A system and method to distinguish normal cells from cells having undergone a biochemical change. A pre-determined vector space is selected where the vector space mathematically describes a first plurality of reference spectral data sets for normal cells and a second plurality of reference spectral data sets for cells having undergone a biochemical change. A sample is irradiated to generate a target spectral data set based on photons absorbed, reflected, emitted, or scattered by the sample. The target spectral data set is transformed into a pre-determined vector space. A distribution of transformed data is analyzed in the pre-determined vector space. Based on the analysis, the sample is classified as containing normal cells, cells having undergone a biochemical change, and combinations thereof. The method includes treating the sample with a pharmaceutical agent prior to irradiating the sample and using the classification to assess the efficiency of the pharmaceutical agent.