Anaphylaxis to infliximab, vedolizumab, and possibly the influenza vaccine / anaphylaxis to multiple medications; This literature case received on 30-Jan-2023 and concerned a 43-year-old, female patient.   The aim of this conference abstract was to present a case of suspected macrogol excipient allergy which was found to be due to indolent systemic mastocytosis (ISM).   The patient's concurrent conditions included ulcerative colitis. The patient's concomitant medications were not reported. The patient had no known medication allergies. As reported, the patient tolerated other medications containing polysorbate 80 (sertraline and bupropion) and polyethylene glycol (methylprednisolone and previously a bowel prep).  On an unspecified date, reported as over the last year, the patient was vaccinated with influenza vaccine (brand not specified; dose, route of administration, anatomical location, and indication: not reported). The batch number was not reported.   On an unspecified date, reported as over the last year, the patient started treatment with non-company, co-suspects infliximab (brand not specified; dose, frequency, route of administration and anatomical location: not reported) and vedolizumab (brand not specified; dose, frequency, route of administration and anatomical location: not reported). The batch number for both co-suspects was not reported.  On an unspecified date, unknown amount of time after receiving influenza vaccine and within minutes after receiving non-company, co-suspects infliximab and vedolizumab (reported as both monoclonal antibodies), the patient developed flushing, hypotension, lightheadedness, and in one case syncope. The patient presented for evaluation of anaphylaxis to infliximab, vedolizumab, and possibly the influenza vaccine. The patient was rapidly administered epinephrine, methylprednisolone, and intravenous fluids. The patient underwent macrogol skin prick and intradermal testing which were completely negative. Further investigation revealed elevated baseline tryptase (13.1 ng/mL), negative testing for hereditary alpha tryptasemia, and elevated urine methyl histamine (265 mcg/g Cr) and prostaglandin D2 (3360 pg/mg Cr) metabolites. The D816V KIT mutation was present in peripheral blood suggesting ISM, underlying mast cell disorder. Given that the patient lacked frequent symptoms and had no evidence of end organ dysfunction, the patient deferred bone marrow biopsy to confirm the diagnosis.  On an unspecified date, the patient recovered from the event. As reported, the patient's symptoms resolved with rapidly administered epinephrine, methylprednisolone, and intravenous fluids.  The reporter assessed the events as possibly related to influenza vaccine. As reported, the patient presented for evaluation of anaphylaxis to infliximab, vedolizumab, and possibly the influenza vaccine. Infliximab, vedolizumab, and the flu vaccine each contain polysorbate 80 suggesting an immunoglobulin E (IgE) mediated macrogol allergy. Anaphylaxis to multiple medications with negative skin testing, elevated tryptase suggested an underlying mast cell disorder, ISM. Therefore, suspected macrogol excipient allergy was found to be due to ISM.  The event of 'anaphylaxis' was considered to be medically significant by a Physician within Seqirus's Pharmacovigilance and Risk Management Department.    Company comment: Authors presented a case of a 43-year-old patient with suspected macrogol excipient allergy which was found to be due to indolent systemic mastocytosis (ISM). An unspecified time after vaccination with the suspect product influenza vaccine the patient presented for evaluation of anaphylaxis due to infliximab, vedolizumab, and possibly the influenza vaccine. The D816V KIT mutation was present in peripheral blood suggesting ISM, underlying mast cell disorder. As reported, Infliximab, vedolizumab, and the flu vaccine each contain polysorbate 80 suggesting an immunoglobulin E (IgE) mediated macrogol allergy. Causality may have been confounded by co-suspect drugs infliximab and vedolizumab. Due to unclear temporal relationship and limited information provided, causality is unassessable.; Reporter's Comments: As reported, the patient presented for evaluation of anaphylaxis to infliximab, vedolizumab, and possibly the influenza vaccine. Infliximab, vedolizumab, and the flu vaccine each contain polysorbate 80 suggesting an immunoglobulin E (IgE) mediated macrogol allergy. Anaphylaxis to multiple medications with negative skin testing, elevated tryptase suggested an underlying mast cell disorder, ISM. Therefore, suspected macrogol excipient allergy was found to be due to ISM.; Sender's Comments: Authors presented a case of a 43-year-old patient with suspected macrogol excipient allergy which was found to be due to indolent systemic mastocytosis (ISM). An unspecified time after vaccination with the suspect product influenza vaccine the patient presented for evaluation of anaphylaxis due to infliximab, vedolizumab, and possibly the influenza vaccine. The D816V KIT mutation was present in peripheral blood suggesting ISM, underlying mast cell disorder. As reported, Infliximab, vedolizumab, and the flu vaccine each contain polysorbate 80 suggesting an immunoglobulin E (IgE) mediated macrogol allergy. Causality may have been confounded by co-suspect drugs infliximab and vedolizumab. Due to unclear temporal relationship and limited information provided, causality is unassessable.