Can corticosteroids (steroids) affect the Purified Protein Derivative (PPD) test results?

Effect of Corticosteroids on PPD Test Results

Yes, corticosteroids can significantly affect PPD (Purified Protein Derivative) test results by causing false-negative readings due to their immunosuppressive effects on T-cell function and delayed-type hypersensitivity reactions.

Mechanism of Interference

Corticosteroids suppress the immune system's ability to mount a delayed-type hypersensitivity reaction, which is essential for producing the induration measured in a PPD test:

• They inhibit T-cell function necessary for the inflammatory response that produces visible induration at the test site 1
• This immunosuppressive effect can lead to false-negative results, particularly with high-dose or prolonged therapy

Factors Affecting Test Reliability

The impact of corticosteroids on PPD test results depends on several factors:

• Dose: High-dose oral steroids (≥15 mg prednisone or equivalent daily) are most likely to cause false-negative results 1
• Duration: Corticosteroid use for ≥1 month significantly increases the risk of false-negative results 1
• Timing: The suppressive effect may persist for weeks after discontinuation of steroids 1

Clinical Implications

The American Thoracic Society and CDC recommend several important considerations:

• PPD testing should ideally be performed before initiating corticosteroid therapy 1
• For patients already on corticosteroids, a lower threshold for positivity (≥5 mm induration) should be used instead of the standard ≥10 mm or ≥15 mm thresholds 2, 1
• Clinicians should maintain a high index of suspicion for TB in steroid-treated patients regardless of PPD results 1

Alternative Testing Approaches

For patients on corticosteroid therapy:

• Interferon-Gamma Release Assays (IGRAs) may be less affected by steroid therapy and should be considered as an alternative to PPD testing 1
• Chest radiography should be included in the evaluation of patients on steroids who are at risk for TB 2
• Multiple sputum specimens may be needed for definitive diagnosis in high-risk patients 1

Common Pitfalls to Avoid

• Do not rely solely on PPD results in patients on steroids due to increased false-negative risk
• Do not assume a negative PPD excludes TB infection in patients on corticosteroids
• Do not delay TB screening until after starting steroids when possible
• Do not overlook the need for TB prophylaxis in high-risk patients requiring long-term steroid therapy

Special Considerations

• In a study of patients with corticosteroid-resistant asthma, prednisolone significantly suppressed cutaneous induration in corticosteroid-sensitive patients but not in corticosteroid-resistant patients 3
• Research has shown that inhaled corticosteroids appear to be safe in tuberculin-positive asthmatic children and may not significantly affect PPD test results at standard doses 4

For optimal patient care, consider TB screening before initiating corticosteroid therapy whenever possible, and interpret negative PPD results with caution in patients already on corticosteroids.