The present invention provides a method of determining, in a sample, the proportion of a total amount of a molecule that is derived from catabolism due to the presence of age-related molecular alterations on the molecule, comprising: a) determining the total amount of the molecule in the sample; b) determining the amount of the molecule in the sample that contains D-aspartate; and c) calculating the proportion of the amount of the molecule of step (b) relative to the total amount of the molecule as determined in step (a), thereby determining the proportion of the total amount of the molecule that is derived from catabolism due to the presence of age-related molecular alterations in the molecule. Further provided is a method of diagnosing a musculoskeletal, arthritic or joint disorder in a subject and/or identifying a subject at risk for developing such a disorder, comprising: a) measuring an amount of D-aspartate and/or an advanced glycation end product in a sample of the subject; and b) comparing the amount of D-aspartate and/or advanced glycation end product in the sample of (a) with an amount of D-aspartate and/or advanced glycation end product in a sample of a control subject, whereby an increased amount of D-aspartate and/or advanced glycation end product in the sample of the subject as compared to the amount of D-aspartate and/or advanced glycation end product in the sample of the control subject is diagnostic of a musculoskeletal, arthritic or joint disorder in the subject and/or identifies a subject at risk of developing such a disorder.
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