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