key: cord-282355-urys21ry
authors: Ortega-Quijano, Daniel; Jimenez-Cauhe, Juan; Burgos-Blasco, Patricia; Jimenez-Gomez, Natalia; Fernandez-Nieto, Diego
title: Reply to “Varicella-like exanthem as a specific COVID-19-associated skin manifestation: multicenter case series of 22 patients”: discussing specificity
date: 2020-05-04
journal: J Am Acad Dermatol
DOI: 10.1016/j.jaad.2020.04.156
sha: 
doc_id: 282355
cord_uid: urys21ry

nan

Title: Reply to "Varicella-like exanthem as a specific COVID-19-associated skin manifestation: multicenter case series of 22 patients": discussing specificity Daniel Ortega-Quijano, MD 1 , Juan Jimenez-Cauhe, MD 1 , Patricia Burgos-Blasco, MD 1 , Natalia Jimenez-Gomez, MD 1 , Diego Fernandez-Nieto, MD 1 . Dear Editor, We read with interest the article by Marzano and colleagues addressing the specificity of varicella-like exanthem to diagnose coronavirus disease (COVID-19) 1 . Although this type of COVID-19 associated rash is rare, the authors claim that it is more specific than others without having performed a diagnostic accuracy study 2 . This type of study would calculate the association of COVID-19 status (yes/no) with the type of exanthem (varicella-like/non varicella-like) in a predetermined number of patients (to ensure sufficient statistical power) and in a defined population with a known prevalence of COVID-19 disease.

Starting from the assumption that the data is not sufficient to draw such a robust conclusion, we would like to contribute our vision of such an important issue.

Firstly, for clinicians, specificity is of little value. The positive predictive value, that is, the probability that a person with a varicella-like rash has COVID-19, is much more relevant, as a high value would justify SARS-CoV-2 testing. The positive predictive value is affected by the prevalence of the disease. Consequently, as we are going through the peak of the COVID-19 pandemic, it is evident by applying Bayes' theorem that the conditional probability of COVID-19 given a varicella-like rash is enormous.

However, this probability is as high given a dengue-like, erythematous or urticarial rash. Therefore, the positive predictive value for COVID-19 of skin rashes, regardless of which, is high.

For us, the main contribution of the study by Marzano and colleagues is that, with all exanthems currently having a high positive predictive value for COVID-19, that of varicella-like rash is probably the highest since vesicular rash is more specific for viral disease than others 3 . In addition, at the current stage of the pandemic, COVID-19 is more frequent in adults, where varicella is not. This gives varicella-like rash additional COVID-19 positive predictive value compared to the rest of the rashes. The main factor that at this moment reduces the positive predictive value of rashes for COVID-19 is adverse drug reactions. Unfortunately, this topic is being less discussed in the literature.

To conclude, since things will change fast as the pandemic progresses, it is expected that as the adult population become infected the age at diagnosis of COVID-19 will decrease 4 . At that time, this rash will be an important differential diagnosis of varicella itself. In this context, Tzank test and Varicella-Zoster PCR should be subjected to new diagnostic accuracy studies.

Varicella-like exanthem as a specific COVID-19-associated skin manifestation: multicenter case series of 22 patients

Cutaneous manifestations in COVID-19: a first perspective

The challenge of diagnosing atypical exanthems: a clinico-laboratory study

Modeling Infectious Diseases in Humans and Animals