Is there a reason to repeat HMGCR antibody level for monitoring disease activity once documented positive in patients with IMNM?   

HMGCR antibody persists even when the disease is quiescent, and levels of the antibody correlate with the log of the CPK levels. Therefore, it is not a very sensitive marker for disease activity, so it is not a useful marker to follow longitudinally. Instead, CPK is a cheaper and more sensitive mark...

This is a really great question, but in large part depends heavily on (1) the quality/reproducibility of the HMG CR antibody testing available to the clinician and (2) (possibly) statin exposure history.In one of the early papers looking at treatment of HMGCR autoimmune myopathy by the Hopkins group...