siemens vs. STAGO anti-Factor Xa (aFXa):]

Table 1: Bias and limits of agreement of Siemens^® vs. STAGO^® anti Factor Xa (aFXa):

[Table 1: Bias and limits of agreement of Siemens vs. STAGO anti Factor Xa (aFXa):]

Conclusions: The results demonstrated acceptable agreement of aFXa results after a freeze-thaw cycle in stored frozen samples analysed on 2 different analytical platforms.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/assessing-the-effect-of-freezing-and-thawing-of-samples-for-anti-factor-xa-testing-for-the-determination-of-enoxaparin-activity/