A Common Calibrator Does Not Secure Harmonisation of Commercial t-PA and PAI-1 Antigen Measurements

There is no common standardisation of different commercially available kits for both t-PA and PAI-1 antigen.

The aim of this project was to study whether the exchange of the kit calibrator with the common calibration materials of the WHO would harmonise the results produced by five different commercially available t-PA and PAI-1 antigen kits when analysing the SSC secondary standard.

WHO international standards were used as calibrator and the SSC secondary standard and a commercially available plasma standard were used as test plasma in 5 commercially available kits measuring total t-PA and PAI-1 antigen. For t-PA only, the SSC secondary standard was spiked with purified t-PA and recovery was studied.

There was a large variation in the concentrations of t-PA antigen (ranging from <0.5 to 6.6 ng/ml for the SSC secondary standard and from 3.3 to 10.9 ng/ml for the commercial plasma standard, respectively) produced by the different kits. Also, PAI-1 antigen results of the different kits showed a large variation (ranging from 20.3 to 51.2 ng/ml for the SSC secondary standard and from 41.8 to 89.7 ng/ml for the commercial plasma standard, respectively). Results of the two test samples and spiking with t-PA were not in agreement in all methods, indicating differences in specificity of tests. Data point to a specific effect of the matrix of standards.

The use of a common calibration material does only marginally harmonise data for t-PA and PAI-1 antigen assays. There is a need for improvement of methods to cope with standards and standardisation.