Therapeutic Drug Monitoring for Crohn's: A Patient Guide

If you have ever been told your biologic "stopped working" and felt like the next step was a guessing game, you are not alone. Therapeutic drug monitoring for Crohn's disease is a blood-based tool that can turn that uncertainty into a clear, data-driven decision - helping you and your gastroenterologist understand exactly why a treatment may be failing and what to do about it. Below, we explain what TDM measures, what your numbers mean, and how to use this knowledge at your next appointment.

Key Takeaways

• Therapeutic drug monitoring measures biologic trough levels and anti-drug antibodies to explain why a treatment may be losing effectiveness - eliminating guesswork before switching medications
• The 2017 AGA guideline recommends target troughs of at least 5 mcg/mL for infliximab, 7.5 mcg/mL for adalimumab, and 20 mcg/mL for certolizumab pegol during maintenance therapy (1)(2)
• The TAXIT trial showed that optimizing infliximab doses to a target trough increased clinical remission from 65% to 88% (2)
• Proactive TDM (testing during stable remission) may improve long-term drug durability even though it has not yet been proven to boost remission rates over reactive testing (5)
• A November 2025 international GRADE guideline issued 14 specific TDM recommendations covering infliximab, adalimumab, and vedolizumab - the first guideline developed specifically for TDM of biologics in IBD (6)

What Is Therapeutic Drug Monitoring?

Therapeutic drug monitoring is a blood test that measures how much of a biologic medication remains in your system between doses and whether your immune system has developed antibodies against the drug. Together, these two pieces of information transform dose adjustments from trial and error into targeted, evidence-based decisions.

What TDM Measures

TDM gives your gastroenterologist two critical numbers. The first is the trough level - the concentration of the biologic in your blood drawn just before your next scheduled dose, when the drug is at its lowest point. The second is anti-drug antibody (ADA) status - whether your immune system has produced antibodies that recognize and neutralize the medication.

These two results create a clinical roadmap. Low trough with no antibodies usually means you need more drug. Low trough with high antibodies usually means the drug can no longer work for you. Adequate trough with active symptoms may mean the inflammation is no longer driven by the pathway that biologic targets.

Why Biologics Need Monitoring

Biologics used for Crohn's disease - including infliximab, adalimumab, and certolizumab pegol - are large protein molecules, and their blood levels can vary enormously from patient to patient. Factors like inflammation burden, body weight, albumin levels, and individual immune clearance all influence how quickly your body uses up each dose. Two patients on the same regimen can have dramatically different drug levels. TDM accounts for this variability and helps ensure you are actually getting enough drug to control your disease.

Reactive vs. Proactive TDM: Two Strategies

Not all gastroenterologists use TDM in the same way. Two broad approaches exist, and understanding the difference can help you have a more informed conversation with your doctor.

Reactive TDM (Currently Standard of Care)

Reactive TDM means testing in response to a clinical problem - you are experiencing symptoms, bloodwork or calprotectin suggests active inflammation, or your biologic seems to be losing effectiveness after months or years. The 2017 AGA Institute Guideline suggests reactive TDM in adults with active IBD on anti-TNF therapy, making it the currently endorsed approach when things are going wrong (1).

The advantage is clear: before your doctor switches you to an entirely different medication, reactive TDM can reveal whether a simple dose increase might restore response - saving you from an unnecessary medication change. As we discussed in our guide to biological treatments for Crohn's disease, having more tools to extend the life of each biologic matters when treatment options are finite.

Proactive TDM (Emerging Practice)

Proactive TDM flips the timing. Instead of waiting for symptoms to return, your doctor orders scheduled blood tests during stable maintenance therapy to catch sub-therapeutic levels or developing antibodies before you ever feel a flare coming on.

A 2023 meta-analysis in Alimentary Pharmacology and Therapeutics found that while TDM overall did not improve clinical remission rates compared to standard dosing, proactive monitoring was associated with improved drug durability - meaning your biologic may last longer before losing effectiveness (5). The 2017 AGA guideline does not make a formal recommendation for or against proactive TDM in quiescent disease, but many IBD specialists are adopting it in practice, particularly for patients on combination therapy or those with a history of antibody development.

What the Numbers Mean: Trough Levels and Antibodies

Understanding your own TDM results puts you in a stronger position to participate in treatment decisions. Here are the targets your gastroenterologist is likely working from.

Anti-TNF Trough Targets

The following minimum trough concentrations come from the 2017 AGA guideline and represent the levels associated with better outcomes during maintenance therapy (1)(2):

• Infliximab: at least 5 mcg/mL (AGA guideline); the PANTS prospective study found levels above 7 mg/L were associated with remission at weeks 14 and 54 (4)
• Adalimumab: at least 7.5 mcg/mL (AGA guideline); PANTS data suggest levels above 12 mg/L correlate with remission (4)
• Certolizumab pegol: at least 20 mcg/mL (AGA guideline) (1)

The TAXIT randomized controlled trial demonstrated just how impactful reaching these targets can be. When infliximab doses were optimized to a target trough, clinical remission jumped from 65% before optimization to 88% after (2). That is a meaningful difference for patients who might otherwise be told their drug "failed."

Anti-Drug Antibodies (ADAs)

Anti-drug antibodies are proteins your immune system can develop against biologic medications, treating the drug itself as a foreign invader. ADAs can be:

• Transient - low-titer antibodies that come and go, sometimes resolving on their own or with the addition of an immunomodulator
• Sustained - high-titer antibodies that persistently neutralize the drug, usually requiring a switch to a different biologic

Combining a biologic with an immunomodulator like azathioprine or methotrexate lowers the risk of developing ADAs in the first place. This is a key reason combination therapy is favored in the 2025 AGA Living Guideline on pharmacologic management of moderate-to-severe Crohn's disease (3).

When Will My Doctor Order TDM?

Knowing the common triggers for TDM testing can help you anticipate the conversation and even request testing when you feel it is warranted.

Common Triggers for Testing

Your gastroenterologist is most likely to order TDM in these situations:

• Primary non-response - the biologic never worked well after induction
• Secondary loss of response - the biologic worked for months or years but symptoms are returning
• Before stopping or de-escalating therapy - to confirm drug levels are adequate before reducing doses
• After a drug holiday or pregnancy gap - restarting after a break carries higher antibody risk
• Before switching biologics - to confirm the current drug truly cannot work before committing to a new one

What to Expect from the Test

TDM is a simple blood draw. The key detail is timing: your sample should ideally be taken just before your next scheduled dose so the result reflects the trough (lowest) concentration. If you take your adalimumab injection every two weeks, for example, the blood draw should happen on day 14 - right before the next pen.

Results typically take days to weeks depending on the lab. In the US, insurance coverage for TDM has improved significantly but still varies by plan. Some specialty labs offer rapid-result assays. Outside the US, availability depends on your healthcare system - ask your gastroenterologist which labs they use locally.

If your levels are low and antibodies are absent, the clinical path is often dose intensification - more drug, more frequently - rather than abandoning the biologic entirely. This is one of the most patient-friendly outcomes of TDM: preserving a working medication by simply giving more of it.

TDM Beyond Anti-TNFs: Newer Biologics and Small Molecules

As the treatment landscape expands, so does the question of which drugs benefit from monitoring.

Vedolizumab, Ustekinumab, and IL-23 Inhibitors

TDM evidence is most robust for anti-TNF agents, but the picture is evolving. Reactive testing is generally recommended for non-anti-TNF biologics like vedolizumab and ustekinumab when patients lose response (6). The November 2025 international multidisciplinary GRADE guideline - the first developed specifically for TDM of biologics in IBD - issued 14 recommendations, including concentration thresholds for infliximab, adalimumab, and vedolizumab (6).

Newer IL-23p19 inhibitors like risankizumab, mirikizumab, and guselkumab are now recommended in the 2025 AGA Living Guideline (3), but standardized trough targets for these agents are still being defined in ongoing research. As more data emerge, expect TDM to expand to cover these drugs as well.

JAK Inhibitors and S1P Modulators

Small-molecule oral drugs like upadacitinib and ozanimod work differently from injectable biologics. Because their dosing is standardized and they do not trigger anti-drug antibodies (they are not proteins), blood-level monitoring is not yet part of routine care. Their pharmacokinetics are more predictable, so the dose-response relationship is less variable between patients.

Questions to Ask Your Gastroenterologist

TDM is most useful when you engage actively. Here are evidence-based questions to bring to your next appointment:

• "Are my current trough levels and ADA status documented in my chart? Could I see them?"
• "If I am losing response, would TDM help us decide between dose intensification and switching biologics before making that change?"
• "Does our practice use proactive TDM, and if so, in what circumstances?"
• "If you recommend a switch, can TDM confirm the original drug truly stopped working before we move on?"
• "How often is TDM covered by my insurance (or national health system), and do you use a specific lab?"

These questions signal to your doctor that you want to be involved in the decision-making process - and that you understand there are options between "your drug failed" and "time for something new."

Frequently Asked Questions

Is therapeutic drug monitoring covered by insurance?

In the US, most major insurers now cover reactive TDM when ordered for clinical indication. Coverage for proactive TDM varies more widely. Outside the US, availability depends on your national health system - TDM is standard in many European countries and Australia but may require referral to specialist centers in other regions. Always confirm with your insurance or health system before testing.

How long does it take to get TDM results back?

Most labs return trough level and antibody results within 3 to 10 business days. Some specialty reference labs offer rapid-result assays with turnaround under 48 hours. Your gastroenterologist's office can tell you which lab they use and typical wait times in your area.

Can TDM prevent me from having to switch biologics?

Yes - this is one of its most valuable uses. If your trough level is low but you have no anti-drug antibodies, dose intensification (more drug or shorter intervals) often restores effectiveness without switching medications. The TAXIT trial showed this approach raised remission rates from 65% to 88% (2).

Does TDM hurt or require special preparation?

TDM is a standard blood draw - no special preparation, fasting, or bowel prep is needed. The only important detail is timing: ideally schedule the draw just before your next biologic dose to capture the true trough level. If the timing is off, let your doctor know so they can interpret the result in context.

What happens if I have high anti-drug antibodies?

High-titer, sustained anti-drug antibodies generally mean the biologic can no longer work effectively for you. In this case, your gastroenterologist will likely recommend switching to a different biologic - ideally within the same class (another anti-TNF) if antibodies are drug-specific, or to a different mechanism (IL-23 inhibitor, integrin inhibitor) depending on your history and the latest guidelines (3)(6).

Should I ask for proactive TDM even if I feel well?

This is a reasonable conversation to have with your gastroenterologist, particularly if you have a history of antibody development, are on monotherapy without an immunomodulator, or want to maximize the durability of your current biologic. While proactive TDM has not been proven to improve remission rates, evidence suggests it may extend how long your biologic continues to work (5).

Is TDM available for newer biologics like risankizumab or guselkumab?

TDM assays exist for some newer biologics, but standardized therapeutic targets have not yet been established for IL-23p19 inhibitors in Crohn's disease. Your doctor may still order levels in specific clinical scenarios, but interpretation is less clear-cut than for anti-TNFs. As research progresses, expect formal recommendations to emerge (6).

References

1. Feuerstein JD, Nguyen GC, Kupfer SS, Falck-Ytter Y, Singh S. American Gastroenterological Association Institute Guideline on Therapeutic Drug Monitoring in Inflammatory Bowel Disease. Gastroenterology, 2017. Read study
2. American Gastroenterological Association. Therapeutic Drug Monitoring in Inflammatory Bowel Disease - Clinical Guidance. AGA, 2017. Read guidance
3. AGA. Living Clinical Practice Guideline on the Pharmacologic Management of Moderate-to-Severe Crohn's Disease. Gastroenterology, 2025. Read guideline
4. Crohn's & Colitis Foundation. Role of Therapeutic Drug Monitoring of Biologics in the Treatment of IBD - Clinical Pearls. 2024. Read article
5. Sethi S, Dias S, Kumar A, Blackwell J, Brookes MJ, Segal JP. Meta-analysis: The efficacy of therapeutic drug monitoring of anti-TNF therapy in inflammatory bowel disease. Alimentary Pharmacology and Therapeutics, 2023. Read study
6. Shi C, et al. Therapeutic drug monitoring of biologics in inflammatory bowel disease: An evidence-based multidisciplinary guideline. Acta Pharmaceutica Sinica B, 2025. Read study