Peak Expiratory Flow Rate Prediction Equations
Recommended Reference Equations by Region and Age
For clinical practice, use ethnically appropriate NHANES III reference equations for patients aged 8-80 years in the USA, and Wang equations for children under 8 years. 1
United States
- Ages 8-80 years: NHANES III equations (ethnically appropriate) 1
- Ages <8 years: Wang et al. equations 1
- These equations account for sex, age, height, and ethnicity 1
Europe
- Ages 18-70 years: Combined European Respiratory Society (ERS) reference equations from 1993 1
- Valid height range: 155-195 cm in males, 145-180 cm in females 1
- Pediatric ages: Quanjer et al. equations 1
- Important caveat: The ERS committee does not currently recommend any single specific set of equations for Europe and suggests the need for updated Europe-wide reference equations 1
Key Determinants of Peak Expiratory Flow
Height is the strongest predictor of PEF, followed by age and sex. 2, 3
Primary Variables
- Height: Most important correlating variable for lung volumes and flows 1, 3
- Age: Linear relationship with PEF in children and adolescents 4, 3
- Sex: Males have higher PEF values than females at the same height and age 4, 5, 6
Mathematical Relationships
- PEF has a curvilinear relationship with height but a linear relationship with age 3
- The relationship can be expressed as: ln(PEF) = function of ln(height) + age 3
Population-Specific Prediction Equations
Moroccan Children (Ages 6-19 years)
Boys (Flowmetric): F-PEF = -187 + 24.4(Age) + 1.61(Height) 4
- Correlation coefficient r = 0.86 4
Girls (Flowmetric): F-PEF = -151 + 17(Age) + 1.59(Height) 4
- Correlation coefficient r = 0.86 4
Chinese Children in Hong Kong (Ages 6-19 years)
Males: ln(PEF) = 1.810256 × ln(height) + 0.038297 × age - 3.734139 3
Females: ln(PEF) = 1.525509 × ln(height) + 0.033275 × age - 2.368592 3
Turkish Children in Istanbul (Ages 7-14 years)
Boys: PEF = 3.5(Height in cm) + 9.2(Age in years) - 256.5 5
- Correlation coefficient r = 0.83 5
Girls: PEF = 3.3(Height in cm) + 10.2(Age in years) - 263.7 5
- Correlation coefficient r = 0.81 5
Indian Children (Ages 6-17 years)
- Common prediction equations based on age and height have been developed for both sexes 6
- Boys demonstrate higher PEF than girls at the same height and age 6
- Females show a plateau effect after age 14 years, which is not observed in males 6
Critical Implementation Considerations
Ethnicity Adjustments
- Black populations: Reference values for lung volumes are on average 12% lower than Whites 1
- Asian populations: Generally lower values than Whites, though the magnitude is not well-defined 1
- The difference may be smaller in Asians raised on Western diets during childhood 1
Age-Related Nuances
- Adolescents: Lung growth lags behind height increase during growth spurts 1
- A single equation does not completely describe growth during the complex adolescent period 1
- Elderly patients: The practice of using 0.70 as a lower limit of FEV1/FVC ratio results in false-positive results in males >40 years and females >50 years 1
Avoiding Extrapolation Errors
- Never extrapolate beyond the size and age limits of the reference population 1
- If a patient's age or height falls outside the reference population limits, document that extrapolation was performed 1
- For children <6 years, reference data from older subjects should never be extrapolated 7
Expressing Results: Z-Scores vs. Percent Predicted
Z-scores should be the preferred method for expressing spirometry results in pediatric and young adult patients. 7
Why Z-Scores Are Superior
- Eliminate biases related to height, sex, and ethnicity 7
- Provide statistically robust interpretation across the entire age spectrum 7
- Percent predicted is invalid in adults because scatter around predicted values is constant regardless of height or age 7
Interpretation Thresholds
- Normal range: Z-scores between -2.0 and +2.0 7
- Below normal: Z-scores below -1.64 (5th percentile) 7
- 95% confidence interval: Z-scores from -1.96 to +1.96 7
Equipment and Technique Requirements
Measurement Standardization
- Reproducibility is essential: At least 2 peak flow measurements should be within 10% of each other 2
- Valid PEF can typically be obtained within 3-4 attempts 2
- Within-test PEF variability should be approximately 4.6% 2
Proper Technique
- Patient should be standing or sitting with neck in neutral position 8
- Take a deep breath to total lung capacity before blowing 8
- Exhalation should be forceful and explosive, not prolonged 8
- Record the highest value from three attempts 8
Quality Control
- Peak flow meters should be cleaned regularly per manufacturer instructions 8
- Use disposable mouthpieces in clinical settings to prevent cross-contamination 8
- Equipment should allow overlay display of multiple maneuvers to evaluate repeatability 9
Clinical Pitfalls to Avoid
Common Errors
- Using 80% predicted as a fixed lower limit of normal: This is acceptable in children but leads to important errors in adults 1
- Selecting inappropriate reference equations: A relatively large sample (n=100) is needed to validate that published equations match the local population 1
- Ignoring effort-dependence: PEF is highly effort-dependent and requires proper technique for accurate results 8
Documentation Requirements
- Always document the reference equation used (first author's name and publication date) on every pulmonary function report 1
- Report the number of technically satisfactory maneuvers and repeatability 7
- If only a single satisfactory maneuver is recorded, do not exclude results but document the limitation 7