1. A system for determining cardiovascular information for a patient, the system comprising: at least one computer system configured to: generate an anatomical model of at least a portion of a patient's vasculature using patient-specific images of a geometry of at least a portion of the patient's vasculature; create a computational model of a blood flow characteristic of the patient using the anatomical model; generate or identify at least one patient-specific uncertainty value specifying a range of uncertainty for at least one of an uncertain parameter, uncertain clinical variable, and/or uncertain geometry of the anatomical model; generate or receive a probability model comprising statistics for blood flow characteristic calculations, based on the at least one patient-specific uncertainty value; modify the probability model by iteratively: adjusting one or more of the at least one uncertain parameter, uncertain clinical variable, and/or uncertain geometry of the anatomical model using the at least one patient-specific uncertainty value; using the computational model and the one or more adjusted uncertain parameter, adjusted uncertain clinical variable, and/or adjusted uncertain geometry of the anatomical model to generate a blood flow characteristic value; and using the generated blood flow characteristic value to determine statistical variation of the blood flow characteristic value as a result of adjustment of the at least one patient-specific uncertainty value; receive a target confidence level; calculate a patient-specific blood flow characteristic value and a range of variation for the patient-specific blood flow characteristic value consistent with the target confidence level, using the computational model of the blood flow characteristic and the modified probability model; and output the anatomical model, the calculated patient-specific blood flow characteristic value, and the calculated range of variation for the patient-specific blood flow characteristic value.