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that can change the immunological response to a single allergen or specific allergen component, the choice of allergen extract to use for polysensitised patients is open to debate (15). In USA, AIT is commonly prescribed with all relevant allergens, and the SCIT extract preparations contain an average of eight different allergens (6). However, the superiority of multiple-allergen AIT to single-allergen AIT is debatable. A previous systematic review demonstrated the clinical efficacy of multiple-allergen AIT but did not clearly show superiority of multiple-allergen AIT (18). Furthermore, a previous randomised, double-blind, placebo-controlled study reported that SLIT with multiple-allergen extracts showed a limited response compared to that obtained using single-allergen extract, potentially limiting its use (39). The GA2LEN/EAACI guidelines do no recommend mixtures (40). For grass pollen allergen, two large double-blind placebo-controlled clinical trials showed that the benefit of SLIT with single-allergen grass pollen in polysensitised patients was comparable to that in monosensitised patients (3,20). However, the supporting evidence of the clinical effectiveness of singleallergen AIT with HDM extracts in polysensitised patients compared to monosensitised patients is limited to observational studies (21-29). Therefore, we conducted a systematic review and meta-analysis to integrate the evidence of clinical effectiveness of HDM AIT in polysensitised patients to make recommendations regarding the treatment of polysensitised patients. Nine studies were included in this systematic review and meta-analysis to compare clinical effectiveness of HDM AIT between monosensitised and polysensitised patients. All studies investigated nasal symptom or/and use of rescue medication and quality of life. Among the nine studies, seven (21,22,24-28) were conducted on AR patients with or without asthma, the study of Li (23) was conducted on AR patients with asthma, while the study of Zhang (29) was conducted on AR patients without asthma. AR was diagnosed according to guidelines, such as the Allergic Rhinitis and Its Impact on Asthma (ARIA) guideline (41) in all individual studies except the study of Kim (26) that did not report on the diagnosis of AR. In contrast, the diagnosis of asthma was based on the Global Initiative for Asthma (GINA) guideline (42) in two studies (23,24), whereas, other studies did not report on the diagnosis of asthma; this may be explained by the fact that these studies focused on AR rather than asthma. The presence of significant improvement of clinical symptoms was evaluated before the comparison of clinical effectiveness. Out of nine studies, eight (21-23,25-29) reported a significant clinical improvement in nasal symptom score after AIT from the baseline symptom in both the monosensitised and polysensitised groups, although high heterogeneity among the studies was found. In the study of Soyyigit (24), although VAS score significantly decreased in the both groups, total symptom score significantly decreased in the polysensitised group but not in the monosensitised group. Figure 4. Comparison of the changes in medication score after singleallergen immunotherapy with HDM extract between monosensitised and polysensitised patients. Abbreviations: CI, confidence interval; HDM, house dust mite; SMD, standardised mean difference. Figure 5. Comparison of the changes in medication score after singleallergen immunotherapy with HDM extract between monosensitised and polysensitised patients. Abbreviations: CI, confidence interval; HDM, house dust mite; SMD, standardised mean difference. 357 Immunotherapy in polysensitised allergic rhinitis Considering the changes in nasal symptom scores after AIT, there was a severe heterogeneity in both the mono- and
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