The stability of ‘add-on’ coagulation assays in refrigerated citrated plasma stored on a packed cellular fraction

Introduction: Haematology laboratories are increasingly faced with requests for add-on coagulation testing. This study explores extending the specimen storage proposals by examining coagulation parameters on refrigerated citrated plasma retained on a cellular fraction over a 24-hour period. Methods: Sodium citrate (Sarstedt^® S-Monovette 3.2%) specimens from 206 patients in University Hospital Limerick, Ireland were refrigerated immediately post-analysis and re-analysed in the centrifuged primary container at 4, 8 and 24-hour intervals using the Diagnostica Stago coagulometer and reagent combination. Coagulation assays examined for statistically and clinically significant differences included PT, APTT, D-Dimer, fibrinogen and Protein C. Results: PT, APTT and Protein C values displayed statistical significance from 4 hours. Fibrinogen differences were statistically significant from 8 hours. D-Dimer differences were not statistically significant at any interval over the 24-hour period. The refrigerated storage limit for PT and APTT results was determined to be 4 hours. D-Dimer was the only test parameter to report a mean percentage variance >10%. However, result changes at the threshold region of 0.5 µg/mL FEU were found to be within assay precision limits and desirable bias up to 8 hours. Maximum mean differences for Protein C (−1.3%) and fibrinogen (2.3%) were within assay precision limits and desirable biases up to 24 hours. Conclusion: PT and APTT results are stable in refrigerated citrated plasma maintained on a cellular fraction up to 4 hours post-phlebotomy. D-Dimers results are reliable up to 8 hours, while fibrinogen and Protein C results are stable for at least 24 hours.