Classification by Duration
| Duration | Classification | Common Causes |
|---|---|---|
| <3 weeks | Acute cough | Viral URTI (common cold, influenza, COVID-19), acute bronchitis, pneumonia |
| 3 – 8 weeks | Subacute cough | Post-infectious cough, whooping cough (pertussis), early TB |
| >8 weeks | Chronic cough — investigate | Asthma, GORD/GERD, post-nasal drip (UACS), ACE inhibitor cough, TB, COPD, bronchiectasis, lung cancer |
Blood Tests for Persistent Cough
| Test | What it Detects | When to Order |
|---|---|---|
| CBC with differential | Infection (neutrophilia), allergy/eosinophilia (eosinophilic asthma), lymphocytosis (viral) | All cases of persistent cough |
| CRP / ESR | Active infection or inflammation | When bacterial or inflammatory cause suspected |
| Total IgE + eosinophil count | Allergic / atopic cause, eosinophilic airway disease | Suspected allergic asthma or eosinophilic cough |
| Mantoux (TST) or IGRA (QuantiFERON TB Gold) | Tuberculosis (TB) exposure and infection | Chronic cough >3 weeks, especially with weight loss, night sweats, or TB risk factors |
| Sputum culture + AFB smear | Bacterial infection, TB mycobacteria | Productive cough, suspected TB or recurrent chest infections |
| Spirometry (PFT — Pulmonary Function Test) | Obstructive pattern (asthma, COPD), restrictive pattern | Chronic cough — essential to exclude asthma; reversibility test with bronchodilator |
ACE Inhibitor Cough: If you are taking an ACE inhibitor (ramipril, enalapril, lisinopril) for blood pressure or heart failure, these medications cause a persistent dry cough in 10–15% of patients (more common in South Asian patients). This is a very common and often missed cause of chronic cough. Switching to an ARB (like losartan or telmisartan) resolves the cough within 1–4 weeks.
Red Flags — Urgent Investigation Required: Coughing up blood (haemoptysis), unintentional weight loss, persistent night sweats, hoarse voice, difficulty swallowing, a lump in the neck, cough starting in a smoker over 40. These require immediate chest X-ray and specialist review to exclude TB and lung cancer.
Questions to Ask Your Doctor
- Am I taking an ACE inhibitor that could be causing my cough?
- Should TB be excluded given my symptoms and duration?
- Do I need spirometry to check for asthma?
- Could this be silent acid reflux (GORD) causing my cough?
- Do I need a chest X-ray?
Medical Disclaimer: This information is for educational purposes only and does not constitute medical advice. Persistent cough warrants medical evaluation, especially if associated with red flag features.