Prick test is the approach of choice for type 1 DHRs. The injectable drug is applied on the volar aspect of the forearm by means of a superficial puncture. The test can be performed with the suspect agent, although the most suitable approach is to use the active ingredient and the excipients separately. In the case of urticarial reactions, the test should start with serial dilutions (10-3, 10-2, 10-1). Controls should be applied, as follows: negative (with physiological saline 0.9%) and positive (with histamine at 10mg/mL). A positive response is defined as a wheal at the puncture site that is 3mm in diameter after 20minutes and no reaction with the negative control. This result confirms a type 1 DHR; the presence of specific IgE is shown in vivo. If the drug is a histamine releaser, then the result could be a false positive caused by pseudoallergy. A negative result in the prick test never rules out a causal relationship (Fig. 6).

Figure 6.

Additional tests for the identification of the triggering drug in hypersensitivity reactions. Abbreviations: BAT, basophil activation test; IDT, intradermal testing; Ig, immunoglobulin; LTT, lymphocyte transformation test.

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Intradermal testing is performed if the result of the prick test is negative and there is strong clinical suspicion of a type 1 DHR. The drug should be applied at the concentrations stipulated in international guidelines. Dilutions should be prepared with physiological saline 0.9% within 2hours before application and under aseptic conditions with a laminar flow hood. If the test dilution is not described, then the test should start with a concentration of 10-4, which should gradually be decreased by 1 decimal place to no more than 0.04mL of solution injected by puncture. This generates an intradermal wheal measuring 4 to 6mm in diameter. The reading is at 30minutes, 6hours, and 24hours. The patient remains under observation during the first 6hours, with specific monitoring of heart rate and blood pressure. The test should be performed using a peripheral venous access and infusion of glucose solution. If a 10-mm wheal is generated at 30minutes, the result is considered positive. If there is no reaction after 30minutes, the concentration can be increased until the pure concentration is reached. A reaction during the first 6hours confirms a type 1 DHR. It is also important to take into account the possibility of false positives caused by pseudoallergy. When the reaction is late, then the DHR is delayed, the size of the wheal should be recorded in the clinical history, and the patient should be followed up at 1 week. If an early reaction is negative, then the patient should be contacted at 1 week to confirm the negative result. A negative result does not rule out a causal relationship (Fig. 6).

The patch test is the approach of choice when a delayed reaction is suspected. Although less sensitive than the previous 2 options, it can be an alternative if no injectable formulation is available. Ideally, the active ingredient should be tested separately from the excipients. The concentration to be tested is standardized. If this is not known, the pure drug is formulated at 5% or 10%. If the pure drug is not available (active ingredient), the suspect pharmaceutical formulation can be tested at a maximum of 30%, and the formulation cannot be stored for more than 24hours. If the capsule is available, it should be hydrated and dissolved so that it can be tested separately from its content. In this case, if the suspect pharmaceutical formulation is used in the case of a positive result, we cannot determine whether the allergen is an excipient or the active ingredient. In order to reduce the risk of false positives caused by irritation, the drugs should be formulated in petrolatum and/or distilled water. If a commercial system is not used, then the in-house product should be prepared with care (Fig. 6). The formulation should be applied on healthy skin of the forearm or back, and the first reading should be taken after 20minutes. This rules out the possibility of immediate allergy or irritant reaction. The patch is occluded for 48hours, after which time it is uncovered, the residue is removed without rubbing, the skin is left open to the air for 20minutes, and the first late reading is made. Readings are then taken at 72 or 96hours and at 1 week. If drug-induced photoallergy is suspected, the drug should be tested in a patch and with photostimulation at 48hours (UV-A 5J/cm2) in a separate patch. In the case of fixed pigmented erythema, one patch should be placed over healthy skin and another in the area of the fixed eruption. A positive response indicates a reaction ranging from a mild reaction with erythema and edema to an intense reaction with vesicles-blisters or erosions. Irrespective of the grade of the reaction, a positive patch test result confirms DHR. However, a negative result does not rule out a causal relationship. The patch test is useful for proving reactions such as maculopapular rash, acute generalized exanthematous pustulosis, drug reaction with eosinophilia and systemic symptoms, and symmetrical drug-related intertriginous and flexural exanthema. Sensitivity varies with the drug and the clinical presentation of the reaction. In a series of 134 patients, Barbaud et al.14 demonstrated that the overall sensitivity of the test was close to 60%. Sensitivity decreases in the case of macular exanthema (fixed pigmented erythema), urticarial reactions, and exfoliative forms (SSJ-TEN). Patch testing is not useful in organ-specific reactions.14

Controlled exposure testing is contraindicated in cases of severe cADR and should only be applied when it seems clinically improbable that the suspect drug is the trigger. In most cases, this exposure test occurs accidentally (ie, not under controlled conditions), when the patient is re-exposed to the drug inadvertently and reports recurrence of the symptoms. Controlled exposure is the only test in which a negative result rules out a causal relationship with the suspect drug. The drug is administered at increasing doses under the supervision of an allergologist or trained physician. Controlled exposure testing is the gold standard for establishing a causal relationship, despite the difficulties involved in performing it.

The basophil activation test is useful when the pharmaceutical formulation is not available in injectable solution and we need to test for a type 1 DHR. This test involves placing the patient's basophils in culture against the drug and measuring the expression of activation receptors (CD63, CD203) using flow cytometry. The probability of a causal relationship is expressed as positive or negative according to the percentage of cells that are activated.

Measurement of serum tryptase is useful for type 1 DHRs. Tryptase is found in mast cell granules and is released after activation. The curve of the measurements is plotted at 15minutes, 3hours, and late at 24hours after initiation of symptoms.

Specific IgE panels are more frequently used for foods or environmental allergens based on techniques such as radioimmunoassay or enzyme-linked immunosorbent assay. The panel for diagnosing drug allergy is fairly limited. In fact, a type 1 DHR does not always progress with increased IgE; therefore, a negative result does not always rule out a causal relationship.

The lymphocyte transformation test is useful in the case of suspected delayed reaction. It consists of the incubation of lymphocytes extracted from peripheral blood and the suspect drug after 48hours to 7 days. The result is positive if there is proliferation of lymphoblasts. This is the test of choice for severe exfoliative delayed DHRs (SSJ/TEN) or specific organ reactions, since controlled exposure testing is contraindicated.