Multi-drug-resistant tuberculosis classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Ammu Susheela, M.D. [2]

Overview

Classification of drug resistant tuberculosis is based on drug susceptibility , pathophysiology, clinical classification and molecular classification. According to drug susceptibility it is classified as monoresistance, poly resistance, multidrug resistance, extensive drug resistance and rifampicin resistance. Clinically it is classified as acquired and primary resistance. The restriction fragment length polymorphism helps for molecular classification into unique and cluster type.

Classification

Based on the drug susceptibility testing of cinical isolates confirmed to be Mycobacterium tuberculosis, the strains are classified as follows.

Classification	Description
Monoresistance	Strain resistant to any one of the first line anti tubercular drugs
Polydrug resistance	Strain resistant to more than one first line anti tubercular drugs. (Other than both rifampicin and isoniazid)
Multidrug resistance	Strain resistant to both rifampicin and isoniazid
Extensive drug resistance	In addition to the multi drug resistance, strain is resistant to any fluroquinolone and to atleast one of the three second line injectable anti TB drugs like amikacin, capreomycin and kanamycin.
Rifampicin resistance	Using genotypic or phenotypic method, strain has been detected to have resistance to rifampicin with or without resistance to other anti TB drugs. It can include monoresistance, multi drug resistance, poly drug resistance or extensive drug resistance.

Clinical classification

Clinically drug resistant tuberculosis can be classified into acquired drug resistance and primary drug resistance according to pathophysiology.

Acquired resistance

According to WHO , acquired resistance is defined as the isolation of drug resistant M. tuberculosis from a patient who has been treated for TB for one month or longer. Strains can acquire resistance against anti tubercular drugs in the following conditions.

• Inadequately treated by errors by caregivers in prescribing or administering drugs or supplying a poor quality drug
• Patients not completing the full course of treatment
• Patient receiving only partial treatment.

Such conditions can lead to the bacterial population to live for several months during the course of treatment thereby developing resistance against such drugs.

Primary resistance

WHO defines primary resistanc as the isolation of a drug resistant strain from a patient without a history of previous treatment. When a patient gets infected with strains already resistant to anti tubercular drugs it is called as primary resistance. It could be against one drug of 1st line anti tubercular drugs or many drugs in the first and second line of drugs.

Molecular classification

Unique

The restriction fragment length polymorphism helps in molecular classification of drug resistance into unique strains of drug resistance.

Cluster

The restriction fragment length polymorphism helps in molecular classification of drug resistance into cluster of strains of drug resistance.

Classification based on pathogenesis

Below is a table with the current classification system of tuberculosis (TB), based on its pathogenesis:

Class	Type of Tuberculosis	Description
0	No TB exposure Not infected	No history of TB exposure Negative result to a TST or IGRA
1	TB exposure No evidence of infection	History of TB exposure Negative result to a TST (given at least 8-10 weeks after exposure) or IGRA
2	TB infection No TB disease	Positive result to a TST or IGRA Negative smears and cultures (if done) No clinical or x-ray evidence of active TB disease
3	Clinically active TB	Positive culture (if done) for M. tuberculosis Positive result to a TST or IGRA, and clinical, bacteriological, or x-ray evidence of TB disease
4	Previous TB disease (not clinically active)	Medical history of TB disease Abnormal but stable x-ray findings Positive result to a TST or IGRA Negative smears and cultures (if done) No clinical or x-ray evidence of active TB disease
5	Suspected TB	Signs and symptoms of TB disease, but incomplete evaluation