Background/Purpose: 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, and thus detects only high-avidity antibodies that bind 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, and thus detect 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. We aim to evaluate the sensitivity and specificity of anti-dsDNA measurement by standard and high-salt ELISA assays for the presence of SLE disease activity. We then wished 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. On identification of a positive ELISA, a high salt assay was performed reflexively. Active SLE was classified as SLEDAI ≥4. Those with disease activity 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 patient encounters were evaluated (69 patients). The mean age was 41 (SD 14) years and 64 (92.8%) were female. The group was composed of mostly Caucasian patients, 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 by SLEDAI. Twenty-six (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.

Conclusion: With an increasing focus on novel markers to evaluate SLE disease activity, there has been little priority placed on the optimal use of pre-existing laboratory 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 that avoids the need for radioactivity and can be readily employed.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/antibodies-to-double-stranded-dna-combined-standard-elisa-and-high-salt-elisa-assays-for-the-detection-of-sle-disease-activity/