Understanding the features of the three approved assays for serum levels of infliximab is important in interpreting the results and adjusting therapy, an Australian study has concluded.

Gastroenterologist Dr Susan Connor from the Liverpool Hospital in Sydney, and colleagues used serum from healthy volunteers ‘spiked’ with known concentrations of infliximab to check the performance of the Grifols Promonitor, Theradiag Lisatracker and R-Biopharm Ridascreen ELISA kits.

They also added a commercially-available antibody to infliximab (ATI) to assess how it influenced the results.

“The Lisatracker kit provided the most accurate determination of infliximab drug levels, however it yielded false positive results at low concentrations of infliximab,” they said.

“The average coefficients of variation for the kits were 8% for Lisatracker, 5% for Ridascreen and 11% for Grifols. Infliximab measurements across all kits were affected by interference from ATI.”

The false-positive results with Lisatracker were unlikely to influence clinical decisions, as the highest reading in samples with no infliximab was 0.3 μg/mL.

The presence of etanercept or adalimumab in the sample also influenced the results for infliximab, increasing the reading with one assay and depressing it in the other two.

Although it was unlikely that clinical samples would include more than one TNF inhibitor, the interference could be significant.

“Cohort studies and post-hoc analyses show that serum infliximab trough concentrations correlate with clinical response, remission and mucosal healing in IBD patients,” Dr Lee and her colleagues wrote in Pathology.

“Low infliximab trough levels and presence of ATI are associated with worse clinical outcomes.

“Therapeutic drug monitoring using a combination of trough infliximab concentration and ATI levels can guide therapy by allowing targeted dose intensification in patients who lose response due to low drug concentration, while avoiding dose escalation in patients who have developed immunogenicity.”

According to the research team the results of their study verified the performance of the three commercial kits for measuring infliximab drug levels.

The Lisatracker kit produced the closest results to the true concentration of infliximab, but like the other kits was limited by false positive results where no infliximab was present, they said.