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STUDY DESIGN Prospective, single center. OBJECTIVE To compare maximal voluntary ventilation (MVV) and forced vital capacity (FVC) testing in the outpatient scoliosis clinic to determine their relative feasibility and sensitivity in assessing pulmonary function in the AIS population. SUMMARY OF BACKGROUND DATA Evidence of compromised pulmonary function in patients with severe AIS is well established. The American Thoracic Society (ATS) has established criteria for the reliability and accuracy of pulmonary function tests (PFTs), including MVV and FVC. METHODS A total of 91 AIS patients with thoracic curves of 20° or greater were enrolled in the orthopedic clinic. Patients performed PFTs using the CareFusion MicroLoop Spirometer. MVV and FVC values were collected. Results were considered reliable or "passing" when ATS spirometer guidelines were met. RESULTS Eighty-seven of the 91 patients (96%) met ATS criteria for the MVV test and 43 of the 91 patients (47%) met criteria for the FVC test. Both MVV (r = -0.41, p < .01) and FVC (r = -0.37, p = .01) were significantly correlated with thoracic Cobb angle. The percentile predicted (%predicted) MVV (r = -0.24, p = .03) and %predicted FVC (r = -0.32, p = .04) were also significantly correlated with thoracic Cobb angle. Of those who passed both tests (42/91 patients), 26% had abnormal MVV results with normal FVC results, and 5% had abnormal FVC results with normal MVV results. CONCLUSION MVV and FVC correlated closely with Cobb angle. Twice as many AIS patients could perform an MVV test compared with an FVC test. MVV seems to be a more practical and sensitive PFT than FVC for assessing the pulmonary function of AIS patients in the orthopedic clinic setting. LEVEL OF EVIDENCE Level II.