Non-steroidal anti-inflammatory drugs (NSAIDs) are some of the most commonly used medications to treat pain and inflammation. Many are available over the counter and are frequently used in children to reduce fever and relieve pain. Hypersensitivity (HS) to NSAIDs affects an estimated 0.6% to 5.7% of the general population. These reactions can be caused by immune or non-immune mechanisms. In most cases, they fall into two categories: reactions to a single NSAID or to several different NSAIDs.

Currently, the main way to diagnose NSAID hypersensitivity is through a drug provocation test (DPT)-this involves giving the patient the suspected drug under medical supervision to check for a reaction. It's considered the most reliable method. However, diagnosing these reactions in children can be challenging. Symptoms can vary widely, and there are few accurate alternative tests available to replace the DPT.

This study looks for easier ways-like symptoms or lab tests-to diagnose NSAID reactions, so fewer patients need to go through multiple DPT.

Hypersensitivity reactions (HSR) to NSAID in children, although similar to those in adults, present unique challenges in terms of diagnosis and management. While international classification systems are useful, real-world clinical scenarios often fall outside established criteria. Examples include:

• Multiple NSAID HSR involving both cutaneous and respiratory symptoms.
• NSAID-induced anaphylaxis dependent on food intake.
• Immediate selective reactions to more than one NSAID. These scenarios are frequently observed in pediatric patients, highlighting the need for a regional study to evaluate and properly address them.

This is a single-center, ambispective observational study.

Primary Objective:

To describe the clinical characteristics of children with NSAID hypersensitivity.

Secondary Objectives:

1. To describe the different clinical phenotypes and assess age-related differences.
2. To assess tolerance to alternative NSAIDs in patients with confirmed hypersensitivity.
3. To evaluate the usefulness of in vivo and in vitro tests in the diagnostic process.
4. To disseminate study findings.

The study design is based on 2 components:

• Retrospective Component: include participants diagnosed with NSAID HSR, who underwent DPT in the Pediatric Allergy Department at Hospital Sant Joan de Déu between January 2014 and December 2021. Data will be pseudonymized. A waiver of informed consent will be requested due to the long study period, the absence of risk, and the fact that many patients are no longer under follow-up.
• Prospective Component: participants with suspected NSAID HSR were included. Participants will be assessed during outpatient visits, where the following steps will occur:
• In vivo testing, where appropriate, using validated prick and intradermal techniques for paracetamol and metamizole. Controls will include saline (negative) and histamine (positive).
• A DPT will be performed with the suspected medication and/or suitable alternatives, in accordance with standard clinical practice.
• Nasal DPT using a rhinomanometer for patients suspected of NSAID-exacerbated respiratory disease (NERD).
• Families will be informed about the procedures, and informed consent will be obtained.

On the day of the DPT, a blood test (including total IgE, IgG, IgA, IgM, tryptase, IL-6) and urine sample (uLTE4 levels) will be collected.

If the DPT is positive, a second round of blood and urine samples will be collected.

The NSAID to be used in testing are commonly prescribed in pediatric practice and grouped by chemical structure:

Salicylic acid derivatives: Aspirin Para-aminophenol derivatives: Paracetamol (acetaminophen) Propionic acid derivatives: Ibuprofen, naproxen Acetic acid derivatives: Diclofenac Enolic acid derivatives: Pyrazolones, oxicams Fenamates (fenamic acid derivatives) COX-2 inhibitors: Celecoxib

Inclusion Criteria:

• Children aged 0-17 years
• Completed DPT with the suspected and/or alternative NSAID

Exclusion Criteria:

• Patients who did not undergo or complete the DPT

The study population consists of pediatric patients (aged 0 to 17 years) with a clinical suspicion or confirmed diagnosis of hypersensitivity NSAID.

Retrospective Cohort: Includes all patients who underwent at least one drug provocation test (DPT) for suspected NSAID hypersensitivity between January 2014 and December 2021 at the Pediatric Allergy Department of Hospital Sant Joan de Déu.

Prospective Cohort: Includes newly referred patients to the outpatient Pediatric Allergy Clinic from August 2022 to December 2024, presenting with clinical symptoms suggestive of NSAID hypersensitivity. Eligibility is determined based on:

• Clinical manifestations (cutaneous and/or respiratory) following NSAID intake.
• A high likelihood of an adverse drug reaction based on history.
• Informed consent signed by parents or legal guardians.
• Patients with incomplete DPTs or those whose guardians decline consent (for the prospective arm) are excluded.

Data will be available beginning 6 months after publication and will remain accessible for 5 years. The results will be available in clinicaltrials.gov with supporting information (Clinical Study Report (CSR)).

Data will be available beginning 6 months after publication and will remain accessible for 5 years.

Data will be shared with qualified researchers affiliated with academic or healthcare institutions, for purposes of academic research or meta-analysis, and can be used for further analysis of the intervention's efficacy, safety profiling, or inclusion in systematic reviews. Study protocol, statistical analysis plan, and informed consent forms (in anonymized format) will also be made available. Requests should be submitted to the corresponding author by email. A data access agreement must be signed, and the research proposal will be reviewed by the study steering committee before approval is granted.