Why Do I Need Blood Tests on a Biologic?

Blood tests on a biologic are doing three different jobs at once, and most of the confusion comes from treating them as one. Safety bloods (blood counts, liver and kidney tests) look for side effects, very often from a second drug you take alongside the biologic. Drug levels and antibodies check whether there is enough medicine in your blood. Inflammation and nutrition markers (CRP, albumin, haemoglobin, iron) show how your Crohn's or colitis is behaving.

The part almost nobody tells you: for an anti-TNF, vedolizumab or ustekinumab taken on its own, the current US labels ask for tuberculosis and hepatitis B checks but set no fixed routine blood schedule. So when one clinic bleeds you at every infusion and another once a year, neither is breaking a rule. They are following local practice, a partner drug's rules, or the specific drug you are on. If your first infusion is coming up, see what to expect at your first biologic infusion.

The short answer: three questions hiding under one needle

Every tube you give is answering one of three questions, and knowing which one turns a scary-looking result into something you can ask sensible questions about. People in r/CrohnsDisease describe exactly this muddle. One person a few months into vedolizumab saw raised liver numbers and could not tell whether it meant liver damage or a blip (r/CrohnsDisease thread). Another, on infliximab for years with ongoing symptoms, felt the team kept pointing to "fine" bloods (r/CrohnsDisease thread). Those are two different questions, and the bloods only answer one of them well.

The rest of this page goes through each line. Drug levels and antibodies get the lightest treatment here, because we cover them in depth in what it means when a biologic stops working. That post covers why antibodies form, which are neutralising, why target numbers differ and why labs disagree.

What your drug's own label actually asks for

The legal prescribing information for most injectable IBD biologics asks for far less routine blood testing than people assume. We read the current US labels on DailyMed for ten advanced therapies and pulled out what each one says about tests before and during treatment. This is the US label, which the manufacturer and the FDA agree on. UK and EU product information may word things differently, and we did not read those.

Three things stand out.

The IL-23 drugs have a liver window because of single cases. Each of the three labels describes one trial participant who developed drug-induced liver injury. For guselkumab and mirikizumab it followed a higher or longer than recommended induction regimen (Tremfya label, Omvoh label). For risankizumab it was a person with Crohn's who had ALT 54 times the upper limit of normal, with a rash, after two 600 mg intravenous doses, which is the label's standard Crohn's induction dose. It resolved with steroids (Skyrizi label). That is why your team may check liver tests more often in the first few months of one of these drugs and then ease off.

The pill-form drugs carry the most specific blood rules. The tofacitinib label gives explicit intervals, and both JAK labels list counts at which treatment should be paused or not continued, for example an absolute lymphocyte count under 500 cells/mm³ or haemoglobin under 8 g/dL (Xeljanz label, Rinvoq label). Many people call these drugs "biologics" because they sit in the same part of the treatment ladder. Strictly they are small molecules, and their monitoring is different.

For anti-TNFs, vedolizumab and ustekinumab, routine bloods are a clinical choice rather than a label requirement. UK guidance acknowledges the gap directly. Its Statement 94 suggests that pre-treatment screening and blood monitoring on vedolizumab and ustekinumab "should at present follow recommendations for anti-TNF drugs due to insufficient long-term safety data", rated a weak recommendation on very low-quality evidence (Lamb 2019). We searched that guideline's full text and could not find a fixed blood-test interval for anti-TNF monotherapy. A Scottish health board guideline on biologics in Crohn's told patients they "will need to attend for regular blood tests" but, in the version we read, also gave no interval for them (NHS GGC 2021). That version was due for review in 2024, so a newer one may exist.

The reason many people are tested so often: the second drug

If you take azathioprine, mercaptopurine or methotrexate alongside your biologic, that drug, not the biologic, usually sets the pace of your blood tests. UK guidance Statement 89 recommends that anyone starting a thiopurine or methotrexate has a baseline full blood count, kidney tests and liver tests, then repeats them "at least at weeks 2, 4, 8 and 12, and then at least 3-monthly", rated a strong recommendation on low-quality evidence with 100% panel agreement (Lamb 2019). The same guideline says bloods should be repeated two weeks after every thiopurine dose increase, because toxicity "can occur at any stage during therapy". It also cites a Spanish database of nearly 4,000 patients in which 26% had a thiopurine side effect, including liver toxicity in 4% and bone marrow toxicity in 4%.

That matches what people report. In the same bloodwork thread, the commenters on the most frequent schedules were nearly all on a combination: adalimumab with methotrexate every three months "to confirm nothing wrong with my liver", azathioprine every two months for liver and white cells, and mercaptopurine started alongside ustekinumab bumping one person from three-monthly tests to every two weeks (r/CrohnsDisease thread).

Why take the second drug at all, if it brings the extra needles? Because in Crohn's, combining helps the biologic. In the SONIC trial of 508 adults new to both drugs, 56.8% (96 of 169) on infliximab plus azathioprine were in steroid-free remission at week 26, against 44.4% (75 of 169) on infliximab alone (Colombel 2010). In the UK PANTS cohort, using an immunomodulator was linked to a much lower chance of developing antibodies with undetectable drug levels, a hazard ratio of 0.40 on infliximab and 0.42 on adalimumab (Chanchlani 2024). PANTS was funded by patient charities and several drug companies, including AbbVie and Pfizer, which the paper declares.

So the blood tests are partly the price of a strategy that makes the biologic last longer. Whether that trade is right for you is a conversation with your team, and nothing here is a reason to change what you take.

Line by line: what each result is watching for

Full blood count

The blood count watches your bone marrow and your iron, and on a biologic alone it is mostly a disease marker rather than a side-effect alarm. The anti-TNF labels describe medically significant low counts as "infrequently reported" with an unclear causal link, and they put the emphasis on symptoms: persistent fever, bruising, bleeding or paleness should prompt immediate medical attention (Humira label). Thiopurines, methotrexate and the JAK and S1P pills are where the count is watched for drug effects.

The haemoglobin line is the one most likely to change how you feel. In one r/CrohnsDisease thread, someone on biologics for two decades described months of breathlessness and a racing heart that they blamed on age, until a delayed blood test showed iron-deficiency anaemia. One reply described getting winded on 12 steps and a haemoglobin of 8; others described having to press for tests before a problem was found (r/CrohnsDisease thread). Iron deficiency in IBD has its own rules, including why ferritin can look normal during inflammation. Our post on fatigue and low iron in ulcerative colitis goes through them, and the same logic applies to Crohn's.

Liver tests

Liver enzymes go up and down on biologics more often than people expect, and most rises are mild and settle. The best IBD-specific data we found come from a US cohort of 1,753 people starting an anti-TNF (1,170 infliximab, 575 adalimumab and 8 certolizumab). Of these, 102 (6%) developed a new ALT of 60 U/L or more. In 54 of them (53%) there was another explanation. Among the unexplained rises, the median time to onset was 18 weeks and the median peak ALT was 96 U/L. Most people (85%) returned to normal over a median of 17 weeks, including 28 of the 34 who carried on with their anti-TNF (Shelton 2015). The six people who had a liver biopsy all showed hepatitis with autoimmune features, so these rises are not always trivial.

The infliximab label shows how much of this is background noise. In the Crohn's trials, ALT between 1 and 3 times the upper limit of normal was recorded in 39% of people on infliximab and 34% of people in the placebo maintenance group, which had received a single initial infliximab dose. ALT 3 or more times normal was 5% against 4% (Remicade label). The disease itself, other medicines, alcohol, fatty liver and viral illness all move these numbers.

The same label carries a sobering line for anyone who treats normal bloods as a guarantee. In post-marketing reports of severe liver reactions, "elevations in hepatic aminotransferase levels were not noted prior to discovery of the liver injury in many of these cases" (Remicade label). Routine liver tests catch the common mild rises. They do not replace telling your team promptly about yellow eyes or skin, dark urine, pale stools or feeling suddenly very unwell.

CRP and albumin

CRP and albumin are disease-activity markers, and albumin has a second job: it affects how long infliximab lasts in your blood. In an analysis of 728 people with ulcerative colitis from two infliximab trials, people with higher albumin kept higher drug levels, cleared the drug more slowly and had a longer half-life. Those whose albumin was below the normal range had much lower drug levels and lower response rates (Fasanmade 2010). The authors suggested a shared recycling pathway for albumin and antibody drugs as the reason. The first author worked at Centocor, the company that developed infliximab, according to the PubMed affiliation. A low albumin on your report can therefore be part of why your team checks a drug level.

TB and hepatitis B

These are checked because anti-TNFs and other advanced therapies can let a sleeping infection wake up, and one negative test does not settle the question for good. Both the Humira and Remicade labels ask for tuberculosis evaluation before treatment and "periodically during therapy", and both carry warnings about hepatitis B reactivation in people who carry the virus (Humira label, Remicade label). UK guidance goes further, recommending that everyone with IBD is screened for hepatitis B and C and HIV, and that people without a clear history of chickenpox, shingles or two doses of varicella vaccine are tested for immunity. It suggests doing this at diagnosis so a later move to stronger treatment is not delayed (Lamb 2019).

Does repeating the TB blood test find anything in a low-incidence country? A US study using the Crohn's & Colitis Foundation's SPARC IBD cohort looked at 687 people with at least one interferon-gamma release assay. First tests were positive in 0.9% (6 of 687) and indeterminate in 5.4%. After a negative first test, 1.9% (5 of 269) of second tests were positive, a median of about 1.1 years later, which was not significantly different from the first-test rate. After two negatives, 1 of 142 third tests was positive (Lapp 2024). The authors concluded that one test is not enough. Five people also went from positive to negative, which they said may reflect false results in one direction or the other. Oral steroids were associated with more indeterminate results, though not significantly.

If tuberculosis or night sweats are on your mind, our post on Crohn's disease night sweats covers the TB risk numbers on biologics in detail.

Cholesterol on JAK inhibitors

If you take upadacitinib or tofacitinib, a lipid check is part of the plan because these drugs raise cholesterol. The Rinvoq label reports increases in total, LDL and HDL cholesterol, says LDL fell back to pre-treatment levels with statin therapy, and says the effect of these rises on heart disease and death "has not been determined". It asks for lipids about 12 weeks after starting and then according to standard cholesterol guidelines (Rinvoq label).

Vitamins and minerals, and a CRP catch

Some nutrient results cannot be read properly while you are inflamed. UK guidance gives CRP thresholds below which a result is reliable: under 20 mg/L for plasma zinc, under 10 mg/L for selenium, vitamin A and vitamin D, and under 5 mg/L for vitamin B6 and vitamin C. It also notes that serum magnesium is a poor measure of body stores, because less than 1% of the body's magnesium is in the blood (Lamb 2019). If a vitamin result was taken during a flare, it may be worth asking whether it should be repeated when things are calmer.

Drug levels and antibodies: a different test with a different job

A drug level measures the medicine, not you, and it is usually taken just before your next dose, when the level is lowest. That timing is called the trough. The Scottish guideline puts it plainly: the test is "usually performed immediately prior to the next scheduled biologic treatment", and "it is important to consider what clinical question the test is intended to answer before the test is performed" (NHS GGC 2021).

Two practical points come up again and again in the forums:

• Results take longer. People who work in laboratories explained in one thread that drug-level and antibody tests are specialist tests usually sent to a reference laboratory rather than run in-house, and waits of up to about 10 days were described (r/CrohnsDisease thread).
• The number depends on the laboratory. UK guidance notes that "drug levels vary according to the assay used, and consensus has not yet been achieved on the optimal therapeutic ranges" (Lamb 2019). An expert panel advised that patients "should ideally be followed and managed over time with the same TDM assay" (Cheifetz 2021).

What a low level or a positive antibody result means, which antibodies matter and why the target numbers you read online disagree are all covered in what it means when a biologic stops working. One extra finding belongs here, because it surprises people. In PANTS, 6.3% of people with Crohn's (96 of 1,525) already had detectable antibodies to infliximab before they had ever received it, and 2.4% to adalimumab. None of these pre-existing antibodies neutralised the drug in laboratory tests, and they were not linked to drug levels, side effects or response (Nice 2025). A positive antibody result is information to interpret, not a verdict.

Why your clinic might check drug levels when you feel fine

Testing levels on a schedule in people who are well, called proactive monitoring, is genuinely contested, and the trials split. This is the question behind "why are they testing me if nothing is wrong?", so here is the ledger.

A few honest caveats sit under that table. The two positive trials are not the typical adult with IBD. NOR-DRUM B pooled rheumatoid arthritis, spondyloarthritis, psoriatic arthritis, psoriasis, ulcerative colitis and Crohn's, and its abstract does not report the result for IBD separately. It was publicly funded through Norwegian regional health authorities, and its lead author declared personal fees from Thermo Fisher. PAILOT was small, unblinded and in children, and far more of the proactive group had their dose increased (87% vs 60%), so part of the benefit may simply be more drug.

Despite the mixed trials, a 2021 US expert panel recommended proactive drug-level testing for anti-TNFs "after induction and at least once during maintenance" (Cheifetz 2021). It was a Delphi consensus of 10 gastroenterologists, which is expert opinion rather than trial evidence. Its disclosures list consulting fees from several drug and diagnostics companies, including LabCorp, which sells drug-level tests, and Prometheus. None of that makes the advice wrong. It is context for weighing it against the trials above.

So if your team checks levels when you feel well, they are following one reasonable reading of the evidence. If they do not, they are following another.

"My bloods are fine but I feel terrible": normal bloods and remission

Normal blood tests are reassuring about safety, but they are a poor test of whether your bowel is healed. A meta-analysis of 19 studies covering 2,499 people with IBD symptoms compared blood and stool markers against endoscopy. CRP picked up active inflammation with a pooled sensitivity of only 0.49, meaning it missed about half of it, though when it was raised it was usually right (specificity 0.92). Faecal calprotectin was far more sensitive, at 0.88 (Mosli 2015).

That is the experience in the forums too. The person on infliximab with two years of symptoms noted that their bloods had also been normal at diagnosis, when they had several ulcers in the ileum. A reply described labs and imaging that looked fine while a scope showed inflammation, and how hard it was to get a new drug approved with "fine" labs (r/CrohnsDisease thread). A parent in another thread described a child with very good bloodwork for two years, a calprotectin around 200 and biopsies still showing granulomas (r/IBD thread).

Long-term numbers explain why this matters. In PANTS, only about a third of people with active Crohn's who started an anti-TNF were in remission three years later: 34.7% on infliximab and 28.9% on adalimumab (Chanchlani 2024). Many people on a biologic are not in full remission, and the blood panel alone will not always show it.

If you have a calprotectin result and want to understand it, our free explainer reads it against your own earlier results:

For the fuller picture, our posts on whether you are in a Crohn's flare and on high calprotectin with no symptoms cover the gap between how you feel and what tests show, in both directions.

When one result comes back off: what usually happens next

One abnormal result usually leads to a repeat test and a look for other causes, not an immediate change of treatment. These are typical patterns described in the sources above. They are not instructions, and your team will know things about you that these do not.

A mildly raised ALT. Expect a repeat in a few weeks and questions about alcohol, new medicines and supplements, recent infections and weight. The Shelton cohort found half of new ALT rises had another explanation and most unexplained ones settled even on treatment (Shelton 2015). On an IL-23 drug in the first months, the labels ask for prompt investigation and say to interrupt treatment if drug-induced liver injury is suspected, until it is excluded (Omvoh label). In the vedolizumab thread, two replies described levels that kept rising and a switch of drug, after which they came down (r/CrohnsDisease thread). That is two people's experience, not a prediction.

A low white count or lymphocytes. On a thiopurine, this is exactly what the UK schedule is designed to catch early (Lamb 2019). On a JAK inhibitor, the label gives specific thresholds for pausing (Rinvoq label). Infection symptoms while counts are low deserve a same-day call.

A low haemoglobin. This points to bleeding, iron or B12 deficiency, or ongoing inflammation more often than to the biologic. It usually leads to iron studies and a closer look at disease activity.

A low drug level, with or without antibodies. UK guidance suggests that people with loss of response "may have serum drug and anti-drug antibody concentrations measured to inform appropriate changes in treatment" (Lamb 2019). If you feel well, a low level alone is a judgement call, as the trials table above shows.

A worked example: the first year on infliximab plus azathioprine

Here is how the three kinds of test stack up over a year for one common combination. It is assembled from the label and UK guidance cited above to show the logic. It is not a protocol, and your clinic's will differ.

The UK good-practice recommendation is that everyone is reviewed 2 to 4 weeks after completing anti-TNF loading doses, using measures such as "serum drug and anti-drug antibody concentrations, blood inflammatory markers, faecal biomarkers or endoscopy" (Lamb 2019). Notice that most of the needles in this year belong to azathioprine. If the azathioprine were stopped, the routine safety bloods could reasonably thin out, while the TB, disease-activity and occasional drug-level checks would remain.

Myths about blood tests on biologics

"The biologic needs monthly blood tests." Not according to the US labels for anti-TNFs, vedolizumab or ustekinumab, which set no routine blood schedule. Monthly tests usually reflect a partner drug, a pill-form drug, a recent abnormal result or local practice.

"Normal bloods mean the drug is working." They mean nothing obvious is being harmed and CRP is not raised. CRP misses about half of active inflammation (Mosli 2015), so stool tests, scans and scopes still matter.

"A raised liver enzyme means I have to stop." Usually not on its own. Mild rises are common, often have another cause and often settle on treatment (Shelton 2015). Very high values, jaundice or symptoms are a different matter, and that decision belongs to your team.

"I tested negative for TB, so I am done." The labels ask for periodic evaluation, and repeat tests in a US IBD cohort turned positive at a similar rate to first tests (Lapp 2024).

"Antibodies mean the drug is finished." Some people have antibodies before they ever start, and those did not affect response in PANTS (Nice 2025). Whether antibodies matter depends on their level, whether they persist and what the drug level is.

"More testing is always better." In three adult trials, adding scheduled drug levels did not improve the main outcome. Testing helps when someone has a question the result will answer.

Keeping your own record of results

Your results are most useful as a series, next to how you felt at the time. A drug level means more beside the date of your last dose. An ALT of 70 means more if you can see it was 30 three months earlier and that you had a viral illness that week. The IBD symptom diary guide has a treatment-record template with a line for drug levels and antibodies, including the point in the dosing cycle when each was taken. For handing results to a new clinician, our post on making a symptom tracker your doctor will read covers what fits on one page.

Clairop's one-page GI report includes lab results alongside your symptom score trend and medication adherence, so a result and how you were feeling sit side by side. It is a record you keep, not a diagnostic tool. You can see how the report is put together on the how it works page.

If you are preparing for a review where results will be discussed, it can help to write your questions down in advance. Our questions to ask about Crohn's disease list has a section on treatment.

When to contact your team promptly

Do not wait for your next scheduled blood test if any of these happen. The labels for these drugs list most of them as reasons to seek medical attention, and the rest are standard IBD red flags:

• Yellowing of the skin or eyes, dark urine, pale stools, or itching with feeling very unwell
• Fever, a persistent cough, or an infection that is not settling, especially on an anti-TNF or a JAK inhibitor
• Unusual bruising or bleeding, or new paleness and breathlessness
• Blood in your stool that is new or increasing, unexplained weight loss, or night-time symptoms
• Severe abdominal pain, repeated vomiting or being unable to keep fluids down

If you feel seriously unwell, use urgent or emergency care rather than waiting for the IBD team. A blood test can be done the same day if needed.

The honest bottom line

Blood tests on a biologic are not one test done for one reason. Some are there because of the biologic's own label, especially in the first months of an IL-23 drug and throughout treatment with the JAK pills. Many are there because of a partner drug. Some track the disease, and those are the ones most likely to give false comfort. Drug levels are a separate tool with their own debate. If your schedule confuses you, the most useful question at your next appointment is short: "Which drug is this blood test for, and what would you do differently if it came back abnormal?"