DST : Drug Susceptibility Testing for Tuberculosis; HOW & WHY?

Introduction: 

DST is used to detect resistance of Drugs used in Tuberculosis. So through this test we try to find out which drug is sensitive to TB Bacteria.

Pre-test Requirement: 

For DST positive cultured sample is required.

Culture Medium:

Mainly two medium is used to perform DST testing:

1. MGIT(Liquid Culture): Fast Method(15-28 Days) 

Mycobacteria Growth Indicator Tube (MGIT) is intended for the detection and recovery of mycobacteria. The MGIT Mycobacteria Growth Indicator Tube contains 4 mL of modified Middlebrook 7H9 Broth base. The machine used for this purpose is of two main types:

• MGIT 960: 

MGIT 960

This instrument is produced by Becton Dickinson (BD). The BD BACTEC MGIT 960 system is for high-volume mycobacteria growth, detection and susceptibility testing and is designed to meet the needs of medium- and high-volume laboratories.

 It is specially designed to accommodate Mycobacteria Growth Indicator Tube (MGIT) and incubate them at 37°C. The instrument scans the MGIT every 60 minutes for increased fluorescence. Analysis of the fluorescence is used to determine if the tube is instrument positive means test sample contain viable organism

• MGIT 320:

 The BD BACTEC MGIT 320 system is designed to help smaller-capacity laboratories quickly and accurately detect TB.

MGIT 320

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. LJ Culture(Solid Culture): Slow Method(6-8 Weeks)

Drugs Availabale for DST:

These Drugs are available on  MGIT(Liquid Culture)

1.  Clofazimine
2. Moxifloxacin
3. Linezolide
4. Rifbutin
5. Levofloxacin
6. PAS
7. Ethionamide
8. Ofloxacin
9. Ciprofloxacin
10. Kanamycin
11. Campreomycin
12. Amikacin
13. Streptomycin
14. Pyrazynamide
15. Ethambutol
16. Rifampicin
17. Isoniazid

Other than these drugs are available on LJ Culture

Why DST is used?

It helps to diffenciate between primary and acquired drug resistance and confirm choice of the initial course of therapy.
Detection of Drug resistance using liquid culture system can be accomplished in 15-28 days rather than 6-8 weeks by conventional LJ culture.
This method is Extremely useful to test drugs not yet available on genotypic platforms.

Basically DST is used to detect MDR TB or XDR TB:

A) Multidrug-resistant Mycobacterium tuberculosis (MDR-TB):

The WHO estimates that 5% of all TB cases worldwide harbor a multidrug-resistant Mycobacterium tuberculosis (MDR-TB) strain.

Definition: MDR-TB is defined as resistance against, at least, rifampicin (RMP) and isoniazid (INH). In 2007, approximately 511,000 new MDR-TB cases were reported

B) Extensively drug-resistant TB (XDR-TB):

In the latest report on anti-TB drug resistance worldwide, conducted by the WHO and the International Union Against Tuberculosis and Lung Disease (IUATLD), high rates of resistance have been shown, with values reaching more than 20% of MDR cases among new cases in certain regions. Extensively drug-resistant TB (XDR-TB) cases have been detected in at least 55 countries.

Definition: XDR-TB is defined as MDR-TB plus resistance to any fluoroquinolone, and to at least one out of three injectable second-line anti-TB drugs (capreomycin, kanamicin and amikacin)

FAQ:

Q1. What is sample required for this test?
Answer: Positive sample of already cultured specimen. It may be sputum, tissues or tissue fluids like CSF, Peritoneal fluid etc

Q2. Is any other fast method available to know Drug resistance?
Answer: Yes. Genotypic platform is also available. It is also called LPA(Line Probe Assay) or Hains Test. But Genotypic platform is only available for Rifamapicin, Isoniazid, fluroquinolones, aminoglycosides, Cyclic peptides, Ethambutol.

Refrences:
1. “Molecular Line-Probe Assay for the Detection of Resistance to Second Line Anti-TB Drugs”, WHO, www.who.int/tb/areas-of-work/laboratory/policy_statements
2. https://www.tbfacts.org/drug-susceptibility/
3.http://www.bd.com/en-us/offerings/capabilities/microbiology-solutions/mycobacteria-testing/bd-bactec-mgit-automated-mycobacterial-detection-system
4. https://www.youtube.com/watch?v=RwifN15FrOc
5. Siddiqi, Salman H.; Sabine Rüsch-Gerdes (2006). Procedure Manual For BACTEC MGIT 960 TB System.
6. Siddiqi, Salman H.; Sabine Rüsch-Gerdes (2006). Procedure Manual For BACTEC MGIT 960 TB System.
7. "Use of BACTEC MGIT 960 for Recovery of Mycobacteria from Clinical Specimens: Multicenter Study" Enrico Tortoli, Paola Cichero, Claudio Piersimoni, M. Tullia Simonetti, Giampietro Gesu, and Domenico Nista Journal of Clinical Microbiology, November 1999, p. 3578-3582, Vol. 37, No. 11 [1]
8. "Evaluation of Automated BACTEC MGIT 960 System for Testing Susceptibility of Mycobacterium tuberculosis to Four Major Antituberculous Drugs: Comparison with the Radiometric BACTEC 460TB Method and the Agar Plate Method of Proportion" Enrico Tortoli, Marta Benedetti, Alessandra Fontanelli, and M. Tullia Simonetti Journal of Clinical Microbiology, February 2002, p. 607-610, Vol. 40, No. 2 doi:10.1128/JCM.40.2.607-610.2002
8. "Evaluation of the fully automated Bactec MGIT 960 system for the susceptibility testing of Mycobacterium tuberculosis to first-line drugs: a multicenter study" Fanourios Kontosa, Maria Maniatia, Christos Costopoulosb, Zoe Gittic, Stavroula Nicolaoub, Efymia Petinakia, Spyridoula Anagnostoub, Ioannis Tselentisc and Antonios N. Maniatis Journal of Microbiological Methods Volume 56, Issue 2, February 2004, Pages 291–294 doi:10.1016/j.mimet.2003.10.015