Serial cardiac troponin differences measured on four contemporary analyzers: relative differences, actual differences and reference change values compared.

INTRODUCTION: The diagnosis of myocardial infarction is in part predicated on a rise and/or fall in cardiac troponin (cTn). z-Values incorporate analytical and biological variation to standardize serial differences: z=Δ/√[2SD²(Analytical) + 2SD²(Biological)]. We investigated the theoretical distributions of actual differences (Δ), relative differences (%Δ) and z-values and compared the agreement in classification of differences measured on four contemporary platforms.
METHODS: cTn was measured in 575 sample pairs on the Abbott Architect cTnI, Beckman Coulter Access2 cTnI, Roche Cobas cTnT and Siemens ADVIA Centaur cTnI methods.
RESULTS: Good agreement was obtained amongst all methods with a universal z-value cut-off (κ>0.79) and method specific fixed Δ cut-offs (κ>0.82) while suboptimal agreement was obtained between cTnI and cTnT with fixed %Δ cut-offs (κ<0.50).
CONCLUSION: Fixed Δ cut-offs will perform well at low cTn concentrations while fixed %Δ cut-off values are predicted to perform poorly. z-Values are independent of the cTn concentration, present differences as a continuum of probability and a universal decision level can be used for all cTnI and cTnT methods. Crown