Autoantibodies as autoimmune markers
An early reliable diagnosis is essential for a helpful treatment of autoimmune diseases. Connective tissue diseases are difficult to diagnose, because their symptoms vary widely and may be misleading.
Antibodies are disease markers and are often not pathogenic. In other words, antibodies are often a consequence of the disease and 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 specific disease. Tissue transglutaminase antibodies is a good example of 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. For example, antibodies to Sm are found in only 10-30 % of SLE patients but almost never in other diseases.
Autoimmune markers can be highly sensitive, but not too specific. For example, antibodies to cardiolipin are markers for the antiphospholipid syndrome but also are detectable in other diseases.
Phadia, now Thermo Fisher Scientific, products are tests that detect autoimmune markers in sera of patients suspected to have autoimmune diseases. The assays are of highest quality and guarantee high sensitivity and specificity.