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Before interpreting the results, one should ascertain that the test was acceptable and reproducible and that the patient’s demographic data are correct. Pattern recognition is key. A low FEV1/FVC ratio (the forced expiratory volume in 1 second divided by the forced vital capacity) indicates an obstructive pattern, whereas a normal value indicates either a restrictive or a normal pattern. If the FEV1/FVC ratio is normal, a low forced vital capacity value indicates a restrictive pattern, whereas a normal value indicates a normal pattern.
Pulmonary function test can help differentiate obstructive and restritive pulmonary defects and guide on the severity. Its reproducible and combines the forced vital capacity and forced expiratory volume in 1 second, FVC/FEV1 is given as a percentage .IN restrictive disease both values are low hence the percentage. In obstrctive disease there is proportionately lower FEV1 hence values below 70% signifies airway narrowing. It can be used to grade to mild , moderate and severe airway narrowing with values less than 30%. Complex lung volume loops when plotted are are useful in characterisiing lung disease
First we check the validity of the test , if valid we assess the FEV1/FVC ratio if below than 70 % signifies obstructive defect then we assess the degree of obstruction through FEV1 value , if FEV1/FVC ratio is normal or high with decrease of all volumes restrictive defect is there.
we also check FEF25-75 for assessing small airway function
if obstructive defect is found we proceeed to postbronchodilator testing to assess the reversibilty
First of all, The test should be acceptable according to the ATS criteria of acceptability ( free from artifacts, cough, glottic closure), as well as good start and no early termination
then, we look to FEV1/ FVC , if below 70, this means airway obstruction which necessitates reversibility testing to determine whether it is reversible or irreversible airway obstruction
if FEV1/FVC more than 70, this may be either normal spirometry, restrictive dysfunction or small airway disease
the next step is to look at the FVC , if below 80% , this indicates restrictive dysfunction which should be confirmed by body box (TLC), if FVC> 80 this can be either normal spirometry or small airway disease (SAW)
lastly we look at the MIF50 to diagnose SAW disease
