EQA: There's not a glitch in the matrix. Investigation of CRP bias on the Roche Cobas c701.

Background: The Roche Cobas c Tina-quant C-Reactive Protein (CRP) IV method has shown persistent, significant negative bias compared to other methodologies in the UK NEQAS external quality assurance (EQA) scheme. The aim of this study was to compare CRP concentrations determined by this restandardized 4th generation assay with another commercially available method to assess the extent of any bias and impact on clinical cut-offs.

Methods: CRP concentrations in 38 anonymized patient serum samples were determined by a particle-enhanced immunoturbidimetric assay on the Roche Cobas c701 module and a latex assay on the Beckman Coulter AU5800 analyser over a concentration range of 1-308 mg/L.

Results: 97.4% samples analysed by the Roche Tina-quant CRP IV method demonstrated a negative bias compared to the Beckman AU latex method. The mean absolute bias was -4.12 mg/L (range: -19.13-10.93 mg/L; 95% CI: -17.25-9.01 mg/L). The mean relative difference was -15.5% (range: -40-4%; 95% CI: -33.03-1.94%). This bias was seen across the range of samples assayed, including at clinically significant cut-offs of 5 mg/L (-24% bias), 20 mg/L (-5%) and 100 mg/L (-13%).

Conclusion: The negative bias of the Roche method demonstrated in the EQA scheme appears to reflect genuine differences in patient results, rather than an EQA matrix effect, despite re-standardization of the CRP assay. This report indicates that clinical cut-offs and reference ranges may be method-dependent, which should be reflected in published guidance and interpretation provided by laboratories.