see also:

• respiratory medicine
• Aust. asthma Foundation QRG to spirometry for GPs
• Aust. Asthma Foundation's buying guide for spirometers for GPs
  • eg. amongst the least expensive are the very light, compact PC-based spirometers such as:
    • MIR MiniSpir 2 at $AU999 excl. gst but includes 60 free turbine mouthpieces

• spirometry provides an assessment of lung function by measuring:
  • FEV₁ = Forced Expiratory volume in 1 sec
  • FVC = Forced vital capacity
  • lung volume vs time curve
  • lung flow vs volume curve
• the spirometer should be calibrated - often with a special 2L or 3L calibration syringe

• GLI (Global Lung Function Initiative):
  • the current benchmark endorsed across Australia and New Zealand
  • built on a massive international repository of over 74,000 healthy non-smokers across more than 30 countries
  • features a single, seamless, and continuous equation that tracks a person from age 3 all the way to 95
  • uses advanced Z-scores -it mathematically accounts for the fact that a 20-year-old's lung variance is very narrow, while an 80-year-old's natural variance is much wider
  • GLI provides multi-ethnic and continuous age-adjusted equations that fit modern populations far better than the older ECSC (European Community for Steel and Coal) or US NHANES datasets
  • avoids the diagnostic discrepancies and biases that occur when swapping between regional or outdated reference sets
  • provides specific mixed/other equations recommended when testing First Nations (Aboriginal and Torres Strait Islander) Australians

• USA (NHANES):
  • better than ECSC for some demographics as it reflects a population much closer to modern health and physical heights
  • but local and international consensus has shifted away from single-country reference ranges.
  • drawn entirely from a sample of the US population, categorized tightly into three racial/ethnic brackets: Caucasian, African American, and Hispanic
  • the traditional US datasets stop calculating adult equations at age 80.
  • NHANES III uses separate mathematical models for children (under 18) and adults, creating an artificial mathematical “jump” in predicted values as a patient crosses into adulthood
  • compared to GLI, in patients older than 70, or those who are significantly taller or shorter than average, the differences in predicted FEV₁ can diverge by up to 400 mL, and Lower Limit of Normal (LLN) ratios can shift by more than 3.5% - NHANES III tends to predict a higher LLN for older or taller males compared to GLI, keeping an older patient on NHANES III can accidentally over-diagnose airflow obstruction (false positive), whereas GLI corrects for natural, age-related lung elasticity loss
• SBPT (Sociedade Brasileira de Pneumologia e Tisiologia - Brazilian Society of Pulmonology and Phthisiology)
  • primarily based on Brazilian population studies (such as the landmark data by Pereira et al. in 2007)
  • for adult males, the local SBPT equations generate significantly higher predicted volumes (FVC and FEV₁) compared to what GLI calculates for White/Caucasian cohorts
  • the Lower Limit of Normal (LLN) for the FEV₁/FVC ratio is mathematically higher in the SBPT guidelines than in GLI
  • because GLI assumes a lower normal baseline for volume and a lower threshold for the ratio, switching a Brazilian patient from SBPT to GLI can result in under-diagnosing obstruction or misclassifying the severity of a restrictive defect
• ECSC (European Community for Steel and Coal):
  • outdated 1993 standard; tends to under-call obstruction or misclassify values in modern diverse cohorts.
  • data was collected from European working-class cohorts decades ago
  • human populations have grown taller and healthier over the last half-century (the secular trend), ECSC significantly underestimates what a normal, healthy lung capacity should be today
  • GLI predicted values for FVC and FEV₁ are generally 3% to 9% higher than ECSC values for the exact same patient
  • as it sets the “normal” bar too low, it frequently misses early or mild respiratory disease
  • traditionally relied heavily on a fixed percentage (e.g., “arbitrarily defining anything below 80% of predicted as abnormal”). This misclassifies shorter, taller, and older adults
• Knudson
  • an older, historical set of spirometry reference equations used to calculate a patient's predicted normal lung function values (such as FVC, FEV₁, and FEF₂₅₋₇₅%) based on their age, height, and sex
  • first published by Dr. Ronald Knudson in 1976 and updated in 1983, it was one of the most widely implemented standards in pulmonary function software across North America and parts of the world throughout the late 20th century
  • relies on standard linear regression models divided into distinct age groups. For example, the formulas shift drastically between growing children, young adults, and aging populations
  • sample bias: the original 1976 model was built on a very small sample size of only 746 healthy, non-smoking Caucasian individuals in Tucson, Arizona
  • systematically overestimate % predicted values in younger patients, meaning some real respiratory deficits are missed (false negatives)
  • the mathematical formulas switch abruptly at arbitrary age cut-offs (e.g., transitioning from a 24-year-old equation to a 25-year-old equation), a patient's “predicted baseline” can drop unnaturally overnight on a screen just because they had a birthday

• assess shape of flow-volume curve to fit with a clinical pattern:
  • poor quality
    • free from coughs, leaks, or a slow start
    • ensure at least three repeatable measurements are captured within an acceptable range
    • volume-time curve should extend longer than six seconds, and there are no signs of early termination or cutoff
    • test session is finished when the difference between the two largest FVC measurements and between the two largest FEV1 measurements is within 0.2 L
  • normal
  • obstructive - unable to blow out quickly
    • asthma
    • COPD
  • restrictive - small lung capacity
    • pulmonary fibrosis
    • interstitial lung disease
    • pleural or chest wall disease
    • weak inspiratory muscles
    • rib deformity
    • obesity
  • mixed pattern
    • eg. cystic fibrosis
• FEV₁/FVC
  • is this above lower limit of normal (LLN) which is usually ~0.7?
    • is the FVC below LLN?
      • possible restriction
    • is the FVC above LLN?
      • normal spirometry
  • is this below lower limit of normal (LLN)?
    • this suggests an obstructive issue
    • severity grading of obstructive disease as determined by % of predicted FEV₁:
      • > 70% = mild
      • 60-69% = moderate
      • 50-59% = moderately severe
      • 35-49% = severe
      • < 35% = very severe
    • is FVC < LLN?
      • could be a mixed pattern
    • mid-expiratory flow rate (FEF_25–75%) is the average forced expiratory flow rate over the middle 50 percent of the FVC
      • in the correct clinical situation, a reduction in FEF_25–75% of less than 60% of that predicted and an FEV1/FVC ratio in the low to normal range may confirm airway obstruction
    • assess response to bronchodilator:
      • positive response indicated by:
        • FEV₁ increasing by > 12%
        • FVC increasing by > 12% and, in adults, by more than 200mL
• maximal voluntary ventilation (MVV) maneuver
  • this is another test that can be used
  • patient is instructed to breathe as hard and fast as possible for 12 seconds
  • result is extrapolated to 60 seconds and reported in liters per minute
  • MVV generally is approximately equal to the FEV1 × 40
  • if the MVV is low but FEV1 and FVC are normal, poor patient effort, a neuromuscular disorder, or major airway lesion must be considered

• predicted values are based on age, sex, height, and ethnicity
• PEFR:
  • Young Adults (20–40 years)
    • Males: ~500 to 600 L/min (8.3 to 10.0 L/sec)
    • Females: ~380 to 480 L/min (6.3 to 8.0 L/sec)
  • Middle-Aged Adults (40–60 years)
    • Males: ~450 to 550 L/min (7.5 to 9.1 L/sec)
    • Females: ~340 to 440 L/min (5.7 to 7.3 L/sec)
  • Older Adults (60+ years)
    • Males: ~350 to 450 L/min (5.8 to 7.5 L/sec)
    • Females: ~250 to 350 L/min (4.2 to 5.8 L/sec)
• FEV₁/FVC
  • within 5% of the predicted ratio (usually 0.7)
  • Young and Middle-Aged Adults (20–50 years):
    • Normal ratio generally falls between 75% and 85%.
    • Peak lung function occurs in early adulthood, after which values remain relatively stable before a very gradual decline begins
  • Older Adults (50–70+ years):
    • Normal ratio naturally drifts downward, often resting between 70% and 80%.
    • For individuals over age 65, a ratio down to around 65% to 70% can still fall within expected limits depending on personal height, sex, and baseline health
• FVC usually 80-120% of predicted (values below 80% suggest increasing severity of restriction as it decreases)
  • Ages 20–30 years:
    • Males: 4.0 to 5.5 liters
    • Females: 3.25 to 4.5 liters
  • Ages 40–50 years:
    • Males: 3.5 to 4.5 liters
    • Females: Lower than the 20-30 age group bracket, mirroring a slow natural decline
  • Ages 60+ years:
    • Males: 3.0 to 4.0 liters
    • Females: Continues a gradual age-related reduction
• FEV₁ 80-120% of predicted
  • 20–30 years: Normal values fall between 80% and 100% (sometimes up to 120% in peak young adults).
  • 30–40 years: Normal values fall between 70% and 100% of the predicted value.
  • 40–60 years: Normal values fall between 75% and 100% of the predicted value, though a slight natural decline can start after age 50.
  • 60–70 years: Normal values fall between 70% and 90% of the predicted value.
  • Over 70 years: Normal values fall between 60% and 90% of the predicted value
  • absolute volumes:
    • Ages 20–30: FEV1 varies between 3.5 and 4.5 liters.
    • Ages 40–50: FEV1 varies between 3.0 and 4.0 liters.
    • Note: Healthy non-smokers naturally experience an FEV1 decline of about 30 mL each year as they age
• FEF_25–75%
  • it is dependent on FVC making it highly variable, so by age 50, the normal reference range for FEF_25–75% can statistically span from 40% up to 160% of the predicted value
  • if FEF_25–75% < 60% it may provided add confirmation of obstructive airways disease
  • absolute values for average height healthy adults:
    • Adult Males (Ages 20–40):
      • 3.5 to 5.5 L/s, peaking in the late 20s.
    • Adult Females (Ages 20–40):
      • 2.5 to 4.5 L/s.
    • Older Adults (Ages 60+):
      • values naturally decline with age as lung tissue loses elasticity, often dropping into the 1.5 to 3.0 L/s range for both sexes
• TLC (Total lung capacity) 80-120% of predicted
• FRC (Functional residual capacity) 75-120% of predicted
• RV (Residual volume) 75-120% of predicted