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An Algorithm for Interpreting PFTs

Question 1: Obstruction

Is there obstruction to airflow present?
FEV1/FVC - Reduction in this ratio
Men Lower limit of FEV1/FVC < Predicted- 8%
Women Lower limit of FEV1/FVC < Predicted- 9%
How severe is it?
Mild Obstruction FEV1 65 - 80% predicted
Moderate Obstruction FEV1 50 -65% predicted
Severe Obstruction FEV1 < 50% predicted
Is it reversible or fixed?
  • Does the FEV1 or FVC increase by at least 15% after inhalation of a bronchodilator?
Is it possibly consistent with emphysema?
  • Are lung volumes increased consistent with air-trapping
  • Is the DLCO reduced consistent with loss of alveolocapillary membrane

Question 2: Restriction

Is there a restrictive process present?
  • TLC less than 80% predicted
How severe is it?
Mild TLC or FVC 65 - 80% predicted
Moderate TLC or FVC 50 -65% predicted
Severe TLC or FVC < 50% predicted
Is it a parenchymal process?
  • Reduction in DLCO is consistent with parenchymal destruction
Is it an extra-parenchymal process (kyphosis, muscle weakness)?
  • DLCO will usually be normal.
  • Maximal Inspiratory and expiratory pressures reduced
Is there a combined obstructive restrictive disorder present ?
  • TLC is low and FEV1/FVC ratio is low

Question 3: Combined Obstructive and Restrictive Disease

Concomitant reduction in TLC and FEV1/FVC
Quantitation of severity
  • Obstruction:Use FEV1
  • Restriction: Use TLC or VC
  • Sarcoidosisis, CF, obliterative bronchiolitis
  • COPD + muscle weakness

Question 4: Isolated Pulmonary Vascular Disease

Is there an isolated gas exchange abnormality?
  • Normal PFT’s other than reduction in DLCO
  • Think of:
    • Pulmonary embolism
    • Pulmonary vascular disease – (e.g.,, pulmonary artery hypertension)
    • Early interstitial lung disease

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How To Report PFTs

The report includes

  1. the tabulation of results of the tests performed, juxtaposed with the predicted values for the subject, generated by the technician and
  2. an analysis and
  3. summary generated by me.

I attempt to keep the report short.  The longer, the less likely to be read.  In the analysis, I do not repeat the findings except as significant positives or negatives and I always state them in the context of the analysis.  For example, "The increase in the RV and the decrease in the indices of forced expiratory flow and the specific airways conductance indicate obstructive airways disease."

I attempt to make the logic explicit.  For example, "The decrease in TLC indicates restriction.  The markedly diminished MIP suggests that this is due to chest wall disease while the normal diffusing capacity suggests that it is not due to a parenchymal process, such as interstitial fibrosis".  This keeps me intellectually honest, and communicates more meaningfully. 

Prior tests can be very valuable because comparison with self is inherently more sensitive than comparison with population norms and may give essential information about the progress of the disease or the positive or negative response to treatment.  I always look at all the previous results.  I often select out specific items for tabulation (my secretaries are very good at pulling out the numbers in the finished report if I simply say "please make a table showing the TLCs, the VCs, and the DLCOs for all of those tests") when progression is worth reviewing.  I do, however, analyze the findings in the current test on its own merits before turning to comparison with previous tests, which, I suspect, has on occasion kept me from propagating a prejudice.

The Summary gives the major conclusions including qualifications, important outstanding questions, and suggestions for how one might proceed.  It is brief (shorter than the analysis) and does not repeat the findings or the logic.  It is intended to tell the referring physician what I think is going on and to help him or her to decide what to do.

First, I decide what my bottom line is going to be and how to qualify it.  For example, "Moderate restrictive process probably due to a parenchymal disease, with an independent obstructive component." 

Second, I try to envision what this report will do for the referring physician.  The physician may have posed a particular question such as "Preop for bronchogenic carcinoma" which warrants a specific comment.  If the referring physician has questioned asthma and is not in a subspecialty that handles asthma often, I may say "These findings do not rule out the clinical diagnosis of asthma". In an extremely obese patient who has perfectly normal pulmonary function tests, obstructive sleep apnea and obesity hypoventilation spring to mind and should be mentioned.  If pulmonary fibrosis is suspected, I may suggest that "if clinically indicated, we could probe the possibility of gas exchange abnormality more finely with oximetry, arterial blood gases, and steady state diffusing capacity during rest and exercise".  If a test result is very surprising or potentially urgent (a preoperative patient, or a PaO2 of 43), I contact the physician directly by phone!

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< How To Interpret Pulmonary Function Tests

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