Liquid biopsy of atherosclerosis using protoporphyrin IX as a biomarker. Protoporphyrin IX (PPIX), a derivative of hematoporphyrin, can accumulate in rapidly growing tissues, including tumors and atherosclerotic plaques. The objective of this study is to employ PPIX fluorescence to detect the changes in blood caused by the formation of atheromatous plaques in arteries; this measurement can function as a liquid biopsy. For this purpose twenty four rabbits were randomly divided into groups: control group (CG)--fed with a normal diet, and an experimental group (EG)--fed with a hypercholesterolemic diet (1% cholesterol). Blood samples were collected before (0 time) and after 22, 43, 64 days to measure biochemical factors. The aortas were removed after 22, 43 and 64 days to assess the atherosclerotic plaques. PPIX was extracted from the blood and fluorescence was measured in the 550-750 nm range from samples that were excited at 405 nm. Aminolevulinic acid (ALA) was administered intravenously to increase the PPIX fluorescence intensity in the arteries and consequently in the liquid biopsy of the atherosclerotic plaques. The results have shown that the PPIX fluorescence increased as the atheromatous plaques grew. The aorta fluorescence and the PPIX fluorescence increased in the animals in the experimental group that received ALA. PPIX that accumulates in atheromatous plaques transfers to the blood and can be analyzed by extracting porphyrin from total blood. Therefore, this method can aid in the early diagnosis of atherosclerosis with high sensitivity.