🦠 Tuberculosis — Patho, Signs & Diagnostics
Why it hides for years, what makes it contagious, and the three tests that get confused.
🧬 What it is & why it hides
Mycobacterium tuberculosis is an aerobic bacillus with a waxy mycolic-acid coat. That coat is the whole story: it resists drying, resists ordinary stains, and resists being digested by the macrophage that swallows it.
Aerobic means it settles where oxygen is highest — the upper lobes. That is why cavitation shows up at the apices on X-ray.
🦠 What happens after you breathe it in
The distinction the exam is really testing. Latent infection is a walled-off bacillus: no symptoms, not contagious, normal chest X-ray, but a positive skin test or blood test. Active disease is the wall breaking down: symptoms, abnormal X-ray, contagious.
| Latent TB infection | Active TB disease | |
|---|---|---|
| Symptoms | None | Cough, fever, night sweats, weight loss |
| Contagious? | No | Yes — airborne |
| Skin test / IGRA | Positive | Usually positive |
| Chest X-ray | Normal | Infiltrate, upper-lobe cavitation |
| Sputum smear/culture | Negative | Positive |
| Treated? | Yes — to prevent reactivation | Yes — multi-drug |
👀 How it shows up
The classic four. A cough lasting 3 weeks or longer, night sweats, unexplained weight loss, and a low-grade afternoon fever.
A cough of three weeks or more is the screening trigger. Less than that is not TB until proven otherwise; three weeks or more, think about it.
- Hemoptysis — blood-streaked sputum, from a cavity eroding a vessel. A late sign, not an early one.
- Fatigue and anorexia — vague, which is why it goes undiagnosed for months.
- Pleuritic chest pain if the pleura is involved.
- A normal chest X-ray does not rule out latent infection — that is the point of latent.
🤵 Who is at risk
Risk of catching it
- Close household contact of an active case
- Born in, or long travel to, a high-incidence country
- Congregate living — shelters, prisons, long-term care
- Health-care workers
Risk of latent turning active
- HIV — the single strongest risk factor
- Immunosuppressants, TNF-alpha blockers, long-term steroids
- Diabetes, chronic kidney disease, malnutrition
- Very young, very old, recent infection within 2 years
🧪 How it is confirmed — the three tests
Three different tests get mixed up constantly. They answer different questions, and knowing which question each answers is most of the marks.
| Test | What it answers | Watch for |
|---|---|---|
| Mantoux / TST (skin test) | Have you ever been infected? | Read at 48–72 h. Measure induration, not redness. A previous BCG vaccine can cause a false positive. |
| IGRA (blood test) | Same question, from blood | One visit, not affected by BCG. Preferred if the patient had BCG or is unlikely to return. |
| Sputum AFB smear & culture | Do you have active disease now? | Culture is the gold standard. Three specimens, 8–24 h apart, at least one early morning. |
| Chest X-ray | Is there lung damage? | Upper-lobe infiltrate or cavity. Supports, never confirms. |
| NAAT | Is it TB, quickly? | Result in hours instead of weeks; culture still needed for drug sensitivity. |
📏 Reading the Mantoux — the cut-off depends on the person
≥5 mmis positive in the most immunocompromised: HIV, recent contact of an active case, organ transplant, chest X-ray showing old TB.≥10 mmis positive with a moderate risk factor: born in a high-incidence country, IV drug use, congregate living, health-care worker, diabetes, child under 4.≥15 mmis positive in someone with no known risk factor.
The sicker the immune system, the smaller the reaction needed to count. That single sentence reproduces the whole table.
The trap. The skin test and the blood test both stay positive for life once positive. Do not repeat a skin test on someone already known to be positive — it tells you nothing new and can produce a severe local reaction. Follow up with a chest X-ray and symptom screen instead.
🩺 What you do — airborne precautions
Active or suspected TB is airborne. Suspicion is enough — you do not wait for the culture.
- Negative-pressure room, door kept closed, 6–12 air exchanges an hour.
- N95 respirator for every person entering — fit-tested. A surgical mask is not enough for staff.
- Surgical mask on the patient whenever they leave the room, and they leave only when necessary.
- Teach: cover the cough, tissues into a lined bag, hand hygiene.
Staff wear N95. The patient wears a surgical mask. Getting those the wrong way round is a classic wrong answer.
✅ When precautions can come off
Three things together, not any one alone:
- Clinical improvement on treatment
- Three consecutive negative AFB sputum smears, collected 8–24 h apart
- Effective therapy taken for an adequate period
Reporting. TB is a notifiable disease. The public health department is contacted so contacts can be traced and tested. This is a legal duty, not a choice, and it is asked.