Antinuclear Antibodies (ANAs)
The antinuclear antibody (ANA) test was developed to help in the diagnosis of Systemic Lupus Erythematosus (SLE).
While ANA is a test that is almost 100 percent sensitive for SLE, its specificity is less reliable. That is to say that there is a significant false positive rate. The reason that children appear to have more false positive results than adults is not clear, but it may relate to the frequency of viral infections in childhood. There seems to be a link between recurrent strep throats with positive ASO titres and false positive ANA tests.
There is new technology in the community in the performance of ANA tests. The classic ANA is a labor intensive test. Therefore, many commercial laboratories will screen sera with a less laborious EIA (Enzyme Immunoassay). Positives are confirmed and titres are performed with the traditional IFA (immunofluorescent antibody) method. If the results are positive by EIA and negative by IFA, the correct interpretation is that the patient’s serum is negative.
Titres of ANA less than 1:160 are not clinically significant. Also the reported pattern of the fluorescence is not very important.
When to Order an ANA
If a patient has arthritis or symptoms of a multiorgan disease, an ANA is indicated.
When Not to Order an ANA
When fatigue is the main symptom, unless the patient reports symptoms of joint pain or multiorgan symptomatology. In patents with Chronic Fatigue Syndrome, fibromyalgia, or depression, there is a fair probability of finding a low titred positive ANA. As 90 percent of SLE patients have an elevated erythrocyte sedimentation rate (ESR), it is more cost-effective to order this first while freezing some serum which can have an ANA tested if the ESR is elevated.
To further define the sera whose titre is greater than 1:160, an ANA profile should be ordered. While less sensitive for SLE, Sjogren’s of Mixed Connective Tissue Disease, positive results are much more specific for these rheumatic diseases. Also at this time a urinalysis should be performed because the renal disease of SLE is silent in terms of symptoms.