PULMONARY FUNCTION TEST-ALGORITHM

SUMMARY

1. Anything < 80% of normal is an abnormal finding

2. Look for restrictive disease: TLC < 80% or (if TLC is unknown) FEV₁/FVC ≥ 80% & FVC < 80%

3. If restrictive: if the decrease in DLCO is proportional to the decrease in TLC, then it's extra-thoracic; if the decrease in DLCO is disproportionately low compared to the decrease in TLC, then it's intra-thoracic

4. Look for obstructive disease: defined by a disproportionately low FEV₁, both FEV₁ and FEV₁/FVC < 70%

5. If obstructive: if TLC is high, no response to β₂-agonist & DLCO is low then it's emphysema; if DLCO is normal and significant response to β₂-agonist then it's asthma

6. If both restrictive & obstructive pictures are present, then it's a combination (such as in patients with concomitant asthma & obesity or asthma & sarcoidosis/Langerhans’s histiocytosis)

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].