Latent TB screening with IGRA in BCG-vaccinated dialysis patients

Hemodialysismedium

A 61-year-old man with CKD stage 5 from ADPKD and heart failure is admitted with anorexia, nausea, vomiting, and weight loss. A left forearm arteriovenous fistula was placed 6 months ago. He takes lisinopril, furosemide, and atorvastatin. He is afebrile, with oxygen saturation 94% on room air, bibasilar crackles, 3+ pretibial edema, and asterixis. HD is started for uremia. Before outpatient placement, hepatitis B serologies and tuberculin skin testing (TST) are ordered. He declines TST, explaining that he was raised in Morocco and has tested positive since childhood because of vaccination. What is the most appropriate next step?

  1. A.Chest radiograph
  2. B.Tuberculin skin testing (TST)
  3. C.Sputum testing for acid-fast bacilli (AFB)
  4. D.Interferon-γ release assay (IGRA)Correct

Explanation

The most appropriate next step is an interferon-γ release assay (IGRA). Screening for latent tuberculosis infection is recommended for patients starting hemodialysis, who have a risk of TB reactivation many times that of the general population because of uremic immune dysfunction. He was born and raised in Morocco, a country with moderate TB incidence, and likely received bacille Calmette-Guérin (BCG) vaccination, which causes false-positive tuberculin skin tests. IGRAs measure interferon-γ released by T cells stimulated with ESAT-6 and CFP-10, antigens present in Mycobacterium tuberculosis but absent from BCG strains and most nontuberculous mycobacteria. They are therefore unaffected by prior BCG vaccination, require a single blood draw, and avoid the return visit and reader variability of skin testing. In patients with kidney failure, IGRAs are also less affected by anergy than TST, and CDC and ATS/IDSA/CDC guidance prefers IGRA for BCG-vaccinated persons. A positive IGRA should be followed by a chest radiograph and symptom review to exclude active disease before treatment of latent infection. A chest radiograph is required if the IGRA is positive or symptoms suggest active disease, but it is insensitive for latent infection and does not replace an immunologic screening test. Repeating the tuberculin skin test is not helpful: he reports prior positive results, BCG vaccination reduces its specificity, and uremia reduces its sensitivity through cutaneous anergy. A two-step TST would add little information. Sputum testing for acid-fast bacilli is appropriate when active pulmonary TB is suspected from cough, fever, night sweats or an abnormal chest radiograph. His symptoms and crackles are explained by uremia and volume overload, so sputum testing is not the first step.

Why each option

A.
A chest radiograph follows a positive screening test or symptoms of active disease and cannot by itself diagnose latent tuberculosis infection.
B.
Skin testing is confounded by BCG vaccination and by uremic anergy, so a repeat TST would add little reliable information here.
C.
Sputum AFB testing is for suspected active pulmonary TB, and his symptoms and crackles are explained by uremia and volume overload.
D.
Correct. IGRA uses antigens absent from BCG, so it is unaffected by childhood vaccination and preferred for latent TB screening at dialysis initiation.

Reference: Lewinsohn DM, Clin Infect Dis 2017 (Official ATS/IDSA/CDC clinical practice guidelines: diagnosis of tuberculosis in adults and children); Sterling TR, MMWR Recomm Rep 2020 (Guidelines for the treatment of latent tuberculosis infection)

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