Ustekinumab (UST), a monoclonal antibody that blocks the p40 subunits of interleukins 12 and 23,1, 2 has demonstrated efficacy in inducing and maintaining clinical remission in patients with Crohn's disease (CD)3, 4 and ulcerative colitis (UC).5 Data from trials UNITI-1 and UNITI-2 in CD confirm the relationship between UST exposure and clinical efficacy. For CD, the reference levels at week 8 are 3.9 μg/mL (range 2–7.3 μg/mL) for clinical remission and greater than 11.1 μg/mL for endoscopic remission.6, 7, 8 In patients with UC, the trough concentration associated with clinical response at week 8 is ≥3.7 μg/mL.8, 9 For maintenance therapy, recommended UST trough levels are between 1.5 and 3 μg/mL for clinical remission and greater than 4.5 μg/mL for endoscopic response.8

UST, like other biological drugs, exhibit high interindividual variability, significantly affecting their clearance and, consequently, drug exposure.10 Several factors are associated with increased UST clearance in IBD patients, including higher body weight, low serum albumin levels, immunogenicity, previous exposure to biologics, increased fat-free mass, male gender, Asian race, and elevated C-reactive protein (CRP) levels. In patients with inflammatory bowel disease (IBD), this variability can lead to drug underexposure, increasing the risk of therapeutic failure.9, 11, 12, 13, 14

Therapeutic drug monitoring (TDM) for biological drugs involves measuring blood drug concentrations and adjusting doses to maintain therapeutic levels.15, 16, 17 It uses pharmacokinetic and pharmacodynamic (PK/PD) analyses, integrating population data with individual patient data via Bayesian estimators and decision algorithms.8 A critical step in TDM is the analytical quantification of drug concentration in blood. The accuracy and precision of these analytical methods are essential, as they provide the necessary data for PK/PD analyses that guide personalized dosing, ensuring adequate drug exposure and optimizing therapeutic outcomes.

Various analytical techniques are employed to measure serum drug concentrations and anti-drug antibodies. The enzyme-linked immunosorbent assay (ELISA) is often considered the gold standard due to its established accuracy and precision in quantifying biologic drug levels.18, 19 However, ELISA has limitations, including a longer turnaround time (4–8 h), which can delay clinical decision-making. Alternatives like chemiluminescence immunoassays (CLIA) and electrochemiluminescence immunoassays (ECLIA) offer faster results and have shown strong correlation and agreement with ELISA in several studies.20, 21, 22

Point-of-care (POC) assays offer several significant advantages in TDM of biological drugs. One primary benefit is the rapid turnaround time, with results available within 15–20 min compared to the several hours required for ELISA tests. This speed enables immediate clinical decision-making, facilitating prompt adjustments to therapy during the same patient visit. Additionally, POC assays are designed to analyze single samples efficiently, which contrasts with ELISA assays that typically require batch processing for cost-effectiveness. This characteristic of POC assays can reduce delays in obtaining test results, enhancing the responsiveness of patient management.22 Despite some limitations, such as slightly lower sensitivity and the need for external and internal quality controls, the convenience and immediacy of POC assays make them a valuable tool in clinical practice. Moreover, it is crucial to evaluate their comparability with established ELISA methods. Previous studies have shown good agreement between various POC assays and ELISA for monitoring infliximab and adalimumab, though discrepancies have been noted at higher drug concentrations.23, 24, 25 However, to our knowledge, there are no studies comparing POC-AFIAS for UST with ELISA-based techniques.

For this reason, the objective of this study was to evaluate the analytical performance and clinical utility of the POC-AFIAS assay, a newly introduced rapid test. This evaluation was conducted in comparison with two established assays: the Promonitor® ELISA assay (ELISA-PRO) and the ELISA Ridascreen® (ELISA-RDSC), for quantifying serum concentrations of UST.