Reasoning GP · Algorithm · 1-page summary
Allergy test abnormalities
For: children and adults in primary care with a positive specific IgE (sIgE) or skin prick test (SPT), a raised total IgE or tryptase, or a drug-allergy label. Not for: full anaphylaxis, urticaria or rhinitis management (use the anaphylaxis, urticaria and allergic rhinitis protocols).
Core rule: a positive sIgE or SPT shows sensitisation, not allergy. Interpret it only with a compatible history: never prescribe an adrenaline auto-injector (AAI) or an elimination diet for a positive test alone, and never defer AAIs after a reaction compatible with anaphylaxis.
0Stop — call 999 now — anaphylaxis: adrenaline first, never delayed
- Sudden airway, breathing or circulation problems, usually with skin or mucosal changes (skin may be absent) · tongue or laryngeal angioedema · recurrent or biphasic symptoms after a reaction → 999 say “anaphylaxis”.
- IM adrenaline 1:1000, anterolateral thigh, without delay: adult and child >12 years 500 micrograms · 6–12 years 300 micrograms · 6 months–6 years 150 micrograms · <6 months 100–150 micrograms. Repeat after 5 minutes if airway, breathing or circulation problems persist.
- While waiting: remove the trigger; lie flat, legs raised (sit up cautiously if breathing is difficult); no standing or walking. Never delay adrenaline for antihistamines, steroids, IV access or tryptase. Hospital transfer after treatment.
1Triage — match the result to the history
| Result + history | Means / next |
|---|---|
| Positive + reaction | Clinical allergy likely → block 2 |
| Positive, food tolerated | Sensitisation only → no avoidance, no AAI |
| Ara h 8, Cor a 1, Mal d 1 | Pollen cross-reactivity; usually oral only |
| Ara h 2, Cor a 9/14, Ses i 1 | Primary, higher risk → specialist advice |
| Drug-allergy label | Structured history; no drug sIgE |
2Screen — history, not the test, triggers these
- Past reaction compatible with anaphylaxis → two AAIs by brand + written plan (not for an easily avoidable drug); Urgent allergy clinic.
- Systemic reaction to a bee or wasp sting → two AAIs; Urgent venom immunotherapy assessment.
- Suspected mast-cell disease or persistently raised baseline tryptase → Urgent allergy / haematology.
- Recurrent angioedema without urticaria → C4, C1-inhibitor; Urgent allergy clinic.
- Severe immediate or delayed drug reaction (DRESS, SJS/TEN, AGEP) → avoid; Urgent allergy clinic.
- Child: severe eczema + multiple food sensitisations + poor growth → Urgent paediatric allergy + dietitian.
3Assess
☐ History: reaction, onset (minutes = IgE), cofactors (exercise, alcohol, NSAIDs, infection), tolerated since? ☐ sIgE only to allergens the history suggests ☐ After adult anaphylaxis: tryptase as soon as possible, second ideally within 1–2 h (≤4 h from onset)
Not in primary care: broad panels, components without specialist advice, food challenge, patch tests. No role: IgG, Vega, hair, kinesiology.
4Classify — only if blocks 0–2 negative
- Inhalant allergy — positive pollen, mite or pet sIgE + matching symptoms → block 5.
- Sensitisation only — positive test, allergen tolerated → explain; no avoidance.
- Pollen-food syndrome — oral itch within minutes of raw fruit or nuts, cooked tolerated → explain; no elimination of tolerated foods.
- Low-risk penicillin label (delayed maculopapular rash, no systemic features) → approved local de-labelling pathway.
- Contact dermatitis — type IV, not IgE → patch testing in dermatology / allergy clinic if not responding.
- Uncertain — specialist allergy advice before any avoidance or AAI.
5Treat today — only these
- AAI: two in-date devices by brand, dose checked against weight, age and product; brand-specific training and written plan; check both are physically carried.
- Hay fever: intranasal steroid (beclometasone 2 sprays each nostril BD, or fluticasone propionate 2 sprays each nostril OD) + cetirizine 10 mg OD. Doses: NICE/SPC, BNF, local formulary.
- Montelukast only for rhinitis with asthma; warn about neuropsychiatric effects (MHRA).
- Optimise asthma control; dietitian if multiple or nutritionally important foods are excluded.
6Refer routinely
- Moderate–severe rhinitis despite pharmacotherapy → Routine specialist immunotherapy (Acarizax per local guidance commissioning).
- Recurrent severe infections + low immunoglobulins → immunology.
- Occupational allergen identified → occupational physician.
7Never reassure by
- A normal tryptase: it does not exclude anaphylaxis, especially food or respiratory reactions.
- “Pollen-food syndrome is always mild”: systemic reactions occur (lipid-transfer protein, nuts, cofactors).
- Antihistamines or steroids: they do not treat airway, breathing or shock problems.
- AAIs on the repeat list: check two in-date devices are carried.
- A beta-blocker in someone at anaphylaxis risk: adrenaline works less well; record it, tell the allergy team.
- A positive test alone: never label, exclude foods or prescribe an AAI on it.
8Safety-net — say it, then document it
999 “If your throat or tongue swells, you struggle to breathe or wheeze, or feel faint: use your auto-injector into the outer thigh straight away, then call 999 and say ‘anaphylaxis’. Lie down with your legs raised, or sit up if breathing is hard; do not stand or walk. Use the second device after 5 minutes if you are not better or symptoms come back.”
Yearly review (food or venom allergy): weight and device dose, expiry, two carried, technique, plan, asthma control, beta-blockers. Montelukast: “stop and contact us promptly if your mood, sleep or behaviour changes.”
✓Safe to leave primary care only if — all five, documented
No block 0–2 feature in the history
Result interpreted against a documented allergy history
Two AAIs, training and written plan where indicated
Referral route and urgency recorded
Safety-net given and documented
Summary of the full Reasoning GP allergy test abnormalities algorithm, which holds the detail and evidence. Sources: NICE NG258 (2026), CG116, CG183, HTG413, TA1045; Resuscitation Council UK 2021; MHRA Drug Safety Updates (adrenaline auto-injectors, montelukast); BSACI guidance; local commissioning. Clinical reference — follow local pathways and check doses: NICE/SPC, BNF.