Apparatus and method for determination of tear osmolarity

An apparatus for analyzing a sample of tear fluid, the apparatus comprising:

• a base;
• a cover movable relative to the base;
• a prism fixed to the base, the prism including a prism surface for contacting the tear sample;
• a first illumination means for illuminating an interface between the first surface of the prism and the tear sample with non-parallel rays;
• a photodetector array arranged to receive illuminating light totally internally reflected at the interface;
• a second illumination means for directing a collimated beam at the tear sample along a path perpendicular to the prism surface, the second illumination means including a light-emitting diode;
• and
• a photodetector arranged to receive light from the collimated beam transmitted through the tear sample;
• wherein one of the light-emitting diode and the photodetector is fixed to the cover for movement with the cover relative to the base.

A method for analyzing a sample of tear fluid, the method comprising the steps of:

• A) measuring an optical absorbance of the tear sample;
• B) measuring a refractive index of the tear sample;
• C) determining a protein concentration of the tear sample corresponding to the measured absorbance;
• D) determining a protein refractive index increment corresponding to the protein concentration of the tear sample;
• E) calculating a refractive index of the tear sample due to salinity alone by computing a difference between the measured refractive index of the tear sample and the protein refractive index increment;
• F) determining a salt concentration of the tear sample corresponding to the refractive index of the tear sample due to salinity alone;
• and
• G) converting the salt concentration to an osmolarity of the tear sample.