Different markers for different autoimmune diseases
Autoimmune diseases are accompanied by autoantibodies, which are sometimes only seen in a particular disease and can therefore act as a marker for it.
Autoantibodies may be classified as (i) primary pathogenic antibodies, which directly cause a disorder by blocking a normal cellular function or by damaging tissue, or (ii) secondary antibodies, which are not pathogenic in themselves but are produced as a result of the disease. Both types may be used as diagnostic markers.
Sensitivity and specificity of autoimmune markers
The quality of a disease marker is defined by its specificity and sensitivity.
A marker with high specificity occurs in only one disease, and not in related diseases or in the patient's relatives.
This is true of antibodies to tissue transglutaminase (tTG), for example, with a clinical specificity for coeliac disease of up to 100%.
A marker with high sensitivity is detectable in all or most patients with a particular disease.
Tissue transglutaminase antibodies are also a good example of a marker with high sensitivity: about 96% of patients with coeliac disease have a detectable titre of anti-tTG.
A disease marker can be highly specific but quite insensitive, such as antibodies to Sm, which are found in only 10-30% of SLE patients but almost never in other diseases. Conversely, a marker can be highly sensitive, but not very specific, such as antibodies to cardiolipin, which are a marker for antiphospholipid syndrome but are also detectable in other diseases.
EliA blood tests detect autoantibodies in the sera of patients suspected of having autoimmune diseases. The assays are of the highest quality, combining exceptional specificity with the highest sensitivity.