Laboratory Tests in Acute Asthma Management
Arterial blood gas (ABG) analysis should always be obtained in patients with acute severe asthma requiring hospital admission, while routine blood tests and chest radiography serve specific secondary purposes. 1
Essential Laboratory Testing
Arterial Blood Gas Analysis (Priority Test)
ABG measurement is the single most critical laboratory test for hospitalized patients with acute severe asthma. 1 This test identifies life-threatening features that require immediate escalation of care:
- Normal or elevated PaCO2 (5-6 kPa or higher) in a breathless asthmatic patient indicates impending respiratory failure and marks a very severe, life-threatening attack 1, 2
- Severe hypoxia (PaO2 < 8 kPa/60 mmHg) despite oxygen therapy 1
- Low pH (acidosis) indicating metabolic decompensation 1
Critical pitfall: A "normal" PaCO2 in an acutely breathless asthmatic is NOT reassuring—it actually signals severe respiratory compromise and potential respiratory muscle fatigue, as the expected response to acute asthma is hypocapnia from hyperventilation 2, 3
Secondary Laboratory Tests
Additional investigations should be arranged after initial stabilization: 1
- Plasma electrolytes and urea - to monitor for complications of treatment (hypokalemia from beta-agonists) and assess hydration status 1
- Complete blood count - to identify infection or eosinophilia 1
- Electrocardiography (in older patients) - to exclude cardiac complications 1
Imaging Studies
Chest radiography should be obtained to exclude pneumothorax, consolidation, or pulmonary edema, particularly in life-threatening presentations 1
Tests NOT Required for Immediate Management
In children with acute asthma, blood gas estimations are rarely helpful in deciding initial management. 1 The guidelines explicitly state that "no other investigations are needed for immediate management" in pediatric cases, with assessment based primarily on clinical features and peak expiratory flow when appropriate 1
Monitoring During Treatment
Peak expiratory flow (PEF) measurement should be repeated 15-30 minutes after starting treatment and at least 4 times daily throughout hospital stay 1
Oximetry should maintain SaO2 > 92% 1
If ABG shows initial PaCO2 was normal or raised, repeat arterial blood gas within 2 hours of starting treatment to assess response 2
Key Clinical Context
The severity of asthma attacks is frequently underestimated due to failure to make objective measurements 1, 2. Laboratory testing, particularly ABG analysis, provides objective data that prevents this dangerous underestimation and guides appropriate escalation to intensive care when deterioration occurs despite treatment 1, 3.