In order for the forced vital capacity to be considered accurate it has to be conducted three times where the peak is sharp in the flow-volume curve and the exhalation time is longer than 6 seconds.
ReproducibilityRepeatability of the PFT is determined by comparing the values of forced vital capacity (FVC) and forced expiratory volume at 1 second (FEV1). The difference between the highest values of two FVCs need to be within 5% or 150 mL. When the FVC is less than 1.0 L, the difference between the highest two values must be within 100 mL. Lastly, the difference between the two highest values of FEV1 should also be within 150 mL. The highest FVC and FEV1 may be used from each different test. Until the results of three tests meet the criteria of reproducibility, the test can be repeated up to eight times. If it is still not possible to get accurate results, the best three tests are used.<ref>{{cite journal | vauthors = Sim YS, Lee JH, Lee WY, Suh DI, Oh YM, Yoon JS, Lee JH, Cho JH, Kwon CS, Chang JH | display-authors = 6 | title = Spirometry and Bronchodilator Test | journal = Tuberculosis and Respiratory Diseases | volume = 80 | issue = 2 | pages = 105–112 | date = April 2017 | pmid = 28416951 | pmc = 5392482 | doi = 10.4046/trd.2017.80.2.105 }}</ref>
