New recommendations from the European League Against Rheumatism on interpreting the results of antinuclear antibody (ANA) testing advised taking the test methodology into account because of differences in performance.
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ANA results vary not only by the test being used but also by the underlying disease they are being used to assess, warned Pier Luigi Meroni, MD, director of the Immunorheumatology Research Laboratory at the IRCCS Istituto Auxologico Italiano in Milan.
“Antinuclear antibody testing is a known diagnostic tool. But the recent advances in methodologies strongly suggests that we have to update our knowledge for a better interpretation of the results,” Dr. Meroni said in his presentation at the annual European Congress of Rheumatology, held online this year due to COVID-19.
There is “no doubt that ANA testing is useful,” he continued, adding that ANA is used as a primary screening tool in many rheumatic diseases, notably systemic lupus erythematosus (SLE), primary Sjögren’s syndrome, and systemic sclerosis. It’s also recently been suggested as an important entry criterion for the classification of SLE.
In fact, the 2019 SLE classification criteria – developed by EULAR in collaboration with the American College of Rheumatology – state that “testing by immunofluorescence on HEp-2 cells or a solid-phase ANA screening immunoassay with at least equivalent performance is highly recommended,” Dr. Meroni said.
The ideas underpinning that recommendation was that “ANA expression is invariable in SLE, and that ANA-negative lupus is quite rare,” he explained. Also, as SLE expression persists over time, ANA testing could be used for classification at any point in the disease course. These assumptions have been borne out in several studies, with very small percentages of patients (6% or less) having ANA-negative lupus, and more than 80% having a positive HEp-2 test over time, even with immunosuppressive treatment.
Which test methodology to use?
There are several methods that can be used to detect ANA, including the preferred HEp-2 indirect fluorescence assay (IFA), several solid-phase assays (SpA), and line- or dot-blot immunoassays. The issue is which assay should be used in which disease?
The performance of a particular assay can depend on the disease in which they are used. For instance, while the HEp-2 IFA and SpA are equivalent in SLE and in other connective tissue diseases, “this is not the case for other autoimmune diseases in which basically we don’t know exactly all the autoantigens,” Dr. Meroni explained. “Most of the autoantigens are undefined. They cannot be found in solid-phase kits, and we have to use the IFA for detecting all these autoantibodies.”
Importantly, neither the IFA nor the SpA is superior to the other. “We just say that one technique can detect relevant antibodies that are not detectable by the other one, and maybe the combination of the two techniques can be the right strategy to get the highest sensitivity,” Dr. Meroni said.
“Clinicians should be aware of the type of assay used for ANA detection,” he said, “because there are strong differences in the performance, for example between IFA and SpA, and such differences can have important clinical and relevant consequences.”
The test selected will depend on if the aim is to exclude or confirm a disease, and the optimal strategy will depend on pretest probability. For instance, IFA is more sensitive than SpA for SLE and scleroderma, whereas IFA is less sensitive than SpA for Sjögren’s. For SLE, it is suggested to use both the IFA and SpA. A combination of both tests is also considered optimal for scleroderma. SpA testing offers the best sensitivity for Sjögren’s.
“The story is a little bit more complicated for inflammatory myopathies in which we don’t have assays able to detect all the autoantibodies,” Dr. Meroni said. In that situation, several different techniques have to be used to check if the SpA results fit with the IFA pattern.
In 2019, the ACR released its own position statement on ANA testing, highlighting that it supported the use of the HEp-2 IFA assay as the preferred option for ANA testing and that labs should specify the methods being used to test for ANA when reporting their results. The ACR position statement also noted that “ordering health care professionals should select specific ANA subserologies based on a patient’s signs and symptoms and when there is a high pretest suspicion for a specific condition.”
Dr. Meroni disclosed serving as a consultant to Inova Diagnostics, Thermo Fisher Scientific, Pfizer, AbbVie, Merck Sharp & Dohme, and UCB.