| Condition | Investigations |
| Cystic fibrosis | Sweat test, nasal potential difference, assessment of pancreatic function, genotyping |
| Immune deficiencies | Differential white cell counts, immunoglobulin levels and subsets, functional antibody responses and lymphocyte subset analysis |
| Primary ciliary disorders | Screening FnNO, saccharine test, cilial ultrastructure and function, culture of ciliated epithelium |
| Protracted bacterial bronchitis | Chest radiography, sputum for culture, exclusion of other causes in this table. Response to 4–6 weeks antibiotic and physiotherapy |
| HRCT scan | |
| Recurrent pulmonary aspiration: Laryngeal cleft or ‘H’ type tracheo-oesophageal fistula Post-TOF repair with swallowing incoordination Neuromuscular or neurodevelopmental disorder GOR, hiatal hernia | Barium swallow, videofluoroscopy, 24 h pH studies, milk isotope scan, fat-laden macrophage index* on bronchalveolar lavage if bronchoscopy indicated. Oesophagoscopy with biopsy may be indicated. |
| NB. There is little evidence that GOR alone is a cause of cough in otherwise healthy children | |
| Retained inhaled foreign body | Chest radiography and HRCT scan may show focal lung disease |
| Rigid bronchoscopy is both diagnostic and therapeutic and is almost always indicated if the history is suggestive of inhaled retained foreign body | |
| Tuberculosis | Chest radiography, Mantoux, early morning gastric aspirates and gamma interferon tests |
| Anatomical disorder (eg, bronchomalacia) or lung malformation (eg, cystic congenital thoracic malformation) | Bronchoscopy and CT scan |
| Interstitial lung disease | Spirometry (restrictive defect), chest radiography and HRCT scan, lung biopsy |
FnNO, fractional nasal nitric oxide; HRCT, high-resolution CT; TOF, tracheo-oesophageal fistula; GOR, gastro-oesophageal reflux.
*Fat-laden macrophages in bronchoalveolar lavage fluid may not be a specific test for recurrent pulmonary aspiration.
Most should have a chest radiograph and an attempt to obtain sputum for microbiology. A high-resolution CT scan may be indicated to determine the extent of disease, eg focal versus generalised.