How to Interpret Pulmonary Function Tests (PFTs)

PFT’s are often taught to medical students by people who are not very good teachers. Today, my medicine attending gave us a quick talk on how to interpret PFT’s and it was brilliantly uncomplicated. I liken it to learning the shortcut to find a derivative of something like 3x^2 in calculus. There is the long way in which you must wade through the viscous quicksands of real math. But, there is a short way in which you just simply manipulate the exponent, coefficient, and variable… arriving almost magically at 6x. So, be forewarned that after you read this post, you may not derive any greater understanding of the lungs. However, you will be able to determine if you’ve got a restrictive or obstructive process and whether it is intrinsic or extrinsic to the lungs.


1) Look first at the FEV1. If it greater than 80%, leave the patient alone. If its less than 80%, look at the FVC and FEV1/FVC ratio.

2) Replace FVC with restrictive and FEV1/FVC with obstructive in your vocabulary.

3) If the FVC is less than 80%, you’ve got an restrictive process.

4) If the FEV1/FVC ratio is less than 80%, you’ve got an obstructive process.

5) If both values are less than 80%, you’ve got both a restrictive and obstructive process.


1) If the DLCO/VA is near 100%, you’ve got an extrinsic process (in relation to the lung).

2) If the DLCO/VA is less than 100%, you’ve got an intrinsic process (in relation to the lung).

FEF 25-75, TLC

Forget about them unless you’re a lung specialist.


2 Responses to “How to Interpret Pulmonary Function Tests (PFTs)”

  1. 1 Steve

    Nice. hahaha. You sound like you want to go into medicine.

  2. 2 Demosthenes

    Hell no. 😛 I just want to know enough to survive internship.

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