AB0799 DO WE OVERDIAGNOSE SERONEGATIVE RHEUMATOID ARTHRITIS? – THE ROLE OF MUSCULOSKELETAL ULTRASOUND IN CLARIFYING SERONEGATIVE INFLAMMATORY ARTHRITIS
Authors: D. Esperança Almeida, E. Costa, F. Guimarães, A. S. Pinto, H. Parente, S. Azevedo, J. Rodrigues, J. Tavares-Costa, C. Afonso, D. Faria, M. Cerqueira, F. Teixeira
Journal: Annals of the Rheumatic Diseases (2021)

Background: Several data indicate that seronegative rheumatoid arthritis (RA-) and seropositive RA (RA+) may have different mechanisms and prognosis, being well established that rheumatoid factor (RF) and anti-citrullinated peptide antibodies (ACPA) have diagnostic and prognostic value. Several conditions, like peripheral spondyloarthritis (SpA), psoriatic arthritis or crystal-related arthropathies may mimic the presentation of RA. Mechanisms and treatment of each of these conditions differ significantly. The authors speculate that RA- may be overdiagnosed in the setting of seronegative chronic inflammatory arthritis and that musculoskeletal ultrasound (US) may help us to better classify these patients through the identification of characteristic lesions of the mentioned diseases. Objectives: To compare the frequency of US lesions found in joints and entheses between RA- and RA+ patients. Methods: Cross-sectional study: systematic US evaluation of ten joints, twelve entheses and both flexor and extensor tendons of four fingers (Table 1) in consecutive RA- patients. RA+ patients matched for sex and age were recruited. RA- patients included met every of the following criteria: history of chronic polyarthritis, RF and ACPA negativity, no extra-articular features of SpA or RA nor family history of SpA, no suspected crystal-related arthritis. Results: Twenty-one RA- patients were included and twenty RA+ patients were recruited as controls. No differences between groups were found in sex, age, body mass index, time of disease evolution or use of biological therapy. RA- patients had a significantly higher number of entheseal structural and/or inflammatory lesions than RA+ patients (median 2.0 vs. 0.5, U 111.5, p=.008**), with triceps enthesitis being significantly more frequent in RA- patients (p=.036*). In total, 18.7% of RA- entheses had enthesitis lesions vs. only 8.3% of RA+ entheses. One RA- patient had ultrasonographic features of dactylitis which was not clinically evident. As expected, considering the role of RF and ACPA in erosive RA, RA- patients had a significantly lower number of joints with erosions compared to RA+ patients (median 0.0 vs. 3.0, U 64.5, p&lt;.001***), with significant differences in every considered joint. Erosions were found in 6.7% of RA- joints vs. 32.0% RA+ joints. Additionally, two RA- patients had hyperechogenic foci in knee cartilage or carpal fibrocartilage suggestive of calcium pyrophosphate deposition. Table 1. Comparative frequency of ultrasound lesions found in joints and entheses of RA- and RA+ patients. RA- (n=21) RA+ (n=20) p -value Mdn number of ENTHESES with any lesion per patient ± IQR 2.0 ± 3.0 0.5 ± 2.0 .008** Triceps – n (%) 8 (38.1%) 2 (10.0%) .036* Quadriceps – n (%) 9 (42.9%) 6 (30.0%) .393 Superior patellar – n (%) 4 (19.0%) 1 (5.0%) .169 Inferior patellar – n (%) - - - Achilles – n (%) 9 (42.8%) 4 (20.0%) .116 Plantar fascia – n (%) 5 (23.8%) 1 (5.0%) .089 DACTYLITIS – 2 nd + 5 th finger – n (%) 1 (4.8%) - - Mdn number of joints with EROSIONS per patient ± IQR 0.0 ± 1.0 3.0 ± 3.0 &lt;.001*** Ulnar styloid process – n (%) 5 (23.8%) 11 (55.0%) .041* Metacarpophalangeal 2 – n (%) 3 (14.3%) 11 (55.0%) .006** Metacarpophalangeal 5 – n (%) 1 (4.8%) 10 (50.0%) .001** Metatarsophalangeal 1 – n (%) - 3 (15.0%) - Metatarsophalangeal 5 – n (%) 3 (14.3%) 11 (55.0%) .006** CHONDROCALCINOSIS – triangular fibrocartilage + knee – n (%) 2 (9.5%) - - DOUBLE CONTOUR – any joint – n (%) - - - RA- – seronegative rheumatoid arthritis; RA+ – seropositive rheumatoid arthritis Mdn – median; IQR – interquartile range; n (%) – absolute number (percentage) of patients with the indicated lesion. Conclusion: We found that some patients diagnosed with RA- had, in fact, ultrasonographic features of different diseases, namely enthesitis/dactylitis and crystal deposition. These data suggest that RA- may be overdiagnosed in clinical practice. Systematic US evaluation of joints and entheses may provide valuable diagnostic information in patients with chronic inflammatory seronegative arthritis and improve patient care. Disclosure of Interests: None declared