What is the normal peak flow (PF) for a 6-year-old male who is 45.5 inches tall and weighs 47 pounds?

Normal Peak Flow for a 6-Year-Old Male (45.5 inches tall, 47 pounds)

The expected normal peak expiratory flow (PEF) for a 6-year-old male who is 45.5 inches (115.6 cm) tall is approximately 150-180 L/min, based on height-derived prediction equations from pediatric reference data.

Height-Based Calculation

The most reliable approach to estimating normal PEF in children uses height as the primary determinant, as it shows stronger correlation than age or weight 1, 2.

Using Published Prediction Equations:

For males, the prediction equation is:

• PEFR = (4.49 × height in cm) - 384.64 3
• For 115.6 cm: (4.49 × 115.6) - 384.64 = 134 L/min

Alternative equation for males:

• PEFR = (3.8 × height in cm) + (10.6 × age) - 313.2 1
• For 115.6 cm and 6 years: (3.8 × 115.6) + (10.6 × 6) - 313.2 = 190 L/min

Expected Range:

Given the variation between prediction equations and populations, the normal range for this child would be approximately 130-190 L/min, with a midpoint estimate of 160 L/min 1, 3.

Clinical Context from Guidelines

The British Thoracic Society guidelines provide reference values showing that PEF varies significantly with height in children 4. While specific values for a 115.6 cm child are not directly tabulated in the guideline evidence provided, the height-based prediction equations from validated studies are the appropriate method for determining expected values 1, 2, 3.

Important Clinical Considerations

Measurement Technique:

• Proper technique is essential in young children, as they often struggle to blow both "hard" and "long" simultaneously 4
• Multiple attempts (minimum 3) should be recorded, with the highest value reported 4
• Visual inspection of the maneuver is critical to ensure rapid rise to peak flow without cough or glottic closure 4

Interpretation Caveats:

• Height is a stronger determinant than age for PEF in children 2
• Approximately 50% of children aged 4-8 years cannot perform PEF maneuvers adequately 4
• Normal values show considerable population variation, with differences noted between ethnic groups and geographic locations 5, 3
• The correlation coefficient for height and PEF in males is strong (r = 0.8-0.86) 1, 3

Clinical Thresholds for Asthma Assessment:

If this child has asthma, the British Thoracic Society defines severity as 4:

• Acute severe asthma: PEF <50% of predicted (i.e., <80 L/min for this child)
• Life-threatening asthma: PEF <33% of predicted (i.e., <53 L/min for this child)

Common Pitfalls:

• Using age-only predictions without considering height leads to inaccurate estimates 2
• Written PEF diaries are unreliable; electronic monitoring shows healthy children have much less variation (95th percentile 12.3%) than previously thought 6
• Weight is not a primary determinant of PEF and should not be used in prediction equations 1, 2, 3