Introduction Asthma poses a diagnostic challenge due to its intermittent symptoms and variable airflow obstruction. Diagnostic assessments such as spirometry and bronchodilator response are frequently non-diagnostic, necessitating confirmatory bronchial provocation testing. Biomarkers of type-2 inflammation —exhaled nitric oxide (FeNO) and blood eosinophil counts (BEC)— are useful in asthma, but their diagnostic values in children and in combination (FeNO+BEC) are unclear. This systematic review will evaluate the diagnostic accuracy of FeNO alone or in combination with BEC in paediatric and adult asthma.

Methods and Analysis This protocol is reported in line with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Protocols. The review will include studies of any design on the diagnostic accuracy of FeNO with or without BEC in asthma compared to the reference standards bronchodilator response and provocation testing in patients ≥ 5 years old selected from MEDLINE, Embase, and Cochrane CENTRAL databases. Screening, study selection, and data extraction will be independently performed by two reviewers. Risk of bias will be assessed using QUADAS-2 and QUADAS-C. Meta-analysis will be carried out by pooling the sensitivity and specificity of FeNO alone or in combination with BEC in a bivariate random effects model allowing the generation of summarised operating characteristic curves and summary points. Further analysis utilising a multiple thresholds model will enable the computation of diagnostic thresholds for FeNO.

Ethics and Dissemination No patient data will be stored without prior approval from ethics committee. The findings will be submitted in a peer-reviewed publication.

Registration PROSPERO CRD42023489738

SOCIAL MEDIA MESSAGE Diagnosing asthma is challenging. Spirometry and bronchodilator reversibility are often non-diagnostic, calling for provocation testing. Our systematic review will explore complementary approaches using FeNO with and without the blood eosinophil count.

AC reports the following interests: she has received non-restricted research grants from the Quebec Respiratory Health Research Network the Fondation Quebecoise en Sante Respiratoire GlaxoSmithKline and Sanofi Regeneron she received speaker honoraria from AstraZeneca GlaxoSmithKline Sanofi Regeneron and Valeo Pharma she received consultancy fees for AstraZeneca GlaxoSmithKline and Sanofi-Regeneron and Valeo Pharma. FMD reports the following: she has received unrestricted research funds from Jamieson Novartis Teva Trudell Medical research funds from Covis Pharma GlaxoSmithKline and MEDteq in partnership with Thorasys Inc. honorarium for consultancy work from Astra Zeneca Covis Pharma Ontario Lung Association Sanofi Teva and Thorasys Inc. honorarium for contribution to advisory boards from Sanofi and and honorarium as an invited speaker from Association des Medecins omnipraticiens du Richelieu Saint-Laurent Covis Pharma Federation des Medecins specialistes du Quebec Sanofi Thorasys Inc and Trudell Medical International. EAG reports the following: he has received investigator led research grants from Circassa Group Gilead Sciences Chiesi Limited and Propeller Health He has collaborated in research with AstraZeneca Helicon Health and Adherium (NZ) Limited. He received speaker honoraria from Circassa Group and Sanofi He lead the publication of European Respiratory Society clinical practice guidelines for the diagnosis of asthma in children aged 5-16 years (2021) for the European Respiratory Journal (ERJ). SC reports the following: he has received non-restricted research grants from the NIHR Oxford BRC the Quebec Respiratory Health Research Network the Association Pulmonaire du Quebec the Academy of Medical Sciences AstraZeneca bioMerieux and Sanofi Genyme Regeneron he is the holder of the Association Pulmonaire du Quebecs Research Chair in Respiratory medicine and is a Clinical research scholar of the Fonds de recherche du Quebec he received speaker honoraria from AstraZeneca GlaxoSmithKline Sanofi-Regeneron and Valeo Pharma he received consultancy fees for FirstThought AstraZeneca GlaxoSmithKline Sanofi-Regeneron Access Biotechnology and Access Industries he has received sponsorship to attend/speak at international scientific meetings by/for AstraZeneca and Sanofi-Regeneron. He is an advisory board member and detains stock options for Biometry Inc a company which is developing a FeNO device (myBiometry). He advised the Institut national d'excellence en sante et services sociaux (INESSS) for an update of the asthma general practice information booklet for general practitioners and is a member of the asthma steering committee of the Canadian Thoracic Society.

This study did not receive any funding

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

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All data produced in the present study are available upon reasonable request to the authors