Undetectable levels of antidrug antibodies (ADA) result in better outcomes than detectable levels in patients diagnosed with psoriatic arthritis (PsA) and treated with adalimumab (Humira), according to findings published in the Annals of Rheumatic Diseases.

Erik Hans Vogelzang, MD, of the Jan van Breemen Research Institute in Amsterdam, and colleagues evaluated patients with PsA being treated with adalimumab as a way to calculate optimal ranges of concentrations of the drug in blood.

Among the 103 patients in the prospective cohort study, adalimumab concentrations were significantly lower at 28 weeks in patients with detectable ADA (1.3 mg/L) compared with patients without detectable ADA (8.7 mg/L; P < 0.001). Results were similar at 52 weeks: 0.9 mg/L for patients with detectable ADA and 9.4 mg/L (P=0.0001) for patients without detectable ADA.

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Adalimumab concentrations and ADA were measured in serum trough samples by enzyme-linked immunosorbent assay (ELISA) and a radio immunoassay. Researchers also monitored the disease activity score of 28 joints (DAS28). These results at 28 weeks (2.16 vs 2.95, P=0.023) and 52 weeks (2.19 vs 2.95, P=0.024) showed a significant difference, with poorer clinical outcomes seen in patients with detectable ADA levels.

The optimal blood concentration range for adalimumab is 5 mg/mL to 8 mg/L, reported the scientists. Patients treated with both adalimumab and methotrexate had higher concentrations of adalimumab in their blood and were less likely to develop antibodies to the biologic agent.

“Patients with detectable ADA had lower adalimumab concentrations and a significantly poorer clinical outcome compared with patients in whom ADA were not detected,” concluded the researchers.


  1. Vogelzang EH. Ann Rheum Dis. 2014 doi:10.1136/annrheumdis-2014-205554.