The disclosure relates to methods for identifying a subject having elevated CD200 levels and/or comprising cells overexpressing CD200, the method comprising the step of assaying a biological fluid from the subject to determine a level of soluble CD200, wherein a level above control indicates the subject has elevated CD200 levels. Diagnostic methods, methods of monitoring prognosis and methods of medical treatment relating to CD200 associated medical conditions are also provided. The disclosure also provides assays and kits useful in the diagnosis of a medical condition associated with elevated CD200 and/or comprising cells overexpressing CD200.