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treatment option for polysensitised AR patients than single-allergen AIT, and some clinicians prefer to perform immunotherapy with multiple allergens, which is common in USA (16). Distinguishing between polysensitisation and polyallergy is crucial when prescribing AIT with multiple allergens. Polysensitised patients do not necessarily have polyallergy. However, determining if these patients are polyallergic or monoallergic is not an easy task for physicians. The period during which the symptoms occur (e.g. spring for tree pollens) is most commonly used to evaluate polysensitized patients, although nasal challenge tests or component-based diagnostics are sometimes used (17). However, determining polyallergy through medical history is not always 353 Immunotherapy in polysensitised allergic rhinitis accurate. Therefore, some physicians prescribe AIT for all allergens identified in a skin prick test (17). For this reason, multipleallergen AIT in a polysensitised patient is not always effective, and the superiority of multiple-allergen AIT to single-allergen AIT has been questioned. Although a previous systematic review by Nelson showed the clinical efficacy of multiple-allergen AIT, its superiority was not clearly demonstrated (18). In addition, the higher cost and limited response to multiple-allergen treatment should be considered when selecting multiple-allergen AIT (19). Several studies show a comparable effectiveness of singleallergen AIT between monosensitised and polysensitised AR patients (20-25). For grass pollen allergen, large double-blind placebo-controlled clinical trials show that the benefits of SLIT with single-allergen grass pollen in polysensitised patients are comparable to those in monosensitised patients (3,20). However, most lines of supporting evidence on a comparison of the clinical effectiveness of single-allergen AIT with house dust mite (HDM) extracts in AR patients polysensitised or monosensitised to HDM, are reported in observational studies (21-29), and there is a lack of systematic reviews and meta-analyses to demonstrate the clinical effectiveness of HDM AIT in polysensitised patients compared to monosensitised patients. This systematic review and meta-analysis aimed to compare the clinical effectiveness of single-allergen HDM AIT in AR patients polysensitised or monosensitised to HDM. Materials and methods Search strategy A comprehensive search was conducted of the PubMed, Cochrane Library, and EMBASE databases in accordance with the Preferred Reporting Items for Systematic Reviews and MetaAnalyses (PRISMA) guideline to identify all relevant studies of AIT with single-allergen extract in monosensitised and polysensitised patients up to March 26th, 2020 (30). Two investigators (J.Y.K. and D.H.H.) independently conducted the literature search to identify all available studies using the terms: (‘rhin*’ OR ‘allerg*’) AND polysensi* AND (‘immunotherapy’ OR ‘desensitisation’ OR ‘immunologic’). Only studies published in English were considered. Selection of studies The two investigators independently screened all abstracts and titles to identify candidate studies and discarded studies that did not satisfy the eligibility criteria; then, eligibility was determined by reviewing the full text. The eligibility criteria for the systematic review and meta-analysis were as follows: 1) the clinical effectiveness of AIT in nasal symptom scores and/or an additional symptom score was directly compared between monosensitised and polysensitised AR patients. 2) single-allergen AIT with HDM extracts was performed in monosensitised and polysensitised patients. Studies in which different doses of AIT were administered to participants and studies lacking full text (e.g. only abstracts) were excluded. Data extraction Data were extracted into standardised forms and
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