The present invention provides a correlation technique for analysis of changes in bodily fluid and/or tissue in order to identify or monitor appearance, progression or treatment of a disease or condition in a subject. The disclosed method involves measuring spectral properties or changes in bodily fluid and/or tissue of a subject using at least two optical techniques; and correlating the measured properties or changes to a corresponding clinical condition or change in clinical condition, respectively. The measure of spectral changes over time can be used as indicators of changes in the clinical condition, for example, in disease treatment and/or disease regulation. This method is particularly useful for identifying a disease state and for monitoring efficacies of therapies used to treat different diseases or disorders, for example, renal dialysis.