In a patient with Crohn’s disease who presents with new skin nodules, which tuberculosis screening tests should be performed before initiating anti‑TNF therapy?

TB Screening Before Anti-TNF Therapy in Crohn's Disease with Skin Nodules

All patients with Crohn's disease should undergo tuberculosis screening before initiating anti-TNF therapy, consisting of clinical examination, chest radiograph, and tuberculosis testing with either tuberculin skin test (TST) or interferon-gamma release assay (IGRA), ideally performed before any immunosuppression begins. 1

Essential Screening Components

The screening protocol must include:

• Clinical history: Document any prior TB exposure, previous TB treatment, travel to endemic areas, and assess for TB symptoms (fever, night sweats, weight loss, cough)
• Chest radiograph: Mandatory for all patients to identify active disease or evidence of prior TB 2
• TB testing strategy: Either TST or IGRA, with specific considerations based on immunosuppression status

Testing Strategy Based on Immunosuppression Status

If NOT Currently on Immunosuppressive Therapy

A tuberculin skin test is helpful and should be performed 2. The test remains reliable when patients are not yet immunosuppressed. IGRA alone may also be appropriate in immunosuppressant-naïve patients 3.

If ALREADY on Immunosuppressive Therapy

This is a critical clinical pitfall. Tuberculin skin testing will not be helpful and need not be undertaken in patients already receiving corticosteroids, azathioprine, 6-mercaptopurine, methotrexate, or other immunosuppressants 2. The immunosuppression significantly reduces test sensitivity.

For immunosuppressed patients, IGRA results are also negatively affected by immunosuppressive therapy 1, 4. Studies show IGRA-positive rates are lower (OR 0.55) and indeterminate rates are higher (OR 2.91) in patients on immunosuppression 4. The negative impact is particularly pronounced with two or more immunosuppressive drugs 3.

When patients are already immunosuppressed, perform an individual risk assessment based on epidemiologic factors rather than relying solely on testing 2. Consider chemoprophylaxis if TB risk exceeds hepatotoxicity risk, particularly for Black African patients over 15 years and all South Asians born outside the UK 2.

Dual Testing Strategy

Both TST and IGRA should be considered in immunosuppressed patients 3, 5. The concordance between tests is only moderate (κ=0.59) and decreases further with immunosuppression (κ=0.51) 3. Using both tests captures more cases: the IGRA-negative/TST-positive rate tends to be higher than IGRA-positive/TST-negative (9.5% vs 5.8%) 4. The "either test positive" strategy identifies 27% of patients with latent TB compared to 22.3% with TST alone or 16% with IGRA alone 5.

Critical Management Points for Skin Nodules

Any patient with an abnormal chest radiograph or previous history of TB must be referred to a TB specialist 2. The skin nodules mentioned in your patient could represent:

• Erythema nodosum (common in Crohn's disease)
• Cutaneous TB manifestations (rare but possible)
• Other inflammatory manifestations

Active TB must be thoroughly excluded before starting anti-TNF therapy 2. If active TB is confirmed, patients require minimum 2 months of full chemotherapy before anti-TNF initiation 2.

Key Clinical Pitfalls

1. Timing matters: Screen before starting any immunosuppression when possible, as both TST and IGRA lose sensitivity once immunosuppression begins 1, 4

2. Don't rely on negative tests in immunosuppressed patients: Up to 9.2% of patients with initially negative screening can have positive conversion during anti-TNF therapy 6, and cases of TB have occurred despite prior negative screening 7, 8

3. Geographic risk stratification: TB risk varies dramatically by region—Portugal shows 18% positive conversion rates versus 1.11% in the USA 6

4. Anti-TNF agents carry different risks: The reactivation risk is 4.7-fold increased with anti-TNF therapy overall 1, with combination therapy (anti-TNF plus methotrexate or azathioprine) showing 13-fold increased risk 1

Practical Algorithm

1. Before any immunosuppression: Perform chest X-ray + either TST or IGRA (or both for maximum sensitivity)
2. Already on immunosuppression: Perform chest X-ray + risk assessment ± dual testing (TST + IGRA)
3. Abnormal findings: Refer to TB specialist immediately
4. Positive latent TB: Initiate chemoprophylaxis (typically isoniazid 6 months) before starting anti-TNF
5. Active TB: Treat with standard chemotherapy for minimum 2 months before considering anti-TNF

Close cooperation with TB specialists is strongly recommended throughout this process 2.