Implementing metrological traceability of C-reactive protein measurements: consensus summary from the Joint Committee for Traceability in Laboratory Medicine Workshop

The Joint Committee for Traceability in Laboratory Medicine (JCTLM) currently lists the secondary commutable certified reference material (CRM) ERM DA-474/IFCC (DA-474) “C-Reactive Protein in Human Serum” and two generic immunoassay-based method principles as the basis for implementing the metrological traceability of C-reactive protein (CRP) measurements by end-user measurement procedures used by medical laboratories. The current metrological traceability has produced well harmonized results for clinical samples among different end-user measurement procedures. New higher-order pure substance and secondary commutable CRMs have been nominated for listing by the JCTLM. However, the data supporting performance of these new candidate CRMs, including use of new mass spectrometry based candidate reference measurement procedures (RMPs), was not clear regarding the influence that introducing these new CRMs would have on the current well harmonized results achieved with the existing metrological traceability to DA-474. The clinically relevant CRP measurand in blood serum or plasma is a pentamer of identical subunits, which adds complexity to the application of higher-order CRMs and RMPs. The JCTLM convened a workshop in December 2022 to review the appropriate implementation of metrological traceability of CRP measurements. The workshop consensus was that the extent-of-equivalence data must include considerations about the impact of a new CRM when used for its intended purpose in the calibration hierarchies of existing end-user measuring systems; and that a new RMP must compare results with another existing well validated candidate RMP or with a globally available end-user measurement system.