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

IntroductionThe 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/2SDAnalytical2+2SDBiological2. 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.MethodscTn was measured in 575 sample pairs on the Abbott Architect cTnI, Beckman Coulter Access2 cTnI, Roche Cobas cTnT and Siemens ADVIA Centaur cTnI methods.ResultsGood 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).ConclusionFixed 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.

► We compared actual differences, % differences and z-values across 4 platforms.
► Z-values are universal across platforms for both cTnI and cTnT.
► Actual and % Δ cut-offs depend on mean cTn levels and the analytical method.
► We provide an explanation for the poor diagnostic performance of fixed % Δ's.