GDF15 and COVID-19: We designed a Mendelian randomization study to evaluate the impact of metformin on COVID-19, using Growth differentiation factor 15 (GDF-15), also known as macrophage inhibitory cytokine-1, as a robust biomarker for metformin use.[25,26] We used genetic variations in GDF-15 as exposure and COVID-19 as the outcome and validated COVID-19 in 3 subtypes: susceptibility, hospitalization, and severe illness.