Tests
1st tests to order
oxygen saturation
Test
Should be performed immediately (ideally before starting supplemental oxygen, depending on clinical urgency) and then closely monitored during acute management.[1]
Pulse oximetry is a way of screening oxygenation noninvasively. Note that pulse oximetry may overestimate oxygen saturation in patients with dark skin color.[1]
Significant hypoxemia with an oxygen saturation (SpO₂) <90% is infrequent during asthma exacerbations and, if present, represents severe airflow limitation with a need for aggressive therapy.
Result
oxygen saturation values measured by pulse oximetry vary with degree of exacerbation: mild/moderate = 90% to 95% (on air); severe = <90% (on air)
peak expiratory flow rate
Test
Peak flow measurements are quick and safe, and can be used to support assessment of exacerbation severity and to monitor the patient's response to treatment.[1]
Normal peak flow values vary with sex, age, and height.
How to use a peak flow meter to obtain a peak expiratory flow rate measurement.
Result
severity of asthma exacerbation can be determined by measuring peak flow as a percentage of predicted or personal best: mild/moderate: >50%; severe ≤50%
Tests to consider
ABG
Test
Assessment of hypercapnia is important in patients with severe exacerbations of asthma (e.g., peak expiratory flow rate or FEV₁ <50% predicted), those who do not respond to initial therapy, and when there is concern for respiratory deterioration.[1]
Increased respiratory drive should correspond to a decreased PaCO₂. However, with an increased work of breathing, a normal or elevated PaCO₂ is an indication of potential respiratory failure.[1]
Result
PaCO₂ of <45 mmHg (6.0 kPa) in mild and moderate exacerbations and ≥45 mmHg in severe exacerbations
chest x-ray
Test
A chest x-ray is not routinely necessary. It should be considered if a complicating or alternative pulmonary pathology is suspected or for patients who are not responding to treatment to rule out a pneumothorax if difficult to diagnose clinically.[1][46]
Result
often normal, even in a life-threatening exacerbation
CBC
Test
Indicated in acute exacerbations when complicating factors (e.g., pneumonia) are suspected from history and exam.
Result
normal or elevated eosinophils and/or neutrophilia
metabolic panel for electrolytes, BUN levels, creatinine
Test
Indicated in complicated or more severe acute exacerbations (e.g., when patients require rehydration).
Result
usually normal
Use of this content is subject to our disclaimer