Comprehensive Agreement Analysis of Colorimetric and Turbidimetric Total Protein Assays in Cerebrospinal Fluid

Background/Objectives: Accurate measurement of total protein (TP) in cerebrospinal fluid (CSF) is crucial for diagnosing various neurological conditions. This study aims to evaluate the concordance between a routine colorimetric method and a recently intro-duced turbidimetric method for measuring CSF TP. Methods: We measured 161 CSF samples using both methods, analysing the whole population and two subgroups: normal (≤500 mg/L) and pathological (>500 mg/L). Agreement was assessed using Lin’s Con-cordance Correlation Coefficient (CCC), Bland-Altman, and Deming regression, while clinical concordance was determined with Cohen’s Kappa. Results: The concentrations obtained from the two methods did not differ significantly and were well-correlated across the population and subgroups. The CCC for the entire dataset was 0.9881 (substantial agreement), while the Bland-Altman analysis showed a mean bias of 4.467 mg/L. For the “normal” subgroup (n=97), the CCC was 0.8722 (poor agreement), with a mean bias of 7.668 mg/L. In the “pathological” subgroup (n=64), the CCC was 0.9858 (substantial agreement) with a mean bias of -3.838 mg/L. Demin regression did not show statistically significant proportional or constant bias in the whole population. However, a stratified analysis revealed a significant negative constant bias in the “normal” subgroup in ab-sence of significant bias in the “pathological” subgroup. Cohen’s kappa was 0.804, in-dicating substantial agreement. Conclusions: Both methods showed substantial agree-ment for quantifying CSF TP and clinical classification, supporting their potential in-terchangeability for diagnostic purposes. Nonetheless, laboratories should note the presence of bias, particularly for samples near the clinical cut-off value.