[1] Univ Aberdeen, Dept Gen Practice & Primary Care, Aberdeen AB25 2AY, Scotland
[2] Univ Groningen, Med Ctr, Dept Gen Practice, NL-9713 AV Groningen, Netherlands
[3] Univ Aberdeen, Sch Med, Dept Publ Hlth, Aberdeen AB25 2ZD, Scotland
[4] Univ Groningen, Med Ctr, Dept Pulm Dis, NL-9713 GZ Groningen, Netherlands