Case Definitions of TB

Standardized case definitions are essential for the proper registration, treatment allocation, and monitoring of tuberculosis patients. Cases are classified based on the history of previous treatment, the basis of diagnosis, and the drug resistance pattern.

1. Classification Based on History of Previous Treatment

Patients are classified into two broad categories to decide the appropriate regimen (though currently, drug sensitivity testing guides therapy more than history alone).

A. New Case

A TB patient who has never had treatment for tuberculosis or has taken anti-TB drugs for less than one month.

B. Previously Treated Case

Patients who have received 1 month or more of anti-TB drugs in the past. They are further classified as:

1. Recurrent TB Case: A TB patient previously declared as successfully treated (cured or treatment completed) and is subsequently found to be a microbiologically confirmed TB case.
2. Treatment After Failure Case: Patients who have previously been treated for TB and whose treatment failed at the end of their most recent course of treatment.
3. Treatment After Lost to Follow-up Case: A TB patient who was previously treated for TB for one month or more and was declared "lost to follow-up" (interrupted treatment for 2 consecutive months or more) in their most recent course of treatment and is subsequently found to be a microbiologically confirmed TB case.
4. Other Previously Treated Case: Patients who have previously been treated for TB but whose outcome after the most recent course of treatment is unknown or undocumented.

2. Classification Based on Basis of Diagnosis

A. Bacteriologically Confirmed TB

A patient with TB diagnosed in a biological specimen by smear microscopy, culture, or a World Health Organization (WHO)-endorsed rapid molecular test (NAAT) adopted by NTEP (such as Xpert MTB/RIF® or Truenat®).

B. Clinically Diagnosed TB

A patient who does not fulfill the criteria for bacteriological confirmation but has been diagnosed with active TB by a clinician or other medical practitioner who has decided to give the person a full course of TB treatment. This includes cases diagnosed on the basis of X-ray abnormalities, suggestive histology, or clinical signs/symptoms without laboratory confirmation.

3. Classification Based on Drug Resistance Pattern

Cases are classified based on in-vitro drug susceptibility testing (phenotypic or genotypic).

1. Mono-resistant TB (MR-TB): A TB patient whose biological specimen is resistant to one first-line anti-TB drug only.
2. Isoniazid-resistant TB (Hr-TB): A TB patient whose biological specimen is resistant to Isoniazid and susceptibility to Rifampicin has been confirmed.
3. Poly-drug resistant TB (PDR-TB): A TB patient whose biological specimen is resistant to more than one first-line anti-TB drug, other than both Isoniazid and Rifampicin.
4. Rifampicin-resistant TB (RR-TB): A TB patient whose biological specimen is resistant to Rifampicin detected using phenotypic or genotypic methods, with or without resistance to other anti-TB drugs. It includes any resistance to R in the form of mono-resistance, poly-resistance, MDR, or XDR.
5. Multidrug-resistant TB (MDR-TB): A TB patient whose biological specimen is resistant to both Isoniazid and Rifampicin with or without resistance to other first-line anti-TB drugs.
6. Pre-extensively drug-resistant TB (Pre-XDR-TB): TB caused by Mycobacterium tuberculosis strains that fulfill the definition of MDR/RR-TB and are also resistant to any fluoroquinolone.
7. Extensively drug-resistant TB (XDR-TB): TB caused by Mycobacterium tuberculosis strains that fulfill the definition of MDR/RR-TB and are also resistant to any fluoroquinolone (levofloxacin or moxifloxacin) AND at least one additional Group A drug (presently either Bedaquiline or Linezolid or both).

4. Pediatric Specific Definitions

A. Presumptive Pediatric TB

Refers to children suspected of suffering from TB based on:

• Persistent fever for >2 weeks.
• Cough for >2 weeks.
• Significant weight loss (>5% loss in the past 3 months) or failure to gain weight in the past 3 months despite adequate nutrition.
• History of contact with an infectious TB case.

B. "Probable MDR-TB" in Children

A term applied when DR-TB is strongly suspected clinically, but bacteriologic confirmation is not feasible. The Nodal DR-TB committee takes the decision to treat. Criteria include:

• Signs/symptoms of active TB PLUS
• Risk factors:
  • Close contact with a known MDR-TB case.
  • Close contact with a person who died during TB treatment.
  • Close contact with a person who failed TB treatment.
  • Non-response/failure of a first-line regimen.
• AND appropriate specimens fail to demonstrate M. tb (culture negative).

C. Severe Disease in Children (for treatment duration)

• Pulmonary: Presence of cavities or bilateral disease on chest radiography.
• Extrapulmonary: Presence of miliary TB, TB meningitis, or forms other than peripheral lymphadenopathy (e.g., pericardial, abdominal, osteoarticular).