## 4 Conclusions
Since, as previously mentioned, biological samples have an extremely complex matrix, there is a high risk that other proteins contained in them will interfere with the correct measurement of the concentration of the protein selected by the analyst. Therefore, a necessary stage of validation was testing the selectivity of the proposed method. For this purpose, solutions of the studied analytes and the most important potential interferents, which were proteins with a similar structure and function, as well as human albumin, were prepared. The latter constitutes as much as 50% of all proteins circulating in the peripheral blood. The mean values of the concentrations were 51.52 \(\pm\) 0.94 pg/mL for VEGF-A and 11.08 \(\pm\) 0.67 pg/mL for FGF-2 and were close to the theoretical (real) concentrations in the samples (50.00 pg/mL VEGF-A; 10.00 pg/mL FGF-2). The recoveries and the CVs were calculated using the mean concentration values and standard deviations. The recoveries for both proteins did not exceed 112%, while the CVs ranged from 1.82% to 6.05%. The results obtained for the analytical parameters prove the high selectivity of the proposed method and suggest that the complexity of the biological sample matrix does not adversely affect the measurement result. The repeatability tests of the proposed method were based on five-fold determination of the analyte in a standard solution of precisely known concentration and in a real sample. The values of recoveries were calculated and lay in the range 98.60-107.35%, while the coefficients of variation (CV) were in the range 0.82-7.65%. The results confirm the high repeatability of the developed analytical method. Finally, to verify the correctness and reliability of the obtained results, the results of the determinations obtained using the developed method were compared with those obtained using commercial ELISA tests. The results underwent an appropriate statistical analysis based on the Mann-Whitney U test. The lack of statistical significance (\(p>0.05\)) of the above-mentioned test indicates the absence of differences between the median concentrations obtained with the use of the SPRi biosensor and the ELISA test. This confirms the correctness and reliability of the results obtained by means of the newly developed method. Conceptualization, L.O. and E.G. ; methodology, L.O. and E.G. ; validation, L.O., A.L., and B.Z.-R.; formal analysis, L.O., E.G., Z.L., and E.L.; investigation, L.O., A.L., and B.Z.-R.; resources, E.L. and M.K.-R.; data curation, L.O. ; writing--original draft preparation, L.O. ; writing--review and editing, E.G., Z.L., and M.K.-R.; visualization, L.O., A.L., B.Z.-R., and E.L.; supervision, E.G. ; project administration, E.G. All authors have read and agreed to the published version of the manuscript. This research was funded by the Dean of the Faculty of Chemistry at the University of Bialystok as part of a competition for small research projects. **Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board (or Ethics Committee) of the Medical University of Bialystok (protocol code APK.002.243.2022). **Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study. **Data Availability Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflict of interest. **Abbreviations**  
AML acute myeloid leukaemia  
CML chronic myeloid leukaemia  
HCL hairy-cell leukaemia  
CLL chronic lymphocytic leukaemia  
ALL lymphocytic leukaemia  
MM multiple myeloma  
NHL non-Hodgkin lymphoma  
CMML chronic myelomonocytic leukaemia  
MDS myelodysplastic syndromes  
\(\mathrm{C_{RM}}\) is the concentration of reference material obtained from standard solutions. It is not a certified reference material. \(\mathrm{X_{C}}\) mean concentration of the analyte  
**Appendix A**  
**Table A1. ** Details for determining the selectivity of the analytical method.