Variability of tear protein levels in normal young adults: between-day variation

Background: An adequate knowledge of physiological variation is important f ur valid comparative studies of tear proteins. The aim of this study was to investigate the between-day variation of the human tear protein levels, in cluding the total protein concentration (TPC and the levels of major protei n fi actions. Two sampling methods, the yawn and the eye-flush methods, wer e used and compared. Methods: TPC was determined by the Bradford method. Th e major protein fractions were separated by sodium dodecyl sulfate-polyacry lamide gel electrophoresis (SDS-PAGE) and their levels were determined by s canning densitometry after SDS-PAGE, The tear protein levels were monitored fur 3 days. Results: The between-day differences in the levels of TPC and the individual protein fractions were not statistically significant in eith er sampling method, but the variations of some proteins were large and woul d be clinically significant. Different variations were observed in differen t proteins. The variations in serum albumin were large, up to 61% and 113% in the yawn and eye-flush methods respectively. The variations in lactoferr in, tear-specific prealbumin and lysozyme were relatively small in the yawn method. The variations in protein levels obtained by the eye-flush method were generally higher than by the yawn method. Conclusion: Although the bet ween-day differences in tear protein levels were not statistically signific ant, the variations in some proteins would be large in magnitude. The varia bility of tear protein levels obtained by the eye-flush method was larger t han that by the yawn method. Therefore, caution should be taken if the eye- flush method is used for sampling tears for quantitative analysis of tear p roteins, although it is easier to collect tear samples using this method. T he rt suits will be useful to exclude normal variation in tear protein leve ls when comparing pre- and post-therapeutic tear protein levels in eyes tre ated for tear-related abnormalities.