Epidermology of tb and investigation psm k park detail easy language
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Tuberculosis (TB) - Epidemiology & Investigations
As per Park's Textbook of Preventive and Social Medicine
1. WHAT IS TB?
TB is a specific infectious disease caused by Mycobacterium tuberculosis. It mainly affects the lungs (pulmonary TB), but can also affect:
- Intestine
- Meninges (brain covering)
- Bones and joints
- Lymph glands
- Skin
Bovine TB (from cattle) can also be transmitted to humans.
2. GLOBAL BURDEN (Problem Statement)
| Fact | Detail |
|---|---|
| Population infected (silent) | ~1/3rd of world population asymptomatically infected |
| Who develops disease | 5-10% of infected develop clinical TB in their lifetime |
| New cases/year (2019) | ~10 million globally |
| Annual infection risk in high burden countries | 0.5-2% |
| One patient spreads to | 10-15 persons per year |
| TB deaths (HIV-negative, 2019) | 1.2 million |
| TB deaths (HIV-positive, 2019) | 208,000 |
| Gender split | Men 56%, Women 32%, Children <15 yrs 12% |
3. INDIA'S TB BURDEN
- India has the highest TB burden in the world.
- 2/3 of cases are males, but >50% of female cases occur in those below 34 years of age.
- TB kills more women in reproductive age than all maternal causes combined.
- 1/3 of female infertility in India is caused by TB.
- ~3 lakh children of TB patients drop out of school.
- A patient loses 3-4 months of income during recuperation.
-
90% of TB's economic burden = loss of life, not just illness.
- Mainly a disease of the poor: migrant labourers, slum dwellers, and backward areas.
4. EPIDEMIOLOGICAL INDICES (How We Measure TB in a Community)
These are important Park's PSM exam points:
| Index | Meaning |
|---|---|
| Incidence | New + recurrent (relapse) TB episodes in a given year |
| Prevalence | Total TB cases (all forms) at a given point in time; best index for case load |
| Mortality | Deaths due to TB (HIV-negative counted separately from HIV-positive per ICD-10) |
| Case Fatality Rate | Risk of death among those with active TB |
| Case Notification Rate | New + recurrent TB cases notified to WHO per 100,000 population per year |
| Case Detection Rate | Notifications of new+relapse cases ÷ Estimated incidence × 100 |
5. CLASSIFICATION OF TB CASES
A. Based on Drug Susceptibility (Drug Resistance)
| Type | Definition |
|---|---|
| Mono-resistance | Resistant to ONE first-line drug only |
| Polydrug resistance | Resistant to >1 first-line drug (but NOT both INH + Rifampicin together) |
| MDR-TB | Resistant to at least BOTH Isoniazid (INH) and Rifampicin |
| XDR-TB | MDR + resistant to any fluoroquinolone + at least one second-line injectable (kanamycin, amikacin, capreomycin) |
| Rifampicin Resistance (RR-TB) | Resistance to Rifampicin by any method (includes MDR + XDR) |
B. Based on HIV Status
- HIV-positive TB
- HIV-negative TB
- HIV-unknown TB
6. INVESTIGATIONS FOR TB (Case Finding)
A. Sputum Smear Microscopy (Most Important - Primary Method)
- Sputum examination by direct microscopy = Method of Choice for field diagnosis
- Stains used:
- ZN (Ziehl-Neelsen) stain - conventional microscope
- LED FM (Fluorescent Microscopy) - more sensitive
- Why it's #1: Cheap, reliable, easy, discovers the most epidemiologically dangerous cases (sputum-positive = biggest spreaders)
- Limitation: Limited sensitivity, especially in children and HIV-positive patients
B. Culture
| Method | Detail |
|---|---|
| Solid culture (LJ medium) | Gold standard but takes 6-8 weeks |
| Liquid culture (MGIT-960, BacT Alert, Versatrek) | Faster; results in up to 42 days |
| Drug Sensitivity Testing (DST) | Available 14-26 days after culture turns positive |
- Used for:
- Follow-up of DR-TB patients
- Long-term follow-up of DS-TB to ensure relapse-free cure
C. Rapid Molecular Tests
| Test | What it detects |
|---|---|
| CBNAAT / TrueNat (NAAT) | MTB + Rifampicin resistance (rpoB gene) |
| Line Probe Assay (LPA) - FL LPA | MTB complex + RIF & INH resistance |
| Line Probe Assay - SL LPA | Fluoroquinolone + Second-line injectable resistance |
| PCR-based methods | rpoB (Rifampicin), katG + inhA (Isoniazid) resistance genes |
D. Tuberculin Skin Test (TST / Mantoux Test)
- Detects cellular immune response to TB proteins (PPD)
- Read at 48-72 hours after injection
- Positive = presence of infection (NOT necessarily active disease)
- Limitations (important for exams):
- Negative test does NOT rule out TB (immune response may be depressed in active TB)
- False negatives: HIV, malnutrition, malignancy, Hodgkin's, measles, chickenpox, steroids, severe bacterial infections, BCG recently given
- False positives: BCG vaccination, atypical mycobacteria cross-reaction
Two-Step Testing (for health workers):
- 1st test → if negative → 2nd test after 1-3 weeks → if 2nd positive = old infection (boosted), if 2nd negative = truly uninfected
E. Interferon Gamma Release Assay (IGRA)
- Blood test, more specific than TST
- Not affected by BCG vaccination
- Used as supportive diagnostic tool
F. Supportive/Radiological Tools
- Chest X-ray - shows cavitation, infiltrates, hilar adenopathy (supportive, not confirmatory)
- Histopathology - caseous granuloma with Langhans giant cells (tissue biopsy)
7. QUICK MEMORY TABLE: Investigations at a Glance
| Test | Use | Key Point |
|---|---|---|
| Sputum Smear (ZN stain) | Primary case finding | Cheap, fast, most used |
| Culture (LJ/MGIT) | Confirmation + DST | Takes weeks |
| CBNAAT/TrueNat | Rapid diagnosis + Rif resistance | Done in 2 hours |
| LPA (FL/SL) | First/second-line drug resistance | For MDR/XDR-TB |
| Mantoux/TST | Infection screen | Does NOT confirm active TB |
| IGRA | Infection screen | More specific than TST |
| Chest X-ray | Supportive | Not confirmatory |
8. TB PREVENTIVE TREATMENT (2019 Data)
- 45% of HIV-positive newly enrolled patients received preventive treatment
- 33% of children under 5 (household contacts of confirmed TB) received preventive treatment
Source: Park's Textbook of Preventive and Social Medicine, 25th edition - Chapter on Tuberculosis (Epidemiology, Investigations, RNTCP/NTEP)
Exam Tips (PSM perspective):
- Sputum smear = method of choice for case finding in the field
- Prevalence = best practical index for case load
- 1/3 of world infected, 5-10% develop disease
- One infectious patient spreads to 10-15 persons/year
- MDR = INH + Rifampicin resistance (minimum)
- Negative Mantoux does NOT rule out TB (immunosuppression causes false negatives)
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