How is osteoarticular TB diagnosed?
Diagnosis may be difficult and is often delayed up to 12 to 18 months due to the insidious onset of nonspecific symptoms. Additionally, the chest radiograph may be normal in 50% of patients with osteoarticular TB. Purified protein derivative (PPD) skin test is useful for establishing infection with TB (latent or active), but this method has a few drawbacks. Some patients will have anergy and a false-negative test may result, this test requires patients return for it to be interpreted at 48–72 hours, and it can be falsely positive in patients who have received the Bacillus Calmette–Guérin (BCG) vaccination. Interferon-γ release assays (QuantiFERON-TB Gold and T-SPOT.TB) (IGRAs) are more sensitive than the PPD test and not affected by previous BCG vaccination.
Pearl: PPD and IGRA may be falsely negative in immunocompromised patients. T-spot has higher sensitivity than quantiferon gold in immunocompromised patients.
A definitive diagnosis is established by demonstrating MTB in the tissue or synovial fluid. The sensitivity of common procedures is as follows:
| Synovial fluid smear for TB | 20% |
| Synovial fluid adenosine deaminase (>31 U/L) | 80% |
| Synovial fluid culture for TB | 80% |
| Synovial biopsy and culture | >90% |
| Nucleic acid amplification tests (NAATs) | 70% |
The advantage of NAAT is related to the rapid result time of 1 to 2 days instead of 3 to 6 weeks with conventional culture. The GeneXpert has been studied in osteoarticular disease and has the ability to detect the presence of TB in addition to rifamycin resistance. Culture and phenotypic drug susceptibility remains the gold standard for diagnosis.
