Tubercolosis Tests & Diagnosis

MDR & XDR is on the rise and is now a major concern. Therefore, a comprehensive TB diagnostic solution is essential.

GeneXpert

GeneXpert MTB/RIF may be used rather than conventional microscopy and culture as the initial diagnostic test in all adults presumed to have TB

• Rapid test for detection of M. tuberculosis genome and Rifampicin resistance (detects rpoB gene mutation)
• Can be performed on all samples: pulmonary & extra pulmonary
• Can be done in smear positive as well as smear negative samples
• Rifampicin resistance is a surrogate marker for MDR-TB in 95% of strains
• Rapid TAT: Reports available same day

GeneXpert test is capable of detecting Rifampicin resistance with 95 % sensitivity and 98 % specificity

TB Culture

• TB Culture using MGIT BACTEC (Liquid Based System) is the Gold Standard in diagnosis of tuberculosis
• All samples- pulmonary & extra pulmonary can be cultured by MGIT BACTEC
• Offers high rate of isolation (with 14% more M. tuberculosis recovered) as compared to solid culture
• TAT using MGIT BACTEC is 14-15 days at the earliest
  Positive cultures are preserved for 12 weeks to allow drug sensitivity testing

 

TB Drug Sensitivity

• Detection of drug resistance can be done using MGIT BACTEC and can be accomplished in 2-3 weeks (after a +ve TB culture)
• Susceptibility to 13 drugs

MDR-TB

• Multi drug-resistant TB: Resistant to Isoniazid (INH) and Rifampicin.
• WHO estimates that 2.2% of new pulmonary tuberculosis (PTB) cases and 15% of previously treated PTB cases in India have MDR-TB (Global Tuberculosis Control 2015, WHO, Geneva, 2015www.who.int/tb/publications/global_report/-)

XDR-TB

• Extensively drug-resistant TB: Resistant to
  (1) INH and Rifampicin
  (2) Fluoroquinolones- Moxifloxacin, Ciprofloxacin, Levofloxacin
  (3) One of the Injectable drugs – Amikacin, Kanamycin, Capreomycin
• The Drug Resistance Surveillance (DRS) survey in India found XDR-TB in 4% of the identified MDR-TB isolates.
• It is estimated that a minimum of 2,000 new cases of XDR-TB emerge in India annually. Ref:http://www.tbonline.info/media/uploads/documents/jmmdraft2015.pdf

MTB Detection Test/ TB PCR

• Test can be performed on paucibacillary extra pulmonary TB (10-15% of all TB cases) samples
• Targets IS6110 gene which is present in multiple (4-15) copies in TB genome, hence sensitivity increases in comparison to other gene targets

Line Probe Assay (HAINS Test)

• Rapid Test for Molecular detection of Mycobacterium tuberculosis genome & resistance to 1st line drugs (Rifampicin & Isoniazid) and 2nd line drugs like Flouroquinolones, Aminoglycosides & Ethambutol
• Line Probe Assay- line 1 (MDR screen): Performed on pulmonary samples and +ve TB cultures
  • RpoB gene associated with Rifampicin resistance
  • KatG and InhA gene associated with Isoniazid resistance \
• Line Probe Assay- line 2 (XDR screen): Performed on smear +ve pulmonary samples and +ve TB cultures
  • GYR gene associated with Fluroquinolone resistance
  • RRS gene associated with Aminoglycoside resistance
  • EMB gene associated with Ethambutol resistance
• Results are available next day

Cytology/ Biopsy

• Gross examination procedure by dedicated histopathologist
• Application of stringent diagnostic criteria/protocols for reporting (as per CAP guidelines)
• Application of special stains-ZN stain

Adenosine Deaminase (ADA)

• Marker of T-cell activation and cell-mediated immune response helps differentiate tubercular from non-tubercular aetiology
• Rapid test useful in accurate and early diagnosis in patients with extra pulmonary tuberculosis
• ADA test showed 100 % sensitivity and 94.6% specificity in tubercular pleural, peritoneal and pericardial effusion with a cut of value of 40 U/L with mean value of 100 U/L, 92U/L and 90 U/L respectively.
  Ref: [Indian J Tuberc 2006; 53:92-95]

Extra Pulmonary (Ascitic Fluid, Pleural Fluid & CSF)

 

Extra Pulmonary (Lymph Node Aspirate OR Tissue)

Pulmonary (Sputum/ BAL)