What is your approach to monitoring a patient with SLE who has persistent hypocomplementemia on hydroxychloroquine, but clinically has resolution of cutaneous symptoms and no other internal organ manifestations of disease?  

Per the subtitle under your question, an undetectable C4 with low C3 makes me wonder about the possibility of a homozygous C4 deficiency, of whom 75-80% of patients have SLE with a high occurrence of lupus nephritis (Yang et al., PMID 15294999). You can consider genetic testing (primarily for geneti...

I agree with Dr. @Thomas's nice explanation. I would just add a few things. Due to copy number variation we see many patients with low C4 without any contribution to their disease activity. Unless we had prior normal values, it's hard to distinguish these individuals. Also complement deficiency (C4...