🩺 FCPS Paediatrics TOACS · Allergen Skin Prick Testing (SPT)

📖 Nelson's Pediatrics · Allergy Testing · IgE-mediated sensitization 📚 paeds.online – Paeds Online

🔬 OBSERVED/INTERACTIVE STATION · CPSP FORMAT · 8 MINUTES · SKIN PRICK TESTING · INDICATIONS + TECHNIQUE + INTERPRETATION

📋 Observed Station – “6-year-old with suspected food allergy – perform and interpret Skin Prick Test”

👧🏻 Clinical Scenario (TOACS – read aloud / displayed):

A 6-year-old girl with history of moderate atopic dermatitis and two episodes of generalized urticaria and lip swelling within 30 minutes after eating peanut butter. She has never required epinephrine. No known asthma. She tolerates egg, milk, wheat, and soy without issue. Mother asks whether she is truly allergic to peanut and wants testing.

🎯 Task (examiner observed): Explain the procedure of Skin Prick Testing (SPT) to the mother, demonstrate the correct technique on a mannequin (or verbalize steps), list required equipment, identify medications to withhold, interpret a mock SPT result (image provided), and discuss the meaning of positive vs negative results.

📸 SPT Result Image (forearm with wheals):

Skin Prick Test on forearm showing wheal and flare reactions

Figure: Skin prick testing on volar forearm. Wheals (elevated, pale) with surrounding flare (erythema). Histamine positive control (top) produces largest wheal; saline negative control (bottom) shows no reaction.

📌 INTERPRETATION OF MOCK SPT (based on image):
- Positive control (histamine): Wheal diameter 8 mm (validates test)
- Negative control (saline): 0 mm (no dermatographism)
- Peanut extract: Wheal diameter 7 mm → POSITIVE (≥3 mm larger than negative control)
- Egg white: 2 mm → negative
- Milk: 1 mm → negative
- Dust mite: 4 mm → positive (sensitization, but no respiratory symptoms reported)

⚠️ Positive SPT indicates SENSITIZATION, not necessarily clinical allergy. Peanut history (urticaria + angioedema) + positive SPT = likely IgE-mediated peanut allergy.

💡 Examiner instruction: Candidate must: (1) obtain consent, (2) identify medications to withhold (antihistamines 72-96h), (3) select appropriate site (volar forearm or back), (4) demonstrate correct technique (drop allergen, prick at 90°, no bleeding, separate lancet per allergen), (5) read at 15 minutes, measure wheal diameter (mean of 2 perpendicular measurements), (6) interpret positive (≥3mm > negative control), (7) explain sensitization vs allergy, (8) discuss next steps (avoidance, epinephrine autoinjector, referral to allergist for possible oral food challenge).

🗨️ Examiner Q&A · Skin Prick Testing · Indications, Contraindications, Interpretation

❓ Q1 (Examiner): “What is the indication for skin prick testing in this child with suspected peanut allergy?”

✅ SPT is indicated to detect presence of allergen-specific IgE to peanut. History of immediate symptoms (urticaria, angioedema within 30-60 min) after peanut ingestion suggests IgE-mediated food allergy. SPT helps confirm the diagnosis. Positive SPT + consistent history = diagnosis of peanut allergy without need for oral food challenge.

❓ Q2 (Examiner): “Which medications interfere with SPT and how long should they be withheld?”

✅ Antihistamines: withhold 72-96 hours (short-acting) or up to 7-14 days for long-acting (cetirizine, loratadine, fexofenadine). Tricyclic antidepressants: 7-14 days. H2 antagonists (ranitidine): 24 hours. Oral corticosteroids (prolonged high-dose) may suppress reactivity. β-agonists, theophylline, cromolyn do not interfere. Topical steroids at test site should be avoided.

❓ Q3 (Examiner): “What is a positive SPT result? How do you measure and interpret?”

✅ Positive result: wheal diameter ≥3 mm larger than negative control after 15 minutes. Measure the longest diameter and the perpendicular diameter; record mean of both. Exclude surrounding flare (erythema) – flare is not measured. Histamine control must be positive (≥3 mm) and saline control negative for test to be valid.

❓ Q4 (Examiner): “Does a positive SPT always mean the child is clinically allergic?”

✅ NO. Positive SPT indicates sensitization (presence of allergen-specific IgE), but not necessarily clinical allergy. Up to 50-60% of positive SPT to foods are false positives (no reaction on oral challenge). Correlation with history is essential. Larger wheal size increases probability of clinical reactivity but does NOT predict severity.

❓ Q5 (Examiner): “What is the negative predictive value of SPT for IgE-mediated food allergy?”

✅ Negative predictive value >95% for IgE-mediated food allergy. A negative SPT virtually excludes IgE-mediated food allergy (very low false negative rate). However, rare false negatives can occur, especially with fresh fruits/vegetables (prick-to-prick needed) or if testing is done too soon after anaphylaxis (4-6 weeks refractory period).

❓ Q6 (Examiner): “What are the contraindications to skin prick testing?”

✅ Absolute: Unstable asthma, history of severe anaphylaxis to the test allergen (consider RAST instead), inability to stop antihistamines. Relative: Widespread active atopic dermatitis (no clear skin), dermatographism (false positives), recent anaphylaxis (wait 4-6 weeks). Age: No absolute lower age limit, but skin reactivity reduced in infants <6 months.

❓ Q7 (Examiner): “What emergency equipment must be available when performing SPT?”

✅ Resuscitation equipment: Adrenaline (epinephrine) 1:1000 (ampoules or autoinjector), oxygen, airway equipment, antihistamines (oral/IV), corticosteroids, β2-agonist inhaler/nebulizer (for asthma), blood pressure monitor, and a trained practitioner capable of recognizing and treating anaphylaxis. Risk of systemic reaction with SPT is ~0.02% (higher with fresh foods, drugs, latex).

❓ Q8 (Examiner): “What is the difference between SPT and intradermal testing? When is intradermal testing used?”

✅ SPT introduces allergen into epidermis via prick/puncture. Intradermal testing injects 0.01-0.02 mL into dermis (100-1000× more concentrated). Intradermal is more sensitive but less specific. Used for venom, penicillin, vaccine allergy when SPT is negative but history highly suggestive. NOT recommended for food allergens (higher risk of anaphylaxis, false positives).

❓ Q9 (Examiner): “What is prick-to-prick testing? When is it used?”

✅ Prick-to-prick testing uses fresh food instead of commercial extract. The lancet is pricked into fresh fruit/vegetable/meat, then pricked into patient's skin. Used when commercial extracts are unreliable (fresh fruits, vegetables, some meats). Important for oral allergy syndrome (pollen-food syndrome). Requires same precautions as standard SPT.

❓ Q10 (Examiner): “What is the role of serum specific IgE (RAST/CAP-FEIA) compared to SPT?”

✅ Both detect allergen-specific IgE with similar sensitivity/specificity. Advantages of SPT: immediate results, lower cost, patient sees reaction. Advantages of serum sIgE: no interference from antihistamines, can be performed in extensive dermatitis, no risk of reaction (except venipuncture). sIgE preferred when SPT contraindicated or patient unable to stop antihistamines.

❓ Q11 (Examiner): “What is component-resolved diagnosis (CRD)? Give an example for peanut.”

✅ CRD measures sIgE to specific allergenic proteins (components). Peanut example: Ara h 2 (major allergen) – positive strongly predicts true peanut allergy. Ara h 8 (cross-reactive with birch pollen) – often associated with mild oral allergy, not anaphylaxis. CRD helps differentiate true allergy from cross-sensitization. Emerging tool, not yet first-line.

❓ Q12 (Examiner): “What unorthodox allergy tests should be avoided? Why?”

✅ Avoid: IgG4 testing (reflects exposure, not allergy), vega testing, kinesiology, hair analysis, cytotoxic testing, provocation-neutralization. These have no scientific validity and may lead to unnecessary dietary restriction, malnutrition, or delayed correct diagnosis. Only SPT, serum sIgE, and oral food challenges are evidence-based.

📢 Examiner probe: “What is the recommended interval for repeat SPT in a child with known food allergy?” → Every 1-2 years to monitor for resolution. Many children outgrow milk, egg, wheat, soy by age 5-6 years. Peanut/tree nut allergy persists in ~80% into adulthood.

🧪 Step-by-Step SPT Technique (BSACI / Nelson's guidelines)

📝 STANDARD OPERATING PROCEDURE – Skin Prick Testing (SPT):

1. Preparation:
- Obtain verbal/written consent from parent/guardian
- Verify no antihistamines taken for 72-96 hours (or longer per drug)
- Select test site: volar forearm (older children) or back (infants/toddlers)
- Skin must be clear of eczema, dermatitis, topical steroids, emollients
- Do NOT clean with alcohol (may suppress reaction)

2. Equipment:
- Individual sterile lancets (one per allergen)
- Allergen extracts (stored at 2-8°C) – check expiry
- Positive control: histamine 10 mg/mL
- Negative control: saline/glycerin
- Timer, skin test measure, tissues, sharp bin, pillow for arm support
- Emergency kit: adrenaline, antihistamines, oxygen, airway equipment

3. Marking & Application:
- Mark skin with pen at 2 cm intervals, label with allergen initials
- Start with negative control, end with positive control
- Place one small drop of each allergen solution next to corresponding mark
- Hold lancet at 90° angle, prick through drop into epidermis (do NOT draw blood)
- Use a new lancet for each allergen to avoid cross-contamination
- Discard lancets immediately into sharps bin

4. Blot & Timing:
- Blot excess fluid with tissue (do NOT cross-contaminate sites)
- Set timer for 15 minutes
- Use distraction (child on parent's lap, toys, videos)

5. Reading & Measurement:
- Measure wheal diameter (pale central elevation) – NOT flare (erythema)
- Measure longest diameter and perpendicular diameter; calculate mean
- Record in mm for each allergen
- Positive: wheal ≥3 mm larger than negative control
- Invalid test: negative control positive (dermatographism) OR histamine negative (poor reactivity)

6. Aftercare:
- Apply hydrocortisone cream to itchy sites if needed
- Oral antihistamine for severe pruritus/swelling
- Document results in medical record (test date, medications withheld, wheal sizes, interpretation)
- Discuss results with family: distinguish sensitization from clinical allergy

🔬 PRICK-TO-PRICK METHOD (for fresh foods):
- Push lancet into fresh fruit/vegetable/meat (fleshy, juicy site)
- Place small amount of food on skin, then prick through it at 90°
- For dry foods: crush/grind, mix with sterile saline to make paste
- Use same interpretation criteria (wheal ≥3 mm > negative control)

⚠️ CRITICAL SAFETY:
- Risk of anaphylaxis: 0.02% (higher with fresh foods, latex, drugs)
- Observe patient for 20-30 minutes after testing
- Must have immediate access to intramuscular adrenaline 1:1000
- Do NOT perform SPT if patient has unstable asthma or prior anaphylaxis to test allergen

📘 Nelson's Textbook of Pediatrics · Allergy Testing Core Concepts

🩺 Indications for SPT
Food allergy, allergic rhinitis, asthma (perennial indoor allergens), venom allergy, penicillin allergy, latex (serum test preferred). NOT for non-IgE mediated disorders (FPIES, proctocolitis).

⚡ Sensitization vs Allergy
Positive SPT = sensitization (presence of sIgE). Clinical allergy requires history of symptoms on exposure. Many sensitized children tolerate allergen (e.g., 8% positive peanut SPT, only 1% allergic).

📏 Diagnostic Decision Points
For certain foods (milk, egg, peanut), SPT wheal size correlates with >95% PPV. Example: peanut SPT ≥8 mm (age <2) or ≥14 mm (older) predicts clinical allergy. But values vary by population.

💊 Medication Interference
Antihistamines: 3-7 days. Tricyclics: 7-14 days. Oral corticosteroids (prolonged high-dose) may suppress. Topical steroids at test site: avoid. β-agonists, theophylline: no interference.

🧒 Age Considerations
SPT can be performed at any age, but infants <6 months may have reduced skin reactivity (smaller wheals). False negatives possible. Specialist should test children <2 years.

🔄 Follow-up Testing
Repeat SPT every 1-2 years for food allergies to monitor resolution. Negative SPT + good history → consider oral food challenge. Peanut/tree nut allergy often persists.

🌾 Cross-reactivity
Positive tests to related foods common (e.g., peanut → other legumes) but clinical cross-reactivity rare (<5%). Pollen-food syndrome: birch pollen → apple, peach, hazelnut (oral itching only).

📉 Limitations
Does NOT predict severity. Does NOT predict threshold. May be negative in non-IgE mediated allergy. Fresh food testing may be needed for fruits/vegetables (commercial extracts less sensitive).

📖 Nelson's Key Quote: “A positive skin prick test only indicates sensitisation (i.e. the presence of allergen specific IgE) and does not always equate to clinically relevant allergy. The overall positive predictive accuracy is <50% with suspected food allergy. If the history is clear, a positive skin test can confirm the diagnosis of IgE mediated allergy. However, if the history is uncertain, the significance of a positive skin test may need to be confirmed by formal oral food challenge.”

⭐ TOACS TAKE-HOME POINTS:
1. SPT detects IgE sensitization, not clinical allergy – always correlate with history.
2. Positive test = wheal ≥3mm larger than negative control at 15 minutes.
3. Negative predictive value >95% for IgE-mediated food allergy.
4. Withhold antihistamines 72-96 hours before SPT.
5. Emergency equipment (especially adrenaline) must be available.
6. Intradermal testing NOT for foods – too risky, high false positives.
7. Unorthodox tests (IgG4, kinesiology, vega) are NOT evidence-based.
8. Refer to allergist for ambiguous cases, oral food challenges, or venom immunotherapy.