The gut microbiome has emerged as a promising target for cardiometabolic disease prevention. Although low microbial diversity has been associated with adverse cardiometabolic health, evidence for a causal relationship remains limited. Prospective studies are needed, yet the scarcity of stool samples in biobanks with long-term follow-up poses a major challenge. To address this gap, we will develop, validate, and apply a biochemical blood test to assess the relationship between microbiome diversity and incident cardiovascular events in large, well-characterized biobanks. The three core objectives are: 1) To develop the GutsyTest, a blood-based biochemical assay for microbiome diversity. 2) To validate the GutsyTest against stool-based microbiome profiling in the Estonian Biobank and the SIMPLER study. 3) And as a proof-of-concept assess the association of the GutsyTest with incident cardiovascular events in the Northern Sweden Health and Disease Study.