What tests confirm clinically vs bacteriologically confirmed tuberculosis (TB) according to the Philippine guideline?

Differentiating Clinically vs Bacteriologically Confirmed TB: Diagnostic Tests According to Philippine Guidelines

Bacteriological confirmation of TB should be sought in all suspected TB cases through appropriate specimens for AFB smear microscopy, culture, and molecular tests like GeneXpert MTB/RIF, while clinical diagnosis relies on symptoms, imaging, and other clinical findings when bacteriological tests are negative.

Bacteriologically Confirmed TB

Bacteriological confirmation is the gold standard for TB diagnosis and requires positive laboratory results from the following tests:

• AFB Smear Microscopy: Should be performed on at least three respiratory specimens from all patients suspected of having pulmonary TB 1

  • Concentrated specimens and fluorescence microscopy are preferred for improved sensitivity 1
  • First morning specimens yield 12% greater sensitivity than spot specimens 1
  • Sensitivity increases from 53.8% with first specimen to approximately 70% with three specimens 1

• Mycobacterial Culture: Remains the laboratory gold standard for TB diagnosis 1

  • Both liquid and solid cultures should be performed on every specimen 1
  • Liquid cultures have higher sensitivity (88-90%) compared to solid cultures (76%) 1
  • Cultures are essential for drug susceptibility testing and genotyping 1

• Nucleic Acid Amplification Tests (NAATs): Recommended for rapid diagnosis 1

  • GeneXpert MTB/RIF should be performed on at least one respiratory specimen, preferably the first diagnostic specimen 1
  • Results should be available within 48 hours of specimen collection 1
  • Sensitivity of 90.2% and specificity of 86.9% for pulmonary TB 2

Clinically Diagnosed TB

Clinical diagnosis is made in the absence of bacteriological confirmation but based on:

• Radiological Findings: Abnormalities consistent with TB on chest radiography or other imaging 1

  • May include computed tomography, ultrasound, magnetic resonance imaging, or positron emission tomography-computed tomography 1

• History of TB Exposure: Contact with an infectious TB case 1

• Evidence of TB Infection: Positive tuberculin skin test (TST) and/or positive interferon-gamma release assay (IGRA) 1

  • Note that TST or IGRA cannot be used to exclude a diagnosis of TB 1

• Clinical Findings: Symptoms and signs suggestive of TB 1

Diagnostic Algorithm for TB

1. Collect appropriate specimens:

   • For pulmonary TB: 3 sputum specimens (preferably early morning) 1, 3
   • For extrapulmonary TB: specimens from suspected sites (pleural fluid, CSF, ascitic fluid, etc.) 1

2. Perform AFB smear microscopy on all specimens 1

3. Conduct molecular testing (GeneXpert MTB/RIF) on at least one specimen 1

   • If NAA positive and AFB smear positive: Presume TB and begin treatment 1
   • If NAA positive and AFB smear negative: Consider additional NAA testing; presume TB if two or more specimens are NAA positive 1
   • If NAA negative and AFB smear positive: Test for inhibitors and test additional specimen with NAA 1
   • If NAA negative and AFB smear negative: Use clinical judgment based on other findings 1

4. Perform mycobacterial cultures (both liquid and solid) on all specimens 1

5. Consider clinical diagnosis if bacteriological tests are negative but clinical suspicion remains high based on:

   • Radiological findings 1
   • History of exposure 1
   • Positive TST/IGRA 1
   • Compatible clinical presentation 1

Important Considerations

• False-negative results are common with AFB smear microscopy, so a negative result does not exclude pulmonary TB 1

• GeneXpert MTB/RIF has higher sensitivity for pulmonary TB (90.2%) than for extrapulmonary TB (81.6%) 2

• Culture remains essential even when molecular tests are performed, as it is required for drug susceptibility testing 1

• The sensitivity of diagnostic tests is lower in HIV-positive patients compared to HIV-negative patients 3

• For children with suspected TB, who often cannot produce sputum, alternative specimens like gastric aspirates should be obtained 1

• Rapid laboratory confirmation can lead to earlier treatment initiation, improved patient outcomes, and reduced transmission 1