IGRA Relevance in Diagnosing Spinal Tuberculosis
IGRAs have limited utility for diagnosing spinal tuberculosis because they cannot distinguish between latent TB infection and active TB disease, and should not be used alone for diagnosing active tuberculosis. 1
Understanding IGRA's Role and Limitations
IGRAs are designed to detect M. tuberculosis infection (both latent and active), but they fundamentally cannot differentiate between latent TB infection (LTBI) and active TB disease. 1 This is a critical limitation when evaluating suspected spinal tuberculosis, which represents active, extrapulmonary disease requiring immediate treatment.
Why IGRAs Fall Short for Active TB Diagnosis
- IGRAs measure cellular immune response to M. tuberculosis antigens, similar to TST, but this response occurs in both latent infection and active disease. 2, 3
- The CDC explicitly recommends that IGRAs should not be used alone for diagnosing active TB. 1
- A positive IGRA only confirms M. tuberculosis infection has occurred at some point—it does not indicate whether disease is currently active or localized to the spine. 1
When IGRA May Provide Supportive Information
While not diagnostic for spinal TB specifically, IGRA can serve as an adjunctive tool:
- In symptomatic patients with radiographic findings suggestive of spinal TB, a positive IGRA supports the diagnosis of M. tuberculosis infection and increases clinical suspicion. 1
- IGRAs have higher specificity than TST, particularly in BCG-vaccinated populations, reducing false-positive results from prior vaccination. 2, 3
- In immunocompromised patients, IGRAs may be more sensitive than TST for detecting M. tuberculosis infection. 1, 4
Critical Pitfalls to Avoid
- Never rely on IGRA alone to diagnose spinal tuberculosis—definitive diagnosis requires microbiological confirmation (culture, molecular testing) or histopathology from spinal tissue/fluid. 1
- Negative IGRA results do not exclude TB infection, especially in immunocompromised individuals where sensitivity is reduced. 1
- In healthy persons with low likelihood of TB infection, a single positive IGRA may represent a false positive and should not be taken as reliable evidence of infection. 1
Practical Algorithm for Suspected Spinal TB
Clinical assessment: Look for back pain, neurological deficits, constitutional symptoms (fever, night sweats, weight loss), and risk factors for TB exposure. 2
Imaging: Obtain MRI of the spine showing characteristic findings (vertebral destruction, paravertebral abscess, disc space involvement).
Microbiological diagnosis: Pursue CT-guided biopsy or surgical sampling for acid-fast bacilli smear, mycobacterial culture, and molecular testing (GeneXpert MTB/RIF).
IGRA as adjunct: Order IGRA (preferred over TST if BCG-vaccinated) to support clinical suspicion, but proceed with treatment based on clinical and microbiological evidence, not IGRA results alone. 3, 1
Treatment initiation: Do not delay anti-TB therapy while awaiting IGRA results if clinical suspicion is high and imaging is consistent with spinal TB.
The bottom line: IGRA is a screening tool for M. tuberculosis infection, not a diagnostic test for active spinal tuberculosis. 1 Diagnosis of spinal TB requires integration of clinical presentation, imaging findings, and most importantly, microbiological or histopathological confirmation from spinal specimens.