How to use it
Choose the drug, when the sample was taken (a trough, just before the next dose) and why: reactive (active disease) or proactive (responding, checking the level). Enter the level and the antibody result as the lab reported it.
Reading the result
| Level | Antibodies | Likely problem | Action |
|---|---|---|---|
| Below target | None or low titre | Rapid clearance | Optimise: higher dose or shorter interval, ± immunomodulator; recheck |
| Low or undetectable | High titre | Neutralising antibodies | Don’t escalate. Switch: another anti-TNF plus an immunomodulator, or a different class |
| At target, active disease | Any | Mechanistic failure | Switch out of class (after optimising to at least 10–15 µg/mL for loss of response) |
| At target, quiet disease | None | Working | Continue; recheck once during maintenance |
Targets (Cheifetz 2021, statements 34–39): infliximab at least 20–25 µg/mL at week 2, 15–20 at week 6, 7–10 at week 14 and 5–10 in maintenance; adalimumab at least 8–12 at week 4 and in maintenance. The consensus says these ranges mostly refer to proactive TDM. AGA 2017’s thresholds for reactive TDM in active disease are ≥5 for infliximab and ≥7.5 for adalimumab. For loss of response, Cheifetz advises not abandoning infliximab or adalimumab until the level is at least 10–15 µg/mL (statements 4–5). The upper end of each range goes with deeper outcomes such as endoscopic healing. The tool flags a level as adequate at the lower end.
Fistulising perianal Crohn’s disease needs higher levels: infliximab ≥10 µg/mL was associated with fistula healing (Yarur 2017). There is no agreed figure for adalimumab.
Ustekinumab and vedolizumab
The evidence is weak. The 2021 consensus supports reactive testing before switching, but found too little data to set a target level. ACG 2025 (KC 37) goes further: it found insufficient evidence to recommend measuring vedolizumab or ustekinumab levels at all. BSG 2025 (GPS 62) agrees, although it summarises exposure data (for IV vedolizumab, about 33–37 µg/mL at week 6, 15–20 at week 14 and 10–15 in maintenance were associated with better outcomes) and notes that dose escalation can recapture response. The tool only says whether drug is detectable. Antibodies to these drugs are uncommon.
Subcutaneous infliximab and vedolizumab
The infliximab targets in this tool are for intravenous dosing. Subcutaneous infliximab (CT-P13 SC, Remsima SC, marketed in Canada) gives higher and steadier levels: after switching from IV, mean levels rose from 8.2 to 14.5 µg/mL, and levels vary little across the 2-week cycle, so a sample can be drawn at any time (BSG 2025; Roblin 2022). There is no consensus target. One registry study suggested a remission cut-off of about 12–13 µg/mL after an IV-to-SC switch, but it discriminated poorly (Iborra 2025). Interpret SC levels against the patient’s own trend, not the IV targets. The same caution applies to subcutaneous vedolizumab, for which there is no target either.
Several infliximab and ustekinumab biosimilars are marketed in Canada (Health Canada DPD). Record which product the patient is on, and keep each patient on the same lab and assay over time.
Assays
Drug levels from different commercial assays generally agree for infliximab, but antibody titres are assay-specific and not interchangeable. Drug-tolerant assays can find antibodies even when drug is present; these are often not clinically relevant. Use the same lab and assay for each patient over time.
Where guidelines disagree
AGA 2017 made no recommendation on routine proactive TDM in quiescent disease. ACG 2025 found insufficient evidence for proactive TDM in all unselected UC patients in remission, and ECCO 2026 (practice point 7) found insufficient data to support proactive TDM with anti-TNF, vedolizumab or ustekinumab in UC; ECCO suggests reactive TDM at loss of response to an anti-TNF. BSG 2025 notes that dose escalation, guided by TDM or not, tends to improve outcomes, and that antibody testing at loss of response helps choose between escalation plus an immunomodulator and a switch (GPS 61). The 2021 consensus (Cheifetz) supports proactive checks after induction and at least once in maintenance. This tool interprets a proactive level but labels the evidence; reactive testing is the use all guidelines agree on.
For acute severe colitis, see the ASUC tool. TDM targets there are not established. In a PREDICT-UC secondary analysis, a day-3 infliximab level of 57.9 µg/mL or less predicted colectomy by 3 months with a negative predictive value of 97% but a positive predictive value of only 24% (Li Wai Suen 2026). ECCO 2026 cites this but still finds insufficient evidence for routine intensified infliximab dosing in ASUC (Rec 46).