Autoantibodies as autoimmune markers
An early reliable diagnosis is essential for a helpful treatment of autoimmune diseases. But especially connective tissue diseases are difficult to diagnose, because their symptoms vary widely and may be misleading.
Autoimmune diseases are accompanied by autoantibodies, which sometimes are only seen in this special disease - thus these autoantibodies can serve as a marker for the disease. If an antibody is a disease marker it is not absolutely necessary also a pathogenic antibody. Often the antibodies are a consequence of the disease, not the trigger.
Sensitivity and specificity of autoimmune markers
The quality of a disease marker is defined by specificity and sensitivity.
High specificity is given for a marker which does only occur in this one disease, not in related diseases nor in relatives of the patient. This is true for example for antibodies to tissue transglutaminase with a clinical specificity for celiac disease of up to 100%.
A marker with high sensitivity is detectable in all or most patients with a special disease. Again tissue transglutaminase antibodies is a good example for a marker with high sensitivity: about 96% of patients with celiac disease have a detectable titer of anti-tTG.
A disease marker can be highly specific but quite insensitive, like for example antibodies to Sm, which are found in only 10-30 % of SLE patients but almost never in other diseases.
The other way round a marker can be highly sensitive, but not too specific - like antibodies to cardiolipin, which are a marker for the antiphospholipid syndrome but also are detectable in other diseases.
Phadia products are tests for the detection of autoimmune markers in sera of patients suspected to have autoimmune diseases. The assays are of highest quality guaranteeing high sensitivity and specificity.