C-reactive protein is a well-studied host response biomarker, whose diagnostic performance depends on its accurate classification into concentration zones defined by clinical scenario-specific cutoff values. We validated a newly developed, bead-based, bound-free phase detection immunoassay (BFPD-IA) versus a commercial CE-IVD enzyme-linked immunosorbent assay (ELISA) kit and a commercial CE-IVD immunoturbidimetric assay (ITA) kit. The latter was performed on a fully automated DPC Konelab 60i clinical analyzer used in routine diagnosis. We classified 53 samples into concentration zones derived from four different sets of cutoff values that are related to antibiotic prescription scenarios in the case of respiratory tract infections. The agreements between the methods were ELISA/ITA at 87.7%, ELISA/BFPD-IA at 87.3%, and ITA/-BFPD-IA at 93.9%, reaching 98-99% in all cases when considering the calculated relative combined uncertainty of the single measurement of each sample. In a subgroup of 37 samples, which were analyzed for absolute concentration quantification, the scatter plot slopes' correlations were as follows: ELISA/ITA 1.15, R2 = 0.97; BFPD-IA/ELISA 1.12, R2 = 0.95; BFPD-IA/ITA 0.95, R2 = 0.93. These very good performances and the agreement between BFPD-IA and ITA (routine diagnostic), combined with BFPD-IA's functional advantages over ITA (and ELISA)-such as quick time to result (~20 min), reduced consumed reagents (only one assay buffer and no washing), few and easy steps, and compatibility with nucleic-acid-amplification instruments-render it a potential approach for a reliable, cost-efficient, evidence-based point-of-care diagnostic test for guiding antibiotic prescriptions.

BioVendor Laboratorní Medicína a s Research and Diagnostic Products Division Karasek 1767 1 Reckovice 62100 Brno Czech Republic

Department of Biomedical Sciences Faculty of Medicine University of Ostrava Syllabova 19 70300 Ostrava Czech Republic

Faculty of Pharmacy Masaryk University Palackeho trida 1946 1 61242 Brno Czech Republic

Hahn Schickard Georges Koehler Allee 103 79110 Freiburg Germany

Institute of Laboratory Diagnostics University Hospital Ostrava 17 listopadu 1790 5 70800 Ostrava Czech Republic

Institute of Virology Freiburg University Medical Center Faculty of Medicine University of Freiburg Hermann Herder Strasse 11 79104 Freiburg Germany

Laboratory for MEMS Applications IMTEK Department of Microsystems Engineering University of Freiburg Georges Koehler Allee 103 79110 Freiburg Germany

Service of Infectious Diseases Lausanne University Hospital University of Lausanne Rue du Bugnon 46 1011 Lausanne Switzerland

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