What labwork is recommended for a patient with a persistent cough?

Labwork for Persistent Cough

For persistent cough, no routine laboratory tests are recommended as mandatory baseline investigations—instead, chest radiograph and spirometry are the essential diagnostic studies required for all patients. 1

Mandatory Initial Investigations (Not Traditional "Labwork")

The evaluation of persistent cough relies primarily on imaging and pulmonary function testing rather than blood tests:

• Chest radiograph is mandatory for all patients with chronic cough to identify structural abnormalities, with 31% showing abnormalities that yield a diagnosis 1
• Spirometry with bronchodilator response is mandatory to identify airflow obstruction from asthma or COPD, measuring FEV1 before and after short-acting β2 agonist (salbutamol 400 mcg by MDI or 2.5 mg nebulized) 1

When Laboratory Tests Are Indicated

Traditional laboratory work is not part of the standard initial workup, but specific blood tests become relevant only in particular clinical scenarios:

For Suspected Immunodeficiency or Bronchiectasis

• Serum immunoglobulins (IgG, IgA, IgM) should be measured in patients with bronchiectasis or recurrent infections to exclude immunodeficiency 2
• Complete blood count to assess for eosinophilia (suggesting allergic disease or eosinophilic bronchitis) or leukocytosis 2

For Suspected Allergic Bronchopulmonary Aspergillosis

• Total IgE and Aspergillus-specific IgE in immunocompromised patients or those with specific risk factors 2

For Suspected Autoimmune Disease

• Autoimmune serologies (RF, anti-CCP, ANA) only if systemic features suggest connective tissue disease with associated bronchiectasis 2

Critical Diagnostic Tests Beyond Traditional Labwork

The following are the actual diagnostic studies that matter for persistent cough:

• Bronchial provocation testing (methacholine challenge) should be performed in patients without clinically obvious etiology who have normal spirometry, as it identifies cough-variant asthma with 88% positive predictive value 1, 3
• 24-hour esophageal pH monitoring if GERD is suspected, though empiric PPI trial is often more practical 4, 2
• High-resolution CT chest may be useful when other targeted investigations are normal, detecting abnormalities in 24-42% of patients with "normal" chest radiographs 1, 2

Common Pitfalls to Avoid

• Do not order routine blood work as a screening panel—persistent cough evaluation is driven by clinical history, imaging, and pulmonary function testing, not laboratory values 1
• Do not rely on peak expiratory flow measurements instead of spirometry, as PEF is less accurate for diagnosing airflow obstruction as a cause of cough 1
• Do not overlook medication review—immediately discontinue ACE inhibitors if present, as no patient with troublesome cough should continue these medications 1, 4, 2
• Do not assume normal spirometry excludes asthma—many patients with cough-variant asthma or eosinophilic bronchitis have normal spirometry and require bronchial provocation testing 1

Practical Algorithm

1. Start with chest X-ray and spirometry (mandatory for all patients) 1
2. Review medications (stop ACE inhibitors immediately if present) 1, 4
3. If spirometry normal: proceed to bronchial provocation testing 1
4. If upper airway symptoms: ENT examination preferred over sinus imaging 4
5. Consider empiric treatment trials based on most probable diagnosis (intranasal corticosteroids for upper airway cough syndrome, PPI for GERD) rather than extensive laboratory testing 1, 4

The key insight is that persistent cough is diagnosed through a combination of targeted diagnostic testing and empirical treatment trials, not through routine laboratory work 1.