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Background The new UK (NICE/BTS/SIGN) asthma diagnostic guidelines for children include the use of House Dust Mite skin prick test (SPT) or Total IgE along with eosinophil count (eos), where exhaled nitric oxide (FeNO), spirometry/reversibility (BDR) or peak flow (PEFv) testing is either unavailable, negative or the child is unable to complete the test. A positive mite SPT (>3 mm wheal diameter) or elevated Total IgE (>120 KU/L) and eosinophil count (>0.5x10 9cells/L) with symptoms suggestive of asthma are sufficient to make the diagnosis in children aged 5–16 yrs. Within an observational asthma diagnostic cohort study we investigated the sensitivity and specificity of these tests in children with suspected asthma.Method Children aged 5–16 yrs, referred from primary care with suspected asthma underwent clinical history, examination, FeNO, spirometry, BDR, PEFv, bronchial challenge, SPT to common inhaled allergens, Total IgE, blood eosinophils, and completed 8 weeks inhaled corticosteroid treatment. A panel of at least 3 asthma specialists using all available data confirmed or rejected the diagnosis of asthma.Results 95 children underwent testing and had a definitive diagnostic outcome (51.6% male; mean (SD) age 10.0 (2.8) years; 73 Asthma, 22 Not Asthma). Positive house dust mite SPT had the lowest sensitivity and specificity of all the tests. Elevated Eos had the highest specificity which was not improved by the addition of Total IgE. However, the standard lung function tests had higher specificity than the newly proposed tests, but not all children could complete these tests. Sensitivity of all the tests was poor. See table 1.Abstract S47 Table 1 Sensitivity (%) Specificity (%) PPV (%) NPV (%) Eosinophil count (n=95) 46.6 81.8 89.5 31.6 Total IgE (n=95) 75.3 68.2 88.7 45.5 Mite SPT (n=94) 61.1 59.0 83.0 31.7 Total IgE and Eos (n=95) 45.2 81.8 89.2 31.0 FeNO (n=80) 45.8 95.0 96.4 38.5 BDR (n=87) 32.3 100 100 33.3 PEFv (n=71) 30.3 100 100 27.8 Conclusion In children who are unable to complete standard lung function tests for asthma, an eosinophil count above 0.5x10 9 cells/L may be an alternative test to consider. In our cohort, the addition of Total IgE did not improve diagnostic accuracy. If children are diagnosed only on the basis of mite SPT, the poor specificity needs to be noted (2 in 5 cases will be wrong) and their response to treatment should be carefully reviewed.