Instrument translation of CVID QoL
To ensure a linguistic equivalence with the original questionnaire, the cross- cultural validation process was conducted according to an international consensual systematic methodology, called standardized linguistic validation21, 22. Permission to translate and validate CVID QoL into Turkish language was obtained from the original questionnaire developer (Dr Pulvirenti and Dr Quinti, author on this manuscript). Two separate forward translations from Italian to Turkish involving two independent both bilingual but native speakers were conducted, and they were reconciled into one version. Then the backward translation of the reconciled version into Italian was made by native speakers. The backward translation was compared with the original Italian questionnaire by the expert committee. Afterwards the pre final Turkish version of questionnaire was obtained. (Figure 1.) In this version, the content validity index was determined for each item by ten experts by using the options of 1= “not suitable”, 2= “partially suitable, applicable by modification”, 3= “the item available as it is”. After this step, the Turkish version of the questionnaire was applied individually to five eligible patients as a pilot study. Meanwhile, we got their comments on whether all 32 items were understandable and suggestions for changes. Finally, the last version CVID QoL was approved.