Biologic Levels & Antibodies in UC: How Dose Changes Actually Get Decided

One thing I see come up in UC is confusion around drug-related bloodwork when you’re on a biologic. People get told “your levels are fine” or “it stopped working” without much explanation of what that actually means.

I wanted to share how GIs generally think about biologic levels and antibodies in ulcerative colitis.

When you’re on a biologic like infliximab, adalimumab, vedolizumab, ustekinumab, or risankizumab, there are usually two things we can measure in the blood:

1. Drug level: This tells us how much medication is still in your system right before the next dose. Most labs are drawn as trough levels, meaning as close as possible to your next infusion or injection.

2. Drug Antibodies: These tell us whether your immune system is forming antibodies against the medication, which can neutralize it or clear it faster.

How those two results get interpreted usually falls into a few common patterns.

Drug level low / antibodies are absent or low = underdosing. The medication can still work, but it’s wearing off too quickly. In UC this often leads to shortening the interval or increasing the dose rather than switching drugs.

Drug level low / antibodies are high = immune rejection of the medication. Pushing the dose higher usually doesn’t help much. Next step is usually switching biologics, sometimes within the same class, sometimes to a different mechanism.

Drug level therapeutic / no antibodies AND inflammation still present = "mechanistic failure". The drug is present, but it’s not working. In this scenario, switching classes makes more sense than continuing to escalate dose.

Drug level therapeutic / no antibodies AND no inflammation = at goal. Even if symptoms fluctuate a bit, this pattern usually supports staying the course rather than changing meds prematurely.

Two additional nuances: timing matters (trough level needed); drug labs shouldn't be interpreted alone (need to correlate with symptoms, CRP, fecal calprotectin, imaging, and scope findings)

It’s reasonable to ask your GI about these labs and their interpretation --> Is dose change needed? Should drug class be switched? Or stay the course?

Curious what others here have experienced with drug monitoring in UC. Has it changed your treatment course?