Selection of SJS, SJS/TEN, and TEN

We used the International Classification of Disease, 9^th edition, Clinical Modification (ICD-9-CM) codes to identify SJS, SJS/TEN, and TEN: SJS, code 695.13;, code 695.14; and TEN, code 695.15). Furthermore, based on a previously validated algorithm, patients who had a length of hospital stay (LOS) shorter than 3 days were excluded to improve the positive predictive value of the diagnostic codes[16].

We included all patients admitted to public hospitals of the NHS between 2010 and 2015 with a primary and/or secondary diagnosis of SJS, SJS/TEN, and TEN according to the ICD-9-CM codes, along with a LOS shorter than 3 days. We excluded those patients with LOS for 2 or fewer days. Patients with missing data were also excluded from the study. Firstly, 1,765 patients with diagnosis of SJS, SJS/TEN, and TEN were selected during the study period. Of these, 297 patients had a LOS shorter than 3 days, and they were excluded. Finally, 1,468 patients were included in the study.

There is growing evidence that SJS/TEN in the pediatric population has different epidemiology and outcomes than adults, so we stratify the sample in pediatric (<16 years old) vs adult (≥16 years old).