Having a positive anti-nuclear antibody (ANA) is not that uncommon. Did you know that 32 million Americans test positive for ANA?! A recent study of people from Belgium evaluated over 68,000 ANA tests found about 9,000 or 13% to be positive. Today we heard an interesting lecture about ANA testing that left me with more questions than answers.
The reference standard for the ANA test is the immunofluorescence ANA using hep2 cells. This test provides a titre and a pattern. It's a worthwhile effort to take some time and visit your local laboratory to see how ANA tests are performed and interpreted. Why? Because ANA testing is so variable. A recent US survey sent a blood sample to labs around the country. The expected result was an ANA of 1:160. The labs across the country reported wide variations from the same sample including results that ranged from negative to an ANA of 1:5120.
The bottom line is this ... It is worth repeating tests that don't support your clinical impression or to send the blood to a different lab. It is also important to talk to your lab director to determine if they use an endpoint control. An endpoint control is a reference which serves as a cut-off for reporting ANA tests. It is important to have an endpoint control as some form of standardization. The funny thing is many labs do not use and endpoint control.
What are the causes of a positive ANA with no clinical evidence of autoimmunity?
Development of autoantibodies years before the onset of disease: It is well reported that autoantibodies are present up to 4-5 years prior to the onset of clinical disease.
Another autoimmune disease such as thyroid or liver disease: Remember to check liver enzymes and thyroid function.
Transient ANA due to a viral infection (e.g., post-viral infection)
Positive end of bell-shaped normal distribution: In other words, these are false positive
Lab factors: This is probably a much bigger issue than any of us realize. As I mentioned above the best thing to do is talk to your lab director.
Benign ANA: a Dense Fine Speckled (DFS70) can be seen in patients who do not have rheumatic disease. The problem is none of the labs report a DFS70. Why? It can be difficult to distinguish a dense fine speckled pattern from a coarse speckled pattern from a homogenous pattern.