Abstract: The invention provides a method of diagnosing arthritis or other joint degrading disease in a subject which comprises determining whether there is a presence or increase of lubricin having a joint tissue posttranslational modification, in a blood sample from the subject, the presence or increase of the lubricin having the joint tissue posttranslational modification indicating arthritis or other joint degrading disease in the subject. The invention further provides a kit or protocol for detecting arthritis or other joint degrading disease by detecting lubricin, the lubricin comprising a joint tissue posttranslational modification.