Evaluation of INSTAND e.V.'s external quality assessment for C-reactive protein and procalcitonin

Harmonization status of procalcitonin measurements: what do comparison studies and EQA schemes tell us?

Sepsis represents a global health priority because of its high mortality and morbidity. The key to improving prognosis remains an early diagnosis to initiate appropriate antibiotic treatment. Procalcitonin (PCT) is a recognized biomarker for the early indication of bacterial infections and a valuable tool to guide and individualize antibiotic treatment. To meet the increasing demand for PCT testing, numerous PCT immunoassays have been developed and commercialized, but results have been questioned. Many comparison studies have been carried out to evaluate analytical performance and comparability of results provided by the different commercially available immunoassays for PCT, but results are conflicting. External Quality Assessment Schemes (EQAS) for PCT constitute another way to evaluate results comparability. However, when making this comparison, it must be taken into account that the variety of EQA materials consist of different matrices, the commutability of which has not yet been investigated. The present study gathers results from all published comparison studies and results from 137 EQAS surveys to describe the current state-of-the-art harmonization of PCT results. Comparison studies globally highlight a significant variability of measurement results that nonetheless seem to have a moderate impact on medical decision-making. For their part, EQAS for PCT provides highly discrepant estimates of the interlaboratory CV. Due to differences in commutability of the EQA materials, the results from different peer groups could not be compared. To improve the informative value of the EQA data, the existing limitations such as non-harmonized conditions and suboptimal and/or unknown commutability of the EQA materials have to be overcome. The study highlights the need for commutable reference materials that could be used to properly evaluate result comparability and possibly standardize calibration, if necessary. Such an initiative would further improve the safe use of PCT in clinical routine.

Development of a primary reference material of natural C-reactive protein: verification of its natural pentameric structure and certification by two isotope dilution mass spectrometry

C-reactive protein (CRP) is one of the most commonly used biomarkers for inflammation. The standardisation of a procedure for the detection of CRP has attracted significant attention globally, and primary reference materials of CRP based on the recombinant expression of E. coli that exist in the form of monomers have been developed. However, a primary reference material of natural CRP is still required to achieve the exact matching of CRP measurements in secondary reference materials (e.g. CRP in frozen human serum). Herein, the development process for a certified reference material of natural CRP is reported, namely GBW09228. The raw material employed in this study was CRP extracted and purified from human body fluid, and exhibits a natural and verified pentameric structure. Through the use of amino acid analysis isotope dilution mass spectrometry (AAA-IDMS) and signature peptide-IDMS, this reference material was certified, and its certification results can be traced to SI units. The developed method was evaluated for its accuracy using the international comparison tests of the National Metrology Institute of Japan (NMIJ) and the Korea Research Institute of Standards and Science (KRISS). Overall, a CRP primary certified reference material (CRM) of well-characterised purity was determined that could be used to calibrate an IDMS-based reference method, that could then be used to assign target values to secondary CRMs. These secondary CRMs could in turn be used to calibrate and verify the accuracy of immunoassays, thereby giving a good foundation for establishing a complete traceability chain for CRP.

Two-center comparison of 10 fully-automated commercial procalcitonin (PCT) immunoassays

Background This two-center study was designed to verify comparability of procalcitonin (PCT) values among 10 different commercial immunoassays. Methods A total number of 176 routine lithium-heparin plasma samples were divided in identical aliquots and simultaneously analyzed with 10 different PCT immunoassays, including Kryptor BRAHMS PCT sensitive, Abbott Architect BRAHMS PCT, Beckman Coulter Access PCT (on Access and DXI), BioMérieux Vidas BRAHMS PCT, Diasorin Liaison BRAHMS PCT, Fujirebio Lumipulse G BRAHMS PCT, Roche BRAHMS PCT (on Cobas E801), Diazyme PCT (on Roche Cobas C702) and SNIBE Maglumi PCT. Results Highly significant correlation was always found across multiple comparisons, with correlation coefficients comprised between 0.918 and 0.997 (all p < 0.001). Bland and Altman plots analysis revealed highly variable bias among immunoassays, ranging between ±0.2% and ±38.6%. Diazyme PCT on Roche Cobas C702 and SNIBE Maglumi PCT displayed the larger overestimation, whilst PCT values were underestimated by Cobas BRAHAMS PCT. The agreement was always >80% (all p < 0.001), but varied largely across multiple comparisons, ranging between 90%-99% at 0.1 μg/L, 81%-99% at 0.25 μg/L, 83%-100% at 0.5 μg/L, 94%-100% at 2.0 μg/L and 90%-99% at 10 μg/L, respectively. The larger disagreement was observed comparing Diazyme PCT and Maglumi PCT with the other methods. Conclusions Although we found acceptable correlation among 10 commercial PCT immunoassays, the limited agreement at clinical decision thresholds remains a major issue, especially at lower end of PCT concentration, thus potentially contributing to jeopardize the clinical value of this biomarker.