How long should a patient with a history of flu, and compromised respiratory system, such as asthma or Chronic Obstructive Pulmonary Disease (COPD), wait before undergoing tuberculin skin testing or initiating tuberculosis (TB) prophylaxis or treatment?

TB Skin Test Timing After Influenza

Wait 4-6 weeks after influenza infection before placing a tuberculin skin test (TST), or alternatively, perform the TST on the same day as symptom onset and read it 48-72 hours later before viral-induced immunosuppression develops. 1

Why This Timing Matters

• Viral infections, including influenza, can temporarily suppress delayed-type hypersensitivity reactions, which is the immunologic basis of the tuberculin skin test 1
• This immunosuppression begins shortly after viral infection and can persist for 4-6 weeks, potentially causing false-negative TST results that could miss true tuberculosis infection 1
• The mechanism is similar to live vaccine-induced immunosuppression, where viral antigens temporarily dampen the cellular immune response needed for tuberculin reactivity 1

Recommended Clinical Algorithm

If TB Testing is Urgent (Cannot Wait 4-6 Weeks):

• Use an interferon-gamma release assay (IGRA) instead of TST, as IGRAs are not affected by viral infections and require only a single blood draw without return visits 1, 2
• IGRAs (QuantiFERON-TB Gold or T-SPOT.TB) are particularly valuable in immunosuppressed patients where TST reliability is already compromised 3, 2

If TB Testing Can Be Delayed:

• Wait the full 4-6 weeks after influenza symptom resolution before placing the TST to avoid false-negative results 1
• This is especially critical in high-risk patients (recent TB exposure, immunocompromised, children <5 years) where missing true infection has serious consequences 3

Special Considerations for Patients with Asthma or COPD:

• These patients may already be on corticosteroids, which independently suppress TST reactivity 3, 4
• If on corticosteroids >1 month, TST cannot be adequately interpreted and should be discontinued for >1 month before testing, or IGRA should be used instead 3
• If on immunomodulators (e.g., biologics for severe asthma), IGRA tests may have superior sensitivity compared to TST 3, 2

Critical Pitfalls to Avoid

• Do not place a TST during the 1-4 week period after influenza onset, as this represents peak viral-induced immunosuppression and will likely yield false-negative results 1
• Do not confuse this 4-6 week waiting period with the 8-10 week window period for TB exposure, which is an entirely different concept related to the time needed for the immune system to mount a detectable response after M. tuberculosis exposure 1, 5
• Do not assume a negative TST during active viral illness rules out TB infection—this is a critical error that could delay necessary treatment 5

When to Proceed Despite Recent Influenza

If there is high clinical suspicion for active tuberculosis (not just latent infection), do not delay diagnostic evaluation:

• Obtain sputum specimens for AFB smear and culture immediately 3
• Chest radiograph should be performed promptly 3
• Initiate empiric TB treatment if suspicion is high, even before TST or IGRA results, as treatment of active disease takes precedence over screening test timing 3