Is serum hemoglobin level an independent prognostic factor for IgA nephropathy?: a systematic review and meta-analysis of observational cohort studies oleh Kang Zhang, Meng-di Wang, Shang-shang Jiang, Long Tang, Yue-fen Wang, Yuan Meng, Zhen Cai, Xue-yan Sun, Fang-qiang Cui, Wen-jing Zhao