PB1148 - Frozen plasmas can replace fresh plasmas in ISI calibrations for secondary thromboplastin standards for multiple reagent/instrument combinations

Background: Accurate ISI calibrations are vital for accurate INRs. Thromboplastins are calibrated against international reference thromboplastins. WHO state a minimum requirement for primary and secondary standard calibration of 20 normal samples and enough patient samples to ensure 55 remain after outlier exclusions, with testing in at least 5 sessions. Samples must be tested within 5 hours after collection. For efficient use of reference thromboplastins, testing is normally done in up to 10 sessions, which is reliant upon collecting enough suitable patient samples in a short time. As vitamin K antagonist (VKA) use decreases it is increasingly problematic to recruit 6-12 suitable patient samples in our centre for a testing session within the relevant time frame.

Aims: To determine whether samples can be frozen and thawed without affecting ISI calibrations.

Methods: Plasma was prepared from citrated blood (Vacutainer Plus, Becton Dickinson). A fresh and frozen sample ISI calibration was run for a variety of reagent/analysers. Some calibrations used the same samples before and after freeze/thaw. Others were performed on a different sample set (table 1). Samples were frozen at -800C for < 3 months and thawed for 3-5 mins at 370C prior to testing. In all cases the interval between collection and completion of testing was < 5 hours, excluding the time spent frozen.

Results: Using frozen/thawed plasmas to replace fresh plasmas for ISI calibration created < 6% and < 3% difference in ISI for different samples and same samples respectively (Table 1). All criteria for valid calibrations were met when frozen/thawed plasma replaced fresh, with no significant effect on number of outliers or CV of slopes (table 2).

Conclusion(s): Freezing samples did not have a significant effect on ISI or MNPT of multiple test systems and would optimise sample recruitment whilst maintaining efficient use of reference thromboplastins if VKA use reduces further.