From Tuberculosis › Chapter 7: Case definitions · Médecins Sans Frontières, Current online edition, 2026
Case definitions for drug-susceptible and drug-resistant tuberculosis for clinicians managing TB programmes.
- Issuer
- Médecins Sans Frontières
- Edition
- Current online edition, 2026
- Audience
- Clinicians, TB programme managers, public health professionals
- Evidence
- WHO-aligned case definitions
- Reviewed
- 22 Oct 2026 · GMJ
What this guideline says
GMJ summary
This section of the MSF Tuberculosis guideline provides standardised case definitions for classifying tuberculosis patients according to drug susceptibility patterns. It defines drug-susceptible TB (DS-TB) as TB caused by strains not resistant to rifampicin and isoniazid, and establishes six categories of drug-resistant TB (DR-TB) based on resistance patterns.
The DR-TB categories include isoniazid-resistant TB (Hr-TB), rifampicin-resistant TB (RR-TB), multidrug-resistant TB (MDR-TB), pre-extensively drug-resistant TB (pre-XDR-TB), and extensively drug-resistant TB (XDR-TB). The guideline also addresses unconfirmed DR-TB cases where treatment is initiated without drug susceptibility test (DST) results, emphasising the importance of reclassification once DST results become available.
These definitions align with WHO standards updated through 2020-2021 and provide a practical framework for recording, reporting and treating TB cases in field settings, particularly in resource-limited environments where MSF operates.
Key points
For clinicians — doses and thresholds as stated in the guideline
- Drug-susceptible TB (DS-TB) is defined as TB caused by strains not resistant to rifampicin and isoniazid
- Isoniazid-resistant TB (Hr-TB) shows susceptibility to rifampicin but resistance to isoniazid
- Multidrug-resistant TB (MDR-TB) demonstrates resistance to at least rifampicin and isoniazid
- Pre-extensively drug-resistant TB (pre-XDR-TB) is MDR/RR-TB with additional resistance to any fluoroquinolone but susceptible to both bedaquiline and linezolid
- Extensively drug-resistant TB (XDR-TB) is MDR/RR-TB with additional resistance to any fluoroquinolone and at least either bedaquiline or linezolid
- Patients with unconfirmed DR-TB (no DST result) should be recorded according to their presumed resistance based on prescribed treatment
- All patients should be reclassified when full DST results from specimens collected prior to treatment initiation become available
Drug-resistant tuberculosis case definitions
Table extracted from the guideline
| Category | Resistance pattern |
|---|---|
| Hr-TB | Susceptibility to rifampicin and resistance to isoniazid |
| MDR-TB | Resistance to at least rifampicin and isoniazid |
| RR-TB | Resistance to rifampicin, with or without resistance to other TB drugs |
| Pre-XDR-TB | MDR/RR-TB with additional resistance to any fluoroquinolone (susceptible to bedaquiline and linezolid or not tested) |
| XDR-TB | MDR/RR-TB with additional resistance to any fluoroquinolone and at least either bedaquiline or linezolid |
Source: Médecins Sans Frontières, Drug Susceptibility Pattern — Tuberculosis — MSF Tuberculosis. Reproduced in summary form.
From the guideline
Verbatim
“Once the result of the full drug susceptibility test (DST) performed on a specimen collected prior to treatment initiation is obtained, patients should be reclassified if the DST shows resistance that could not be detected initially.”
Editor’s perspective
Why this matters for Georgia and the region
Prof. Giorgi Pkhakadze, MD, MPH, PhD
Editor-in-Chief, Georgian Medical Journal · Chair, Public Health Institute of Georgia · WHO consultant, health and migration
As Editor-in-Chief of the Georgian Medical Journal and WHO consultant, I recognise the critical importance of standardised TB case definitions for Georgia, which has one of the highest MDR-TB burdens in the WHO European Region. These MSF definitions align with WHO standards that Georgia's National TB Programme has adopted, ensuring consistency in surveillance, reporting and treatment allocation. I would particularly emphasise to Georgian clinicians the requirement to reclassify patients once full DST results become available—a practice that directly impacts treatment outcomes and programmatic reporting accuracy. The realistic implementation challenge for Georgia remains ensuring timely access to second-line DST, particularly for bedaquiline and linezolid, in peripheral regions outside Tbilisi, where laboratory capacity for phenotypic DST may be limited and specimen transport logistics can delay results by weeks.
ORCID 0000-0001-7609-4515 · Perspective written for the GMJ Guidelines Library, 22 Oct 2026
Applicability
Georgia and the South Caucasus
These case definitions are directly applicable to Georgia and the South Caucasus region, which faces significant TB and MDR-TB burden. National TB programmes in Georgia, Armenia and Azerbaijan require standardised classification systems for surveillance, reporting and treatment assignment. The definitions support rational use of drug susceptibility testing resources and guide appropriate treatment regimen selection. Implementation requires functional laboratory capacity for rapid molecular testing (e.g. Xpert MTB/RIF) and culture-based DST for second-line drugs including fluoroquinolones, bedaquiline and linezolid—capacity that varies across the region and may require strengthening in peripheral facilities.
Questions this guideline answers
Quick answers
What is the practical definition of drug-susceptible TB?
In practice, patients with TB strains not resistant to rifampicin and isoniazid are recorded and treated as having drug-susceptible TB (DS-TB), even if formal susceptibility testing to all first-line drugs has not been performed.
How are MDR-TB and RR-TB patients grouped for treatment purposes?
Patients with MDR-TB (resistance to at least rifampicin and isoniazid) and those with RR-TB (resistance to rifampicin with or without resistance to other drugs) are grouped together as 'MDR/RR-TB' patients for treatment and recording purposes.
What distinguishes pre-XDR-TB from XDR-TB?
Pre-XDR-TB is MDR/RR-TB with additional resistance to any fluoroquinolone but still susceptible to both bedaquiline and linezolid (or not tested for these drugs). XDR-TB has the same fluoroquinolone resistance plus resistance to at least either bedaquiline or linezolid.
How should patients be managed when DST results are not yet available?
Patients prescribed DR-TB treatment without DST results (e.g. known DR-TB contact, DS-TB treatment failure, or when specimens cannot be obtained) should be recorded as unconfirmed DR-TB in the register corresponding to their presumed resistance pattern, then reclassified when DST results become available.
Related on GMJ
Interconnected resources
Dictionary of Public Health
Drugs A–Z
Conditions A–Z
GMJ News coverage
Quality & accreditation
- ASF Laboratory — Standard 1 — Governance & Document Control
- ASF Laboratory — Standard 6 — Pre-Examination: Specimen Pathway
- ASF Laboratory — Standard 7 — Examination: Process & Safety
- ASF Laboratory — Standard 8 — Post-Examination: Reporting & Critical Values
- ASF Hospital — Standard 13 — Radiology
- ASF Laboratory — Standard 4 — Equipment, Reagents & Traceability
Same guideline
- Bacteriological status — MSF TuberculosisChapter 7: Case definitions
- High-risk groups — MSF TuberculosisChapter 6: Screening for active tuberculosis
- MSF Tuberculosis: Chapter 6.1 Introduction — Screening for active tuberculosis — MSF TuberculosisChapter 6: Screening for active tuberculosis
- Guidance for using diagnostic algorithm for pulmonary tuberculosis — MSF TuberculosisChapter 5: Diagnostic algorithm for pulmonary tuberculosis in symptomatic adolescents and adults
- Diagnostic approach — MSF TuberculosisChapter 4: Diagnosis of active tuberculosis in children
- Diagnosis of Active Tuberculosis in Children: Introduction — MSF TuberculosisChapter 4: Diagnosis of active tuberculosis in children
Other guidelines on this topic
- Bacteriological status — MSF Tuberculosis
- High-risk groups — MSF Tuberculosis
- MSF Tuberculosis: Chapter 6.1 Introduction — Screening for active tuberculosis — MSF Tuberculosis
- Cutaneous abscess — Clinical guidelines — Diagnosis and treatment manual — MSF Clinical guidelines
- Guidance for using diagnostic algorithm for pulmonary tuberculosis — MSF Tuberculosis
- Issuer
- Médecins Sans Frontières
- Guideline
- Tuberculosis
- Chapter
- Chapter 7: Case definitions
- Type
- Clinical guideline (chapter)
- Edition
- Current online edition, 2026
- Status
- Current
- Published
- 2026-10-14
- Scope
- Case definitions for drug susceptibility patterns in tuberculosis
- Audience
- Clinicians, TB programme managers, public health professionals
- Evidence
- WHO-aligned case definitions
- Record history
- Created 22 Oct 2026 · updated 22 Oct 2026 · next check Apr 2027
Médecins Sans Frontières. Drug susceptibility pattern. In: Tuberculosis [guideline]. Paris: MSF; 2026. Chapter 7, Section 7.3.GMJ Guidelines Library records are original summaries written by the GMJ editorial team from the issuer’s published document and reviewed by a physician. They do not replace the full guideline. The document is the copyright of its issuer and is linked, not hosted. Spotted an error or a newer edition? Tell the Guidelines Desk.
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