Peak Flow Threshold for Hospital Discharge in Acute Asthma
This patient should not be discharged until peak expiratory flow exceeds 75% of personal best (450 L/min in this case), which corresponds to greater than 75% of his baseline 600 L/min. 1, 2, 3
Discharge Criteria Based on Guidelines
The British Thoracic Society and National Asthma Education and Prevention Program establish clear discharge thresholds that prioritize preventing early relapse and mortality:
Primary Peak Flow Requirement
- Peak expiratory flow must exceed 75% of predicted or personal best value before discharge can be considered 1, 2, 3
- For this patient with a personal best of 600 L/min, discharge requires PEF >450 L/min
- His current PEF of 340 L/min (57% of personal best) falls well below this threshold
Additional Clinical Stability Markers Required
Beyond the 75% PEF threshold, patients must also demonstrate:
- Diurnal PEF variability <25% (or ideally <20% based on outcomes data) 1, 2, 4
- Absence of nocturnal symptoms 1, 2
- Normalized vital signs (respiratory rate <25 breaths/min, heart rate <110 beats/min) 1, 2
Critical Pre-Discharge Medication Requirements
- Patient must be on discharge medications for at least 24 hours before leaving the hospital 2, 3
- Inhaled corticosteroids must be started at least 48 hours before discharge 1, 3
- Nebulizers should be replaced by standard inhalers 24-48 hours before discharge to ensure adequate technique 1, 3
Why the 75% Threshold Matters
The 75% threshold is not arbitrary—it reflects evidence-based risk stratification:
- Research demonstrates that diurnal PEF variability >20% in the 24 hours before discharge strongly predicts major post-discharge exacerbations (p<0.001) and readmission 4
- Patients with PEF 40-70% of predicted have a 6% relapse rate within 10 days even with corticosteroids, while those >70% have a 14% relapse rate without corticosteroids 5
- The high-risk period for sudden death extends 6-12 weeks after admission, making premature discharge particularly dangerous 4
Common Pitfalls to Avoid
Discharging based on clinical improvement alone without objective PEF criteria increases relapse risk substantially. 2, 3 While one study suggested earlier discharge might be safe in highly selected compliant patients 6, this contradicts established guideline recommendations and the weight of evidence supporting the 75% threshold.
Do not rely on patient-reported "personal best" values without verification—45% of inner-city patients reported personal best values that were actually lower than their measured peak flows, which could lead to inappropriate early discharge 7
Mandatory Discharge Preparations
Before this patient can leave at >75% personal best:
- Provide a peak flow meter with written self-management plan specifying exact PEF values for treatment escalation and emergency return 1, 2, 3
- Verify proper inhaler technique and document it 1, 2
- Prescribe prednisolone 30-60 mg daily for 1-3 weeks 1, 3
- Increase inhaled corticosteroid dose above pre-admission levels 1, 3
- Arrange follow-up with primary care within 1 week and respiratory specialist within 1 month 1, 2, 3