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resources:pulm:interpreting_pfts
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Table of Contents
Official PFT Guides
Step-by-Step to Interpreting PFTs
Assess Flow-Volume Loops
- Are the loops similar between attempts?
- YES: Skip to next
- NO: See first page's comment if ATS reproducibility criteria was met
- Roughly speaking, do the loops appear obstructive or restrictive?
Assess Spirometry
Spirometric Severity
ATS Criteria
| Grade | FEV1 z score | Severity |
|---|---|---|
| NA | −1.65 to −2.5 | Mild |
| NA | −2.51 to −4.0 | Moderate |
| NA | <−4.1 | Severe |
GOLD Criteria
| Grade | FEV1 Percent Predicted | Severity |
|---|---|---|
| 1 | ≥80% | Mild |
| 2 | 50% − 79% | Moderate |
| 3 | 30% − 49% | Severe |
| 4 | <30% | Very Severe |
Bronchodilator Response
ERS/ATS Definition
- Required:
- BDR >10%
- Calculation
- $ BDR = \frac{FEV1_{postBD} - FEV1_{preBD}}{FEV1_{predicted}}$
Important NOTE:
- Calculation is not
- $ BDR = \frac{FEV1_{postBD} - FEV1_{preBD}}{FEV1_{preBD}}$
- Does NOT require
- $\Delta FEV1_{volume} >200 mL$
DLCO Severity
ERS / ATS Definition
Gold Definition
| DLCO Percent Predicted | Severity |
|---|---|
| >140% | Abnormally High |
| 76%-140% | Normal |
| 61-75% | Mild |
| 41-60% | Moderate |
| <40% | Severe |
Assessing Restriction
ATS Criteria
- Check spiro
- Is $FVC \cdot z_{score} \leq -1.65$ (e.g. < 5th percentile)?
- Yes: get lung volumes
- No: no restriction
- Check lung volumes
- Is $TLC \cdot z_{score} \leq -1.65$ (e.g. < 5th percentile)?
| Grade | FEV1 z score | Severity |
|---|---|---|
| NA | −1.65 to −2.5 | Mild |
| NA | −2.51 to −4.0 | Moderate |
| NA | <−4.1 |
GOLD Definition
| TLC Percent Predicted | Severity |
|---|---|
| >70% | Mild |
| 50%-69% | Moderate |
| <50% | Severe |
Other Lung Volumes
VC: ↓ in restriction, ↓ in air trapping RV & FRC: ↑ with air trapping RV & FRC: ↓ with restriction ERV: ↓ in obesity
Lung Volume
FRC is the primary thing calculated in lung volume calculations.
Other
Plethysmography
Boyle’s Law
- $P_1V_1 = P_2V_2$
- $\implies V_{tg} = 970\frac{\Delta V}{\Delta P}$
$ FRC = ERV + RV $
$TLC = VC + RV $
Helium
- Helium is used because He doesn’t cross the alveolar-capillary membrane
- $ C_1V_1 = C_2(V_1+V_2)$
- $\implies V_2 = V1(\frac{C_1}{C_2}-1)$
- limitations since inhalation done only for a few minutes, would get into poorly ventilated lungs, so will underestimate FRC
Nitrogen
Method for calculation is similar to Helium method.
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