Best Objective Measurement of Response to Therapy in Acute Asthma Management
Peak expiratory flow (PEF) measured 15-30 minutes after bronchodilator administration is the best objective measurement of response to therapy in acute asthma management, with specific thresholds determining disposition and further treatment. 1
Primary Objective Measure: Peak Expiratory Flow
PEF should be measured before treatment and repeated 15-30 minutes after nebulized bronchodilator therapy to objectively assess treatment response. 1 The British Thoracic Society guidelines consistently emphasize this timing across all acute care settings—general practice, emergency departments, and hospital wards. 1
Specific PEF Thresholds for Clinical Decision-Making
The guidelines provide clear, actionable thresholds based on percentage of predicted or personal best PEF:
- PEF >75% predicted/best: Patient improving adequately; may be managed at home with step-up of usual treatment 1
- PEF 50-75% predicted/best: Partial response; requires oral prednisolone 30-60 mg and close monitoring 1
- PEF <50% predicted/best: Severe asthma; strongly consider hospital admission 1
- PEF <33% predicted/best: Life-threatening asthma; immediate hospital admission required 1
A minimum absolute change of at least 60 L/min is also clinically significant when assessing variability. 1
Complementary Objective Measures
Forced Expiratory Volume in One Second (FEV₁)
While PEF is the primary measure in acute settings due to ease of use, FEV₁ provides complementary information and is considered a fundamental objective measure of asthma control. 1
A bronchodilator response is defined as improvement in FEV₁ by at least 12% and 0.2 L (200 mL) in adults, or more than 12% in children and adolescents. 2 This threshold is well-established, though some evidence suggests a less stringent improvement may be applicable in children. 3
Pre-bronchodilator FEV₁ is a strong independent predictor of future risk of exacerbations, making it valuable for risk stratification beyond the acute episode. 1
Oxygen Saturation
Pulse oximetry should be measured before oxygen administration, with values >92% as the target during treatment. 1 However, oximetry values >90% may be falsely reassuring as they can miss CO₂ retention and low partial pressure of oxygen. 4
Practical Implementation Algorithm
- Measure baseline PEF (if patient able to perform maneuver) 1
- Administer nebulized bronchodilator (salbutamol 5 mg or terbutaline 10 mg) 1, 5
- Repeat PEF at 15-30 minutes post-treatment 1
- Apply decision thresholds as outlined above to determine disposition
- Continue monitoring PEF every 4 hours if improving, or more frequently (up to every 15-30 minutes) if not improving 1
Critical Pitfalls to Avoid
Do not rely solely on clinical signs without objective measurement. The British Thoracic Society explicitly states that "many deaths from asthma are preventable" and identifies "doctors failing to assess severity by objective measurement" as a key preventable factor. 1
Patients with severe or life-threatening attacks may not appear distressed and may not exhibit expected abnormalities on examination, making objective measurement essential. 1
PEF measurements have limitations: While they can reliably exclude airway obstruction (high negative predictive value of 95%), they have lower positive predictive value (46.5%) for detecting obstruction compared to FEV₁. 6 However, in the acute setting, the ease of obtaining serial PEF measurements outweighs this limitation.
Correlation Between Measures
Changes in pre-dose FEV₁ and pre-dose PEF correlate well when both are measured under supervision, supporting the use of either measure. 7 The correlation between supervised clinic and unsupervised home measurements is actually stronger for PEF than for FEV₁. 7
Documentation Requirements
The actual numerical PEF value should be entered into the medical record, not just "normal" or "abnormal." 1 This allows for accurate assessment of response and comparison over time.