Case Studies:Special investigations are rarely essential to make a diagnosis of atopic eczema. Investigations are done mainly to identify trigger factors that flare up or aggravate eczema so that patients may be advised to avoid these. These investigations are useful mainly in the management of atopic eczema in children. The roles of skin biopsy and total IgE levels have been addressed. Formal patch testing can help to diagnose superimposed allergic contact factors , but has no value in diagnosing or excluding the diagnosis of atopic dermatitis. This is probably of more value in the adult-onset subgroup of patients. Skin prick tests (SPT) is the most common procedure used to confirm food allergy. These tests are also used to confirm inhaled or aeroallergens. Fresh food extracts of cow’s milk, egg, wheat, soy, fish and peanuts are used. A standardized test lancet is dipped into the test food solution and the patient’s skin is pricked with the allergen impregnated lancet. A new lancet is used for each skin test. The test site is then observed for 15-20minutes and any wheal reaction measured. A positive control is a wheal of 3mm or greater than the negative saline control. A positive test is a wheal produced by histamine. This should measure at least 3mm. Variants of SPT include the scratch patch test (the skin is scratched first and then the food applied under an occlusive patch) and the skin food application test (food is applied to skin without pricking and examined at 10 minute intervals for a reaction). These tests are not commonly used as they offer no benefit over the routine SPT. Patients have to be off steroids and antihistamines for 72 hours prior to SPTs. It is important to note that SPT antigen kits have a fixed shelf life. Radioallergosorbent (RAST) test for specific food IgE antibodies. The tests measure serum specific IgE to recognized food allergens in kU/l. The tests are less accurate than SPTs. The food mix (egg white, milk, gluten, peanut and wheat) is commonly used in children. In adults egg yolk and gluten is frequently used. RAST tests are expensive. The tests are not affected by steroids and antihistamines. As mentioned previously SPT and RAST have a high negative predictive value (95%) for the diagnosis of food and environmental allergies but positive tests are less reliable. The tests are of greater value in excluding allergies. Positive tests are of far less value with only a 40% predictive value . Atopic individuals typically show numerous positive results on these tests and the exact clinical relevance of these reactions to specific allergens is doubtful . Atopy patch tests (APT): In these tests, common aeroallergens and foods are patch-tested to investigate the role of these allergens in individual cases . Antigens include house dust mite (HDM), cat dander, Bermuda grass and Cockroach. The results correlate somewhat with RAST and skin prick tests, but further refinement is necessary to make it more reliable . Unlike SPTs which measure IgE sensitivity to HDM, the APT elicits a delayed type response mediated by specific CD4+ which clinically resembles eczema. The patch test reaction to aeroallergens is specific to sensitized atopic eczema patients and does not occur in healthy individuals. Although APTs are found more commonly in the presence of high IgE levels, they may also be absent despite them. APT is much more likely to be positive among patients whose eczema is predominantly on exposed parts of the skin. The ATPs are currently being standardized. Combining APT and SPT may improve accuracy. Double-blind placebo controlled food challenge tests are the gold standard for diagnosing associated food allergies, but they are very difficult to perform, time consuming and available to few. In adult patients these are seldom performed. Some foods give rise to clinical symptoms of urticaria, itching, angio-oedema and exacerbation of eczema where the histamine release is by non IgE immunological mechanisms. For these reactions the term food intolerance is suggested. Many “junk” foods appear to aggravate eczema by this mechanism. These foods are best identified by using a basic elimination diet for 2 weeks to assess whether the symptoms improve. During this period, rice cakes, rice, apples, pears, apricots, grapes (no skin), lamb, chicken (free range), potatoes, sweat potato, butternut, beetroot, asparagus, lettuce, squash, olive or sunflower oil and table salt are allowed. Tinned, packet or bottled foods are forbidden. Test foods are introduced one by one on a daily basis. A repeat challenge is necessary to confirm the culprit food. These elimination diets are best supervised by an allergist with the assistance of a dietician. If immunodeficiency is suspected with atopic eczema, tests like HTLV1, HIV, immunoglobulin levels, T and B cell number and functions will need to be done. If secondary bacterial infections are suspected, appropriate swabs for microscopy, culture and sensitivity is necessary. Tzanck smears are recommended for suspected Herpes infection. Special investigations in atopic dermatitis:
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