Reference Values, Interpretation, and Pattern Recognition
Reference Values, Interpretation, and Pattern Recognition
This domain tests selecting appropriate predicted equations, using the lower limit of normal (LLN), and recognizing obstructive, restrictive, and mixed patterns.
Core rules
- Use equations matched to age, sex, ethnicity, and height. GLI-2012 is widely recommended; NHANES III (Hankinson) is common in U.S. occupational settings.
- Define abnormal with the LLN (5th percentile, z = −1.645), not a fixed 80% of predicted.
- Obstructive pattern: FEV1/FVC below LLN. Restrictive pattern: FVC below LLN with a normal or high ratio; confirm with total lung capacity.
- Mixed pattern: both FEV1/FVC and FVC below LLN; interpret cautiously without lung volumes.
- Bronchodilator response: increase in FEV1 or FVC of at least 12% and 200 mL.
Worked example
A 45-year-old man has FEV1 2.80 L (78% predicted), FVC 4.20 L (92%), and FEV1/FVC 0.67; LLN for the ratio is 0.70. Reasoning: the ratio is below LLN, so this is obstruction; the FEV1 is reduced but the FVC is preserved, consistent with obstructive impairment. A post-bronchodilator FEV1 of 3.20 L (+400 mL, +14%) meets the 12% and 200-mL threshold.
Traps
- Calling a ratio of 0.72 normal because it is above 0.70 when the LLN is higher.
- Diagnosing restriction from spirometry alone.
- Reporting a bronchodilator response when the change meets only one of the two criteria.