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Anticonvulsant hypersensitivity syndrome (AHS), also known by the other names drug rash (reaction) with eosinophilia and systemic symptoms (DRESS) and drug-induced hypersensitivity syndrome (DIHS), is a rare and potentially fatal reaction that occurs in susceptible patients after exposure to certain drugs, including aromatic anticonvulsants. Because of its ill-defined clinical picture and resemblance to other diseases, the diagnosis of AHS is often difficult and requires a safe and reliable diagnostic test. The skin patch test has been proven to be very useful for prediction and diagnosis of some types of hypersensitivity reactions such as delayed drug eruptions to beta-lactam antibacterials. However, the diagnostic value of patch testing for AHS is yet to be determined and its negative predictive values (NPVs) and positive predictive values (PPVs) are still unknown. This systematic review attempts to evaluate the usefulness of patch tests in the diagnosis of AHS and to examine different technical aspects of patch testing that may contribute to its performance. We included studies in which aromatic anticonvulsant drugs are the likely causes of the hypersensitivity reaction. Analysis of original publications from 1950 to August 2008 and cited in PubMed, MEDLINE and EMBASE has revealed contradictory findings, possibly due mainly to the use of unstandardized methods. Numerous factors have been suggested to affect the final result of the test, including the following: type of drug tested; concentration of drug and vehicle used; timing of the test after exposure; and the clinical picture of the reaction. The PPV of the test in optimal conditions was as high as 80-90% depending on the drug tested. On the other hand, this value is around 10-20% in many other published studies. Although patch testing may be a useful diagnostic test for AHS, accurate determination of its sensitivity and specificity is yet to be achievable due to the lack of a gold standard test against which the performance of patch testing can be measured. Its PPV appears to be higher than its NPV, a matter that necessitates the use of other confirmatory tests in case of negative patch tests (e.g. careful systemic rechallenge). The benefit of testing appears to be maximal with certain drugs (i.e. carbamazepine and phenytoin) and for specific clinical manifestations (strong reactions). It should be performed 2-6 months after recovery from the date of the ADR for best results, with adequate vehicle control.

Anticonvulsant hypersensitivity syndrome (AHS) is a rare and potentially fatal reaction that develops in susceptible patients following exposure to certain drugs, including aromatic anticonvulsants. Because of its ill-defined clinical picture and resemblance to other diseases, the diagnosis of AHS is often difficult and requires a safe and reliable diagnostic test. Other than systemic rechallenge, which is not always ethically permissible and has its own limitations, no reliable diagnostic test is available for this type of disorder. This systematic review attempts to evaluate the usefulness of the available in vitro tests in the diagnosis of AHS - namely, the lymphocyte transformation test (LTT) and the lymphocyte toxicity assay (LTA) - and to examine the different technical aspects of these tests that may contribute to their performance. We included studies in which aromatic anticonvulsant drugs were the likely causes of the hypersensitivity reaction and either the LTT or the LTA was used to aid the diagnosis of AHS. Analysis of original publications from 1950 to the last week of March 2009 and cited in PubMed, MEDLINE and EMBASE has revealed that there are numerous factors affecting the final result of the test, including the following: the timing of the test after exposure; the clinical manifestation of the reactions; the specific drug; and the test procedure and read-out system. In vitro diagnostic tests have the advantage over in vivo tests of being safe to use; however, in vitro tests for the diagnosis of AHS are not well standardized and their sensitivity and specificity are not yet determined. From the reviewed literature, the sensitivity of the LTT and the LTA seem to be around 70% and 90%, respectively, and the positive and negative predictive values of the tests in highly imputable cases are quite high. However, the lack of a gold-standard diagnostic test to prove drug culpability, along with the paucity of large-scale studies, precludes accurate determination of the epidemiological characteristics of these tests. It appears that without further understanding of the mechanisms underlying the pathophysiology of AHS, and how specific drugs and metabolites differentially affect these mechanisms, the development of more reliable tools for AHS diagnosis will be compromised. Consequently, in the absence of further research, the predictability of these tests will remain questionable and they are unlikely to be utilized on a large scale.

Idiosyncratic drug reactions (IDRs) represent a major health problem, as they are unpredictable, often severe and can be life threatening. The low incidence of IDRs makes their detection during drug development stages very difficult causing many post‐marketing drug withdrawals and black box warnings. The fact that IDRs are always not predictable based on the drug's known pharmacology and have no clear dose–effect relationship with the culprit drug renders diagnosis of IDRs very challenging, if not impossible, without the aid of a reliable diagnostic test. The drug provocation test (DPT) is considered the gold standard for diagnosis of IDRs but it is not always safe to perform on patients. In vitro tests have the advantage of bearing no potential harm to patients. However, available in vitro tests are not commonly used clinically because of lack of validation and their complex and expensive procedures. This review discusses the current role of in vitro diagnostic testing for diagnosis of IDRs and gives a brief account of their technical and mechanistic aspects. Advantages, disadvantages and major challenges that prevent these tests from becoming mainstream diagnostic tools are also discussed here.

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