Active TB disease diagnosis is usually pursued by mycobacterial acid -fast bacilli smear (“AFB smear”), mycobacterial culture (“AFB culture”), nucleic acid amplification tests (e.g., Xpert MTB/RIF), radiological exam, and histopathology.

Phenotypic drug-susceptibility testing (DST) is optimal for patient care and genotyping is paramount for tuberculosis prevention and control. Both DST and genotyping require recovery of an M. tuberculosis isolate from a positive culture. Thus, adequate samples for mycobacterial culture should be obtained whenever possible.

Diagnostic samples can be collected after TB treatment initiation among patients with life-threatening disease or other compelling indications for immediate TB treatment (Figure 1).

TST and/or IGRA are negative in 10-30% of patients with active TB. Thus, a negative TST and/or IGRA do not rule out active TB.

The sensitivity of each test (smear, culture, and nucleic acid amplification) differs by sample source (e.g., sputum vs. cerebrospinal fluid).

ATS/CDC/IDSA guidelines for Diagnosis of Tuberculosis in Adults and Children are a comprehensive source for laboratory tests for TB diagnosis [Lewinsohn et al. Clinical Infectious Diseases, 2017].