Background: Tests for lupus anticoagulant (LA), including silica clotting time (SCT) and diluted Russel's viper venom time (dRVVT) are used to diagnose antiphospholipid syndrome. Due to sample instability, it is recommended that samples are frozen if analysis is postponed >4 h. Shipping on dry ice is common practice to keep samples frozen during transport. Recent data suggest that exposure to dry ice may affect sample pH and results in subsequent analyses. We aimed to determine the effect of dry ice on pH and LA analysis.
Methods: Citrated plasma from eight healthy volunteers was allocated to three preanalytical regimes: 1) storage at -20 °C; 2) dry ice exposure followed by storage at -20 °C; or 3) dry ice exposure followed by storage at -80 °C.
Results: Samples stored at -20 °C after dry ice exposure had significantly lower median pH (1.2 units, p=0.01) and prolonged clotting time ratios (up to 55% for dRVVT tests, p<0.02) in LA analysis, compared to samples not exposed to dry ice. This resulted in poor test specificity (25%). Similar changes were not observed in samples placed at -80 °C after dry ice exposure.
Conclusions: Dry ice may affect sample pH and increase the fraction of false positive LA results. This preanalytical factor should be taken into account by laboratories receiving frozen samples for these tests.