Module 4: Occupational Allergy - Lecture (Continued)

8. INVESTIGATIONS

8.1 Patient:

• history and examination (skin);
• evaluation of risk for atopy:
• phadiotop/SPT.
• identification of specific allergens causing symptoms:
  • SPT/RAST/bronchial challenge test (IgE-mediated health outcomes);
  • patch test (allergic contact dermatitis);
  • precipitating IgG antibodies (extrinsic allergic alveolitis);
• monitoring the effect of the inflammatory response (e.g. nasal smear, total eosinophils, eosinophilic cationic protein);
• target organ assessment:
  • CXR (extrinsic allergic alveolitis);
  • LFT (asthma and extrinsic allergic alveolitis).

a) SKIN PRICK TEST: IgE mediated allergy (link to Skin Prick Test practical).

Types:

• common aeroallergens (HDM, grass, pollens, feathers, cat, dog, moulds);

• occupational aeroallergens.

  Better for high molecular weight agents:

  • grains (wheat, maize, rye, barley);
  • storage mites;
  • seafood;
  • latex (specialist centre).

  Some low molecular weight agents:

  • platinum salts;
  • pthalic anhydride.

Functions:

• establish atopy:
      as a predisposing factor
      as a causal factor.
• diagnostic, with appropriate history.

Method:

performed by trained practitioner; histamine dihydrochloride used as a positive control and a diluent of glycerol/sodium chloride as a negative control; positive test = wheal, flare and itch - read 15 minutes with wheal diameter of 3 mm or more than the negative control (See figure on the left); atopic status considered positive if SPT to one or more common aeroallergens is positive.

Value:

• highly specific, reasonably sensitive (fresh extracts for unstable allergens) and reliable for the diagnosis of aeroallergy especially;
• inexpensive, somewhat time-consuming but immediate results;
• generally safe, but reactions may occur;
• SPT have best positive predictive value and best efficiency to diagnose atopic respiratory diseases when compared to total serum IgE and Phadiotop.

Contra-indications:

• known severe reactions to the identified allergen;
• skin: dermatographism and eczema;
• allergens: dangerous allergens (penicillin, horse, venoms) and unreliable allergens (especially foods and moulds);
• medication: patients on antihistamines and any other drugs that suppress SPT reactions.

b) Radioallergosorbent test (RAST): Specific IgE

Types:

• high molecular weight allergens: proteins, foodstuffs, animal derivatives;
• low molecular weight allergens: isocyanates, formaldehyde, woods, ethylene oxide, pthalic anhydride, drugs - (less sensitive).

Function:

• PHADIOTOP inhalant screening test (grass-, weed- and tree pollens, mould spores, epithelia and HDM): establishes atopy (more than 93% sensitive and 96% specific); useful in parasite endemic areas; important for new recruits into industry;
• RAST for individual inhalant allergens (eg. wheat, latex etc.): specific diagnosis, with appropriate history; available as CAPRAST mixes or individual tests.

Method: (see the illustration)

• based on the ELISA method;
• known allergen is coated to a solid phase medium;
• allergen-coated solid phase is incubated sequentially with patient's serum and radiolabelled anti-human IgE;
• the number of counts bound is proportional to amount of specific IgE in serum (linear response);
• ELISA similar to RAST except that antihuman IgE conjugated to alkaline phosphatase (catalyzes a colorimetric change proportional to amount of specific IgE in serum).

Value:

• highly specific, some cross reactivity;
• less sensitive than SPT's especially for LMW (occupational) agents, unstable allergens;
• comparatively more expensive than SPT;
• requires serum, laboratory;
• convenient and low risk to patient.

See the prototype allergy screening test (e.g CAP RAST) request form.

c) Note: Skin patch testing will be covered in the Occupational Dermatoses section.