The relationship between total tear protein concentrations determined by different methods and standards

Background: It is known that the values of total protein concentration (TPC) of human tears obtained by different methods/standards vary considerably. The variations are due to differences in reactivity of proteins to the methods used. The aim of this study was to investigate the relationships and agreements between TPC values obtained by different methods/standards. Methods: Two experiments were conducted. In experiment 1, 30 normal subjects were recruited and their TPC were determined by the Bradford and the modified Lowry methods, with bovine serum albumin and bovine immunoglobulin G as standards. The relationships (equations) between TPC values obtained by different methods/standards were determined. In experiment 2, another 15 normal subjects were recruited and the TPC were measured as in experiment 1. Measured values of TPC using different methods/standards were compared with their respective values calculated from the equations defined in experiment 1. The agreement and the reliability between the measured and the calculated values were investigated. Results: Experiment 1 showed that the TPC obtained by different methods/standards were strongly correlated to one another, and their relationships can be defined by linear equations. In experiment 2, the measured and the calculated values, using the equations defined in experiment 1, showed good agreement and reliability. This indicates that TPC obtained by one method/standard can be used to estimate the value(s) that would be obtained if another method/standard were used using the equation(s) found in experiment 1. Conclusion: The correlations between TPC values obtained by different methods/standards were strong. Our results also indicated good agreement and reliability between measured and calculated TPC values. Therefore, values obtained by different methods/standards are interchangeable using the appropriate equations.