Procalcitonin (PCT) is useful for differentiating between viral and bacterial 
infections and for reducing the unnecessary use of antibiotics. As the rise of 
antimicrobial resistance reaches "alarming" levels according to the World Health 
Organization, the importance of using biomarkers, such as PCT to limit 
unnecessary antibiotic exposure has further increased. Randomized trials in 
patients with respiratory tract infections have shown that PCT has prognostic 
implications and its use, embedded in stewardship protocols, leads to reductions 
in the use of antibiotics in different clinical settings without compromising 
clinical outcomes. However, available data are heterogeneous and recent trials 
found no significant benefit. Still, from these trials, we have learned several 
key considerations for the optimal use of PCT, which depend on the clinical 
setting, severity of presentation, and pretest probability for bacterial 
infection. For patients with respiratory infections and sepsis, PCT can be used 
to determine whether to initiate antimicrobial therapy in low-risk settings and, 
together with clinical data, whether to discontinue antimicrobial therapy in 
certain high-risk settings. There is also increasing evidence regarding 
PCT-guided therapy in patients with coronavirus disease 2019 (COVID-19). This 
review provides an up-to-date overview of the use of PCT in different clinical 
settings and diseases, including a discussion about its potential to improve the 
care of patients with COVID-19.