Those numbers 40 80 160 etc relate to the concentration of the offending auto-immune antibody in the plasma.

The higher the no the higher the antibody concentration.

They are called doubling dilutions - that is the serum is diluted out further and further until the antibody is sufficiently dilute as to be undetected by the relevant serological test. So a serum diluted to 1 part in 1280 but still reactive has a higher level of antibody than someone whose highest (serum) titre is just 1 part in 40 - for instance.

Dx Oct 2006 B27+ undifferentiated spondlyarthropathy (uSpA) with mild sebhorrhoeic dermatitis and mild Inflammatory Bowel Disease (IBD) controlled by NSD since 2007.