ANA Update 11.5.09 - Weak Positive ANA
Screening for anti-nuclear antibodies can be a valuable test where there is a clinical suspicion of connective tissue disease e.g. SLE.
However, low level ANA can occur in a variety of situations where the immune system is activated e.g. in infection, malignancy or other inflammatory disorders.
The immunology laboratory currently screens patient serum by indirect immunofluorescence for the identification of ANA. Further tests are performed as required, in particular testing for the presence of antibodies to DNA or ENA (Ro, La, SM, RNP, Scl70). This screening process identifies a large number of low level positive sera, many from patients where the likelihood of connective tissue or other autoimmune disease is very low, including many elderly patients, who often have low level autoantibodies in their serum, thought to be part of the aging process of the immune system. Weakly positive ANA occur in both the normal healthy population and increase in frequency in healthy individuals with increasing age. Individuals with intercurrent illness have an increased prevalence of positive ANA, again increasing further with age.
Our audit in the laboratory has identified that only low numbers of these ‘weak’ positive ANA (defined as ones not present when the patient’s serum sample is further diluted) are due to the presence of antibodies to DNA or ENA known to be associated with autoimmune diseases, and where these are quantified, they tend to be present at low levels. Many of these results therefore are likely to be of doubtful clinical significance, and appear unassociated with the presence of autoimmune disease.
Nevertheless, such tests take time to undertake and are not without a considerable cost to the laboratory, and therefore the Trust and local health services.
It is therefore recommended that ANA is only requested where there is a reasonable clinical suspicion of autoimmune disease as the primary pathology. False positive tests are common where testing is undertaken in an individual who is acutely unwell, and where there are no real clinical suspicions of autoimmune disease.
We have introduced a limit on the number of additional tests offered where the ANA is only weakly positive or where no clinical details suggest autoimmune disease. In this situation, where a weak positive ANA is found, no further tests for presence of DNA antibodies or ENA antibodies will be undertaken, with a recommendation that the ANA screen is repeated in three months unless the clinical situation changes.
I am confident that this will not compromise the ability of clinicians to make a diagnosis of systemic autoimmune disease. Users are encouraged to complete clinical details indicating the likelihood of autoimmune disease in order to assist laboratory staff in making decisions about additional tests that may be required.
If you have any questions or wish to discuss this matter further please do not hesitate to contact Dr Philip Wood (Secretary x67256/x65526)