Losing Response to Infliximab: Checking the Level Before Raising the Dose
A single Crohn's disease patient losing response to infliximab, testing whether a therapeutic drug level should decide the next step before the dose is simply raised.
Julian F., a 41-year-old man who teaches high school shop, has been on infliximab for ileocolonic Crohn's disease for just over two years, with the first eighteen months genuinely good — back to running the school's after-hours woodworking club, no bathroom emergencies mid-lesson, a calprotectin that had settled comfortably into normal range. Over the last two months that's reversed: four to five loose stools a day, cramping that wakes him before his alarm most mornings, and a calprotectin back up to 340 at his last check. He hasn't missed a single infusion, isn't on any new medications, and nothing about his diet or stress level has meaningfully changed — by his own account, the drug that was working just seems to have stopped.
Secondary loss of response — a good initial response to an anti-TNF agent that erodes over time despite continued dosing — has more than one real mechanism behind it, and the mechanisms don't all call for the same fix. A trough infliximab level that's come back low with a high anti-drug antibody titer points toward immunogenicity: his own immune system has started neutralizing the drug, and the right response is usually adding an immunomodulator or, if antibody titers are very high, switching to a different anti-TNF agent rather than simply giving more of the same one. A low trough level with no meaningful antibody titer suggests accelerated drug clearance, a mechanical problem dose intensification or interval shortening genuinely fixes.
A therapeutic trough level with no antibodies and ongoing symptoms points toward a different problem altogether — true pharmacodynamic failure, disease no longer responsive to TNF inhibition at all, where raising the infliximab dose further wouldn't be expected to help regardless of how high it's pushed. Three genuinely different findings, three genuinely different next steps, and none of them distinguishable from Julian's symptoms and calprotectin alone.
Clinic, evaluating secondary loss of response
I'd draw a trough infliximab level and anti-drug antibody titer before changing anything. Three genuinely different findings are possible here — a low level with high antibodies, a low level with no antibodies, or a therapeutic level with no antibodies — and each one points to a different fix. Empirically raising his dose without knowing which of those he actually has risks treating the wrong mechanism, especially if it turns out to be true pharmacodynamic failure, where more drug simply won't help.
I understand the mechanistic argument, but he's genuinely symptomatic right now — four to five stools a day, waking him before his alarm — and empiric dose intensification remains common real-world practice for exactly that reason, even though the AGA's own guideline (Feuerstein and colleagues, Gastroenterology 2017) actually favors drawing levels first rather than escalating blind. Guideline-concordant or not, several days waiting on a lab result is real discomfort accumulating in the meantime.
The urgency is real, but I'd weigh it against what guessing wrong actually costs here, not just against how long the test takes. If his level comes back low with high-titer antibodies, raising the infliximab dose mostly means giving his own immune system more drug to neutralize — the symptoms wouldn't meaningfully improve, and we'd have lost the turnaround time anyway while learning nothing new. If it comes back therapeutic with no antibodies, we'd have escalated a patient who has actually failed the TNF-inhibition mechanism entirely, delaying a switch to a different drug class he might actually need. A trough level and antibody titer typically turn around within a matter of days, not weeks, and given how differently each result changes the plan, I think that short wait is worth it here rather than intensifying blind.
Agreed: trough infliximab level and anti-drug antibody titer drawn before his next scheduled infusion, with results expected within several days and an explicit plan for each outcome already documented — add azathioprine and consider switching anti-TNF agents if antibodies are high, intensify the infliximab dose or shorten the interval if the level is low without antibodies, and move toward a different mechanism of action entirely if the level is therapeutic with no antibodies present. Julian's discomfort in the interim is real, and the plan includes symptomatic support while the results are pending rather than asking him to simply wait it out.