Severe cutaneous adverse reactions (SCAR) are life-threatening conditions including acute generalized
exanthematous pustulosis (AGEP), Stevens-Johnson Syndrome (SJS), toxic epidermal necrolysis (TEN) and drug
reaction with eosinophilia and systemic symptoms (DRESS). Diagnosis of causative underlying drug hypersensitivity
(DH) is mandatory due to the high morbidity and mortality upon re-exposure with the incriminated drug. If
an underlying DH is suspected, in vivo test, including patch tests (PTs), delayed-reading intradermal tests (IDTs)
and in vitro tests can be performed in selected patients for which the suspected culprit drug is mandatory, or in
order to find a safe alternative treatment. Positivity of in vivo and in vitro tests in SCAR to drug varies depending
on the type of reaction and the incriminated drugs. Due to the severe nature of these reactions, drug provocation
test (DPT) is highly contraindicated in patients who experienced SCAR. Thus, sensitivity is based on positive test
results in patients with a suggestive clinical history. Patch tests still remain the first-line diagnostic tests in the
majority of patients with SCAR, followed, in case of negative results, by delayed-reading IDTs, with the exception
of patients with bullous diseases where IDTs are still contra-indicated. In vitro tests have shown promising
results in the diagnosis of SCAR to drug. Positivity is particularly high when the lymphocyte transformation test
(LTT) is combined with cytokines and cytotoxic markers measurement (cyto-LTT), but this still has to be confirmed
with larger studies. Due to the rarity of SCAR, large multi-center collaborative studies are needed to better
study the sensitivity and specificity of in vivo and in vitro tests.
Cutaneous adverse reactions after m‐RNA COVID‐19 vaccine: early reports from North‐East Italy