Limitations of Diagnostics for Autoimmune Disease Biomarkers
Author: Karolina Künnapuu
Dysfunction of the immune system can lead to autoimmune disease, in which the body’s defence systems target its own tissues. Over 80 autoimmune diseases have been identified to date and an estimated 5-8% of the world’s population suffers from an autoimmune disease (1), causing socioeconomic strain (2) and arousing an urgent need for prompt detection and treatment of these diseases.
Unfortunately, the aetiology of autoimmune diseases is not entirely clear and indicators of disease are shared across various autoimmune diseases, making diagnosis and treatment difficult. The greatest limitation of currently available diagnostic tests is that they lack sensitivity and/or specificity, which means patients endure a variety of laboratory tests, symptom evaluation and tests like ultrasound and tissue biopsies before healthcare providers can arrive at a diagnosis (3).
Extensive testing can be cost- and time-intensive and is stressful for the patient. Improving on the limitations of tests that are currently in use would lead to autoimmune diseases being diagnosed more quickly and conveniently and allow for more effective treatment, slowing disease development and raising patients’ quality of life.
Figure 1: Examples of autoimmune diseases affecting different organ systems (4)
Biomarkers of inflammation
Autoimmune diseases are associated with changes in the serum levels of various inflammatory proteins, such as C-reactive protein (CRP), fibrinogen, ferritin and haptoglobin (4), meaning that tests that detect the presence of these proteins can be used to detect signs of immune system dysfunction. A change in inflammatory serum protein levels, however, is not specific to autoimmune disease and is simply an indicator of the presence of inflammation. An increase in ferritin, for example, is associated with juvenile arthritis, but also iron overload diseases (5).
The presence of inflammatory biomarkers is thus rarely reason enough to diagnose a patient with an autoimmune disease. Furthermore, the levels of inflammatory proteins indicated by these tests may not accurately reflect a patient’s disease status. One study found that 44% of rheumatoid arthritis (RA) patients in Jyväskylä, Finland, and 58% of RA patients in Nashville, United States, had CRP levels under the disease threshold of 10mg/L (6). This suggests that the sensitivity of the tests is not ideal.
Antinuclear antibody (ANA) test
An antinuclear antibody (ANA) test is commonly the first-line diagnostic test upon suspicion of autoimmune disease, particularly systemic lupus erythematosus (SLE) and rheumatic diseases. ANAs are a group of autoantibodies that associate with components of the cell nucleus. Some ANAs are associated with specific autoimmune diseases, while others are elevated in various diseases (5). ANA presence can be detected through analysis of a variety of ANA assays, including immunofluorescence, ELISA, and multiplex assays.
Although widely used as a diagnostic test, ANA assays are associated with disadvantages that make diagnosis challenging. Not only do up to 20-30% of healthy individuals falsely test positive for an ANA, but false-negative results among people with autoimmune disease are also relatively common and some disease biomarker autoantibodies, such as those for myositis, may not be detected using certain ANA assays (7). The reason for the high number of positive tests in healthy people is not known, but possible reasons involve cross-reactivity and binding to protein DFS70, which is commonly present in healthy individuals (7). Due to ANA tests’ unreliable results, further testing is frequently needed in addition to further symptom evaluation to reach a diagnosis. Even so, incorrect diagnoses based on ANA results are not uncommon (8).
Other autoantibody tests
Rheumatoid factor (RF) and antibodies against cyclic citrulline peptide (CCP) are also frequently tested for in suspected autoimmune disease patients. While commonly associated with rheumatoid arthritis, elevated levels of these two autoantibodies are also present in other autoimmune diseases (e.g. Sjögren’s syndrome, lupus) as well as some infectious and pulmonary diseases (5). Interpreting test results is also made difficult by the fact that autoimmune diseases present differently in different individuals; some patients may test positive for both anti-CCP antibodies and RF, while others may only test positive for one or the other (9). RF and anti-CCP tests have similar sensitivity (around 50-70%), while anti-CCP tests have higher specificity (95% as opposed to 85% for RF) (10). While autoantibody assays can be useful in determining the presence of disease biomarkers in the body, the fact that these biomarkers are absent in many patients (10) means that testing must always be combined with an evaluation of other potential signs of disease as a negative test result may not be reliable.
Conclusion
Although various biomarkers for autoimmune diseases exist, testing for autoimmune diseases can be a time and resource-demanding process. Currently available assays lack either sensitivity and specificity and extensive testing in addition to symptom evaluation are often needed to reach a reliable diagnosis. Improved diagnostic tests that exhibit both sensitivity and specificity would reduce the time and cost of diagnosing autoimmune diseases, allowing diseases to be detected early and making treatment more effective.
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