Risk factors for acute exacerbation of idiopathic pulmonary fibrosis: A systematic review and meta-analysis. 
BACKGROUND: Idiopathic pulmonary fibrosis (IPF) is a chronic and progressive fibrotic disease limited to the lungs. The course of disease varies widely, with  some patients experiencing acute respiratory deterioration, a condition called acute exacerbations of IPF (AE-IPF). The risk factors contributing to AE-IPF are  unclear. This systematic review and meta-analysis investigated the risk factors for AE-IPF. METHODS: Studies of risk factors for AE-IPF were identified in Medline, EMBASE and Cochrane databases. Fixed effects models were used to calculate pooled relative risks and weighted mean differences (WMD).The meta-analysis included seven articles involving 14 risk factors for AE-IPF. RESULTS: Risk factors for AE included reductions in vital capacity (VC; WMD - 10.58, 95% confidence interval (CI) -17.17 to - 3.99), forced vital capacity (FVC; WMD -6.02, 95%CI - 8.58 to - 3.47), total lung capacity (TLC; WMD -4.88, 95%CI -7.59 to - 2.17), and PaO2 (WMD -4.19, 95%CI -7.66 to -0.71) and a higher alveolar-arterial oxygen difference (AaDO2 ; WMD 4.4, 95%CI 0.24 to 8.57). Mechanical procedures, higher serum KL-6 concentration and secondary pulmonary hypertension, might be risk factors for AE-IPF. In contrast, age, sex, body mass  index (BMI), differences in diffusing lung capacity for carbon monoxide (DLCO), exposure to seasonal variations and air pollution, and virus infection might be unrelated to AE-IPF. CONCLUSIONS: Poor pulmonary function, mechanical procedures, higher serum KL-6 and secondary pulmonary hypertension were associated with increased risks of AE-IPF.