Using soybean trypsin inhibitor as an external loading control for Western blot analysis of tear proteins: Application to corneal disease

► Soybean trypsin inhibitor used as external loading control in Western blotting. ► Appropriate for relative protein quantification of biological fluids (e.g. tears). ► High reproducibility with a coefficient of variation <10% was observed. ► Confirmed the presence of secreted frizzled protein 1 (SFRP1) in human tears. ► Relative level of tear SFRP1 was less in keratoconus patients than controls.