Ultracentrifugation is regarded as the gold standard method for exosome isolation. The ultracentrifugation process is time consuming and may introduce protein and nucleic acid contamination. Pre-clearing samples by one or more low speed centrifugation steps will deplete the cells, platelets, and large apoptotic bodies. Larger extracellular vesicles can be pelleted at forces between 10000-20000 x g. Smaller extracellular vesicles can be pelleted at higher speeds, ranging from 100000-200000 x g. Depending on the protocol, it can take up to 24 hours to isolate exosomes by ultracentrifugation. It is impossible to achieve absolute separation due to potential contamination of proteins and nucleic acids.