Systems and methods for determining the osmolarity of a sample are provided. Aspects of the subject methods include contacting a sensing surface of a surface plasmon resonance based sensor with a sample, and generating one or more data sets at at least two wavelengths over a time interval, wherein the data sets are used to determine the osmolarity of the sample. The subject methods find use in determining the osmolarity of a sample, such as a biological sample (e.g., a tear fluid), and in the diagnosis and/or monitoring of various diseases and disorders, such as, e.g., dry eye disease.