Background. Formation of antibodies (Ab) to TNFa blockers (adalimu-mab (ADA), etanercept (ETA) and infliximab (INF)) inversely correlates with functional drug levels and clinical effect. Comparison of drug levels and anti-drug Ab monitoring is hampered by lack of standardization.
Aim. To determine the correlation and agreement between two different assays for measurement of TNFa blockers and anti-drug Ab levels.
Methods. Serum samples of 145 patients with autoimmune rheumatic diseases were evaluated in two different assays performed by Sanquin (Assay A), and a commercially available kit from Progenika Biopharma (Assay B) performed in the Centre of Laboratory Medicine at Vilnius University Hospital Santariskiu Clinics.
Results. Assay A found detectable levels of ADA in 96.3%, ETA -88.9%, INF - 63.6% patients, whereas Assay B these results were 96.3%, 93.7% and 70.9%, respectively. The good correlation was obtained when comparing Assay A vs. B in defecting drug levels. Further analyses showed good agreement of two used assays only in INF detection. Assay A detected anti-ADA Ab in 14.8%, anti-INF Ab - in 30.9% patients. Assay B detected anti-ADA Ab in 3.7%, anti-INF Ab - in 25.4% patients. Both assays did not detect anti-ETA Ab in tested patient's population.
Conclusions. Our study showed good correlation of trough levels measurements of all three TNFa blockers between two tested assays. Agreement between two assays in detecting serum INF levels was good, but not in ADA and ETA cases. In conclusion, the agreement between two assays could be calculated only for detection of TNFa blockers levels, but not for anti-drug Ab as both assays use different laboratory methods for detection of Ab. The authors agree that existing assays to measure levels of TNFa blockers and anti-drug Ab need to be standardized.