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Introduction: Spirometry is a screening tool for evaluating the degree of restrictive lung disease in systemic sclerosis (SSc). Observations indicated that some patients could not complete the test. The aim of the study was to identify the prevalence, causes and clinical predictors of an inadequate pulmonary function test (PFT) in SSc.Material and Methods: A cross-sectional study was performed among SSc patients over 18 years old followed up at Srinagarind Hospital, Khon Kaen, Thailand, during January 2006-December 2012. The adequacy of the PFT was based on the acceptable blow criteria as set out by the American Thoracic Society and the European Respiratory Society 2005 Standardizations of Spirometry.Results: Two hundred and forty-nine patients were included (female to male ratio was 2 : 1). The mean age at performing PFT was 51.4 ±11.1 years (range: 19.6-79.5). Median duration of disease at performing PFT was 2 years (IQR: 0.6-4.4). Inadequate PFT occurred in 73 cases (prevalence 29.3%: 95% CI: 23.6-35.0); the majority (60 cases; 82.2%) had an expiration time < 6 s and the others were due to plateau < 1 s (11 cases; 15%), air leak around mouth piece (1 case; 1.4%) and hesitation (1 case; 1.4%). Thirteen of 73 (17.8%) had an unusable graph with the overall prevalence of 5.2% (95% CI: 2.4-8.0). The factor associated with inadequate PFT was docy mass index (BMI) < 18.5 kg/m(2) (OR = 2.17: 95% CI: 1.49-3.17); the same factor was associated with an unusable graph, which was confirmed by the multivariate analysis (OR = 5.21; 95% CI: 1.60-16.95).Conclusions: One-third of Thai SSc patients had an inadequate pulmonary function test - the majority because of inadequate time for expiring. Low BMI influenced the effectiveness of the test, leading to an incomplete graph for evaluating lung disease in SSc. [ABSTRACT FROM AUTHOR]