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Background Bronchial challenge agents induce bronchoconstriction through distinct pathways, hence varying in sensitivity and specificity across patient cohorts. The BTS/NICE/SIGN 2024 asthma guidance recommends bronchial challenge testing (BCT) for diagnosis, but without specifying challenge agents. In contrast, ERS 2022 recommends a sequential approach: direct challenge followed by indirect if negative. We aim to establish the test concordance between methacholine and mannitol challenges (BCT Mch and BCTMann) and identify predictors of response to each in both adults and children. We will also assess the added value of sequential testing in adults.Methods Symptomatic and untreated children and adults with GP-suspected asthma were recruited into the RADicA study. Clinical history, examination and objective tests including fractional exhaled nitric oxide (FeNO), blood eosinophil counts (BEC), spirometry, bronchodilator reversibility (BDR), skin prick testing (SPT) and both direct BCT Mch and BCTMann were performed (on separate days prior to inhaled corticosteroid treatment).Results Of 95 participants (median [IQR] age: 26.0 [13.2–39.1] years, 41.1% male) who completed both BCT Mch and BCTMann, 35.8% had a positive BCTMann (PD15<635 mg), and 33.7% a positive BCTMch (PD20<0.2 mg), with an overall agreement of 78.9% between the two tests: nine individuals had positive BCTMch but negative BCTMann, compared to 11 BCTMann positive and BCTMch negative. Self-reported exercise-induced symptoms were not associated with either BCTMann or BCTMch outcomes. SPT, FeNO, BEC and BDR were associated with both positive BCTMann and BCTMch (table 1). In adults, following BTS/NICE/SIGN (2024) diagnostic pathway, 57–76% require BCT, of which 14–24% had positive BCTMann and 11–30% for BCTMch. Following ERS (2022) diagnostic pathway, 43–71% patients require BCTMch, 79–89% had a negative result, requiring further BCTMann; of these only 12–16% subsequently tested positive.Abstract P260 Table 1Univariate logistic regression for predicting positive BCTMann and BCTMch in both adults and childrenConclusion BCT is required in over half of cases using current UK and European diagnostic pathways; sequential testing advised by ERS, yields few additional diagnoses. We found no differential predictors of bronchial hyperresponsiveness between mannitol and methacholine. Given their moderate agreement, high cost and limited accessibility, identifying better predictors may help guide more efficient selection of challenge agents. This also highlight the urgent need for more accessible and efficient diagnostic tools.