*2.1. Literature Search and Selection of Studies*

A search of the literature was performed by interrogating the PubMed database (on 27 July 2021) with the keyword combination "PRP AND alopecia AND clinical trial", a

sufficiently specific search string that avoids sampling biases. We selected for meta-analysis the clinical trials that included treatment groups, using as therapy PRP alone, combined with or compared to other procedures. The exclusion criteria comprised clinical trials including patients with other types of alopecia (e.g., alopecia areata, seborrheic dermatitis, iatrogenic alopecia, *a*.*s*.*o*.). The stepwise procedure for selecting clinical trials included in the meta-analysis observed the methods, stages, as well as selected checklist items of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines and its subsequent updates [30,31].
