One aspect of the invention provides a method of diagnosing a disease condition, comprising measuring presence or amount of a targeted protein or a degradation product of said protein in a collected biological sample as a marker for the disease condition. The targeted protein or degradation product is selected for measurement based on a prior identification of a measurable half-life at a predetermined time period, including the time at which said method is conducted, and correlating said measuring with the presence or absence of the disease condition. The targeted protein or degradation product may be identified by selecting a protein known or suspected to be a diagnostic marker for the disease condition, analyzing degradation of the protein in the collected biological sample, and selecting a protein or degradation product that exhibits a measurable half-life at a predetermined period of time. The analyzing may include identifying degradation product(s) of the protein as a function of time, and half-life of the protein and the degradation product(s).