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Hopstaken RM, Cals JW, Dinant GJ.Hopstaken RM, et al. Among authors: cals jw.Prim Care Respir J. 2009 Sep;18(3):227-30. doi: 10.4104/pcrj.2009.00011.Prim Care Respir J. 2009.PMID: 19330269Free PMC article.