ANTIBODIES TO DOUBLE STRANDED DNA: COMBINED STANDARD ELISA AND HIGH-SALT ELISA ASSAYS FOR THE DETECTION OF SLE DISEASE ACTIVITY

Background: Serological markers in systemic lupus erythematosus (SLE) are crucial objective measures included in disease activity indices. Antibodies to double stranded DNA (anti-dsDNA) are included in composite outcomes, and changes in titers in response to therapy are frequently reported in clinical trials. Despite this, there are multiple disparate methods to evaluate anti-dsDNA. The classic Farr immunoassay for anti-dsDNA employs high concentrations of ammonium sulfate, detecting only high-avidity antibodies, binding in high ionic strength. The salt-resistant anti-dsDNA assay is considered highly specific for the diagnosis of SLE and reliable as a disease activity marker. In contrast, most ELISAs are performed with buffers of relatively low ionic strength, detecting both low-avidity and high-avidity antibodies. We screen for anti-dsDNA using the standard ELISA, and follow-up with a modified high salt ELISA which detects only higher avidity antibodies. Use of both forms of anti-dsDNA has not been evaluated as an additive tool for the assessment of disease activity. Objectives: To evaluate the sensitivity and specificity of anti-dsDNA measurement by standard and high-salt ELISA assays for SLE disease activity (SLEDAI ≥4). To gauge the additive value of the high salt assay, performed as a reflex, in those with a positive ELISA evaluation, for the assessment of disease activity. Methods: Patients fulfilling ACR classification criteria for SLE were identified in rheumatology clinic. Demographic data and disease activity (SLEDAI) were recorded. Anti-dsDNA titers were evaluated initially by standard ELISA. With a positive ELISA, a high salt assay was performed reflexively. Those with disease activity (SLEDAI≥4) were compared to those with quiescent disease. Statistical analysis involved the calculation of sensitivity, specificity, positive and negative predictive value of each assay. Results: Seventy-seven encounters were evaluated (69 patients), mean age 41 (SD 14) years, 64 (92.8%) were female. The group was mostly Caucasian, 39 (56.5%), 10 (14.5%) were African-American, 9 (13.0%) Asian and 8 (11.6%) were Hispanic. Forty-two (54.5%) assessments were of active disease, 26 (33.7%) had active renal disease. The sensitivity of antibodies to dsDNA for disease activity, by standard ELISA, was 90.5% with a specificity of 35.1%. The high salt avid assay resulted in a lower sensitivity (47.6%) and higher specificity, (78.4%). When considered in combination, given that both are performed, the sensitivity of our protocol was 90.5% with a specificity of 78.4% for disease activity. The correlation between the standard and high-salt anti-dsDNA assay taking all cases where both was performed was moderate (r=0.53), often with substantial discordance in results in individual patient specimens. Conclusions: With an increasing focus on novel markers to evaluate SLE disease activity, there has been little focus on the optimal use of pre-existing tools. Here we demonstrate the clinical utility of a screening ELISA followed by a reflex high salt anti-dsDNA assay. We show high sensitivity and specificity for SLE disease activity through the use of a standard anti-dsDNA ELISA, and a high-salt modified assay which avoids the need for radioactivity and can be readily employed. Disclosure of Interest: None declared DOI: 10.1136/annrheumdis-2016-eular.3661Citation: Annals of the Rheumatic Diseases, volume 75, supplement 2, year 2016, page 1065Session: SLE, Sjögren's and APS - clinical aspects (other than treatment) (Abstracts Accepted for Publication )