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SCRATCH principles for diagnosis of insect bites

Dr. Mona Abdel-Halim


Insect bite reactions (papular urticaria) are the most commonly encountered skin eruptions in pediatric population. However, in many cases, these eruptions are frequently misdiagnosed and parents usually keep going from one dermatologist to another seeking a correct diagnosis.

An interesting article by Hernandez and Cohen was published in “Pediatrics”, the official journal of the American Academy of Pediatrics has discussed certain clinical features that represent principles for diagnosing insect bite reactions (papular urticaria). The authors have collected these features in the Acronym: SCRATCH as follows:
S: Symmetric distribution (scalp, neck, face, torso, extremities)
C: Crops/clusters of different coloration (erythema, hypo-/hyperpigmentation)
R: Rover not required: pets are not necessary criteria for diagnosis
A: Age specific (usually occurring between 2 and 10 years of age)
T: Target lesions and time (may take weeks to years to resolve)
C: Confused pediatrician/parent: “we don’t have fleas”!
H: Household with single family member affected

On histopathological basis, insect bite reactions may also cause diagnostic confusion to dermatopathologists as one may encounter large atypical CD30 positive T cells leading to confusion with lymphoma, however an interstitial distribution of esinophils is said to be a reassuring sign. Interestingly, a sweet-like reaction has been also reported in cases of insect bite reactions.

So, it is important to give cases presenting with skin eruptions in the pediatric population great attention and to keep insect bites in the background of our minds in such cases taking in consideration the SCRATCH principles in establishing a clinical diagnosis.

[b][i]Raquel G Hernandez and Bernard A Cohen (2006): [/i][/b]Insect bite-induced hypersensitivity and SCRATCH principles: A new approach to papular urticaria. PEDIATRICS, 118 (1): 189-196.

[b][i]Battistella M, Bourant E, Fardet L, Saada V, Janin A, Vignon-Pennamen, MD (2012):[/i][/b] Sweet like reaction due to arthropod bites: A histopathologic pitfall. Am J Dermatopathology, 34(4): 442-445


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Dr. Phillip McKee


Nice blog. Thank you Mona. The only comments that I can add are that flame figures can be a feature and sometimes very severe subepidermal edema may result in a bullous pemphigoid-like appearance. When we lived outside Boston, Grace developed massive (6 cm or more) blisters on the lower legs looking for all the world like BP. The localization to the lower legs, symmetry and our living in the country where was abundant stagnant water reassured me that the features were those of a massive insect-bite reaction. Happily, she lived to tell the tale.
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