Mat, I know about complement proteins C3 and C4 in a research but not clinical context.

They are both on the list of acute-phase response proteins, same as CRP - proteins whose levels are elevated in an acute or chronic infection.

Often a chronic infection has an inflammatory aspect so maybe the possible inflammatory aspect (not the same as an infection) of your primary Sjogren's might explain your elevated C3 and C4 levels - your CRP level is also high(?).

This fits with the inflammatory aspect of OA - C3 and C4 could be elevated in it but PubMed does not shed any light on this.

Thanks for clarifying about compliments Kiwi. Brain shot from insomnia but yes my CRP and PV (Plasma Viscocity) were all high at the last rheum clinic two months ago when compliments were tested. GP was right about the yeast infection being a possible cause but the symptoms have continued regardless of next swab being negative for candida. And my inflammatory markers are always between raised and high. The only time they come down is with immunesuppression or steroids. I'm blaming Sjogren's for everything now - which feels rather good!