Though such blood tests have become remarkably accurate (skin prick is still the most accurate way to diagnose what a person is allergic to), there are profound and significant limitations.
In order to understand why, first a little biology lesson.
In "classic" allergy (Type 1 hypersensitivity), there are antibodies called IgE floating around in the blood. When such antibodies come into contact with what a person is allergic to, it causes mast cells to explode releasing bioactive chemicals that triggers the classic signs of allergy including runny nose, sneezing, and hives.
The timing of exposure to reaction is very short... within minutes.
In this scenario, the allergy blood testing is appropriate... because such testing measures the amount of IgE antibodies floating around in the blood.
Unfortunately, not all contact dermatitis is due to this biologic response.
There is another type of allergic reaction that can cause skin manifestations that is NOT mediated by IgE antibodies at all!
Known as Type 4 Hypersensitivity, special cells called macrophages initiate the "allergic" reaction when it detects an allergic substance.
This type of reaction occurs hours to days AFTER exposure.
Obtaining a blood test for allergies will NOT diagnose this type of allergic reaction because the test measures the level of IgE which is not involved in this type of reaction!
The ONLY way to test for Type 4 Hypersensitivity is skin patch testing, typically performed by dermatologists.
Patch testing is performed on skin where the dermatitis is not apparent (typically upper back). The allergens are mixed with a non-allergic material (base) to a suitable concentration which is then placed in direct contact with the skin within small aluminium discs. Adhesive tape is used to fix them in place. The patches are left in place for 48 hours. The patches should not be exposed to sunlight or other sources of ultraviolet (UV) light. After 48 hours the patches are removed and an initial reading is taken one hour later. The final reading is taken a further 48 hours later. Additional readings beyond 48 hours increase the chance of a positive test patch by 34 per cent. The patient should refrain from washing until the last reading is taken.
Some chemicals produce an allergic reaction only when exposed to light (usually ultraviolet type A). If this scenario is suspected, patch testing is performed, one while exposed to UVA light and the other hidden in darkness. A positive light-reaction is made if the patch exposed to UVA light causes a skin reaction whereas the one hidden in darkness looks fine.
What are some common chemicals/substances that cause Type 4 skin reactions?
- Balsam of Peru: an aromatic mixture made from resins and essential oils. It is found in the haemorrhoid preparation Anusol, some perfumes and certain spices.
- Caine mix: local anaesthetics found in preparations for sore throats, sunburn remedies, haemorrhoid preparations, Wasp-eze. Used by dentists and doctors for minor surgical procedures.
- Carba mix: rubber 'accelerators' (chemicals used to speed up the polymerisation process in the manufacture of rubber). It is found in rubber gloves, shoes, bandages and elastic. Of those allergice to carba, 85 per cent are also allergic to thiuram.
- Chlorocresol: a substituted phenol preservative that kills bacteria. It is widely used in medications and some cosmetics. It cross-reacts with Dettol, which you should also avoid if you have a chlorocresol allergy.
- Chromate: a metal used for plating other metals to prevent rusting and in the manufacture of stainless steel. It is also found in cement and tanned leather.
- Cobalt: found in jewellery, dental implants, artificial joints, jet engines. Most patients are also allergic to nickel, and some are also allergic to chromate.
- Colophony: present in adhesives, plasters, paper, printing inks, medicated creams, glue tackifiers (stamps, labels), and cosmetics.
- Epoxy resin: plastics, used mainly as adhesives in the industrial setting but also by DIY enthusiasts. Found in two-component glues, such as Araldite.
- Formaldehyde: preservative frequently used in household products and in industry. Often found in cosmetics and shampoo.
- Fragrance mix: used in patch testing, this collection of eight individual fragrances detects about 75 per cent of patients allergic to perfume. If you have perfume allergy, you will not be allergic to all fragrances, but you cannot tell from the label which fragranced cosmetics are safe. Avoid all cosmetics listing 'parfum' as an ingredient on the label. Also found in air fresheners, washing powders and candles.
- Lanolin: produced by sheep to protect the fleece from the results of weathering. It is widely used in cosmetics, medical creams and bandages.
- Latex: most commonly found in gloves used by healthcare professionals.
- Mercapto mix/thiazoles: a rubber accelerator found in rubber shoes, insoles, gloves and elastic. It is also a component of balloons and bandages.
- MBT (mercaptobenzothiazole): another rubber accelerator. Neomycin: an antibiotic commonly used in ear and eye drops and creams to treat infected skin problems. Cross-reacts with other antibiotics.
- Nickel: 10 per cent of women and at least 1 per cent of men are affected by nickel allergy. Nickel is released from metals such as alloys or electroplated items. Found in jewellery, keys, coins, zips and buckles, pacemakers and batteries.
- Parabens: preservatives found in cosmetics and topical medical products to inhibit the growth of fungi and prevent slow deterioration. They are commonly used in cosmetics, household products, glue, shoe polish, shampoos and conditioners, sunscreens and medical creams.
- PPD (paraphenylenediamine): a permanent hair dye that is very frequently used in hair salons and at home. Dyed hair cannot cause an allergy but the dye may do during application. Also found in skin paints, such as 'henna' tattoos, and occasionally in fur and leather dyes. Primin: a substance produced by the plant Primula obconica, a common houseplant.
- PTBPF resin (para-tertiary-butylphenol-formaldehyde): a synthetic polymer used as an adhesive. It is often combined with leather or rubber to make shoes, handbags, watchstraps, hats and belts.
- Thiuram (tetramethylthiuram disulphide or TMTD): another rubber accelerator that is also found in pesticides. People who are carba allergic often react to thiuram. Patients who react to the drug disulfiram (Antabuse), used for alcohol dependence, may also be allergic to thiuram.
- Toluene sulphonamide formaldehyde resin (TSF resin): the commonest polymer in nail polish and a frequent allergen.
BUT, the key thing for people to remember is that allergy blood testing and skin prick testing can NOT diagnose all allergies due to intrinsic factors.
Skin patch testing may be the ONLY test to determine what that mysterious hive reaction is due to.
AND, now you know why!
For physicians, skin patch testing supplies can be purchased at www.dormer.com. However, your local dermatologist should be able to perform such patch testing (our office does not).
In our area, Dr. Moshell at Georgetown is the expert in this field.
5530 Wisconsin Ave, Suite 730
Chevy Chase, MD 20815
Another dermatologist in Northern Virginia who performs patch testing is:
Kurt Maggio, MD
7512 Gardner Park Dr
Gainesville, VA 20155
Robert Gurney, MD
11315 Sunset Hills Rd
Reston, VA 20190