Background Hepatitis B (HBV), hepatitis C (HCV), and HIV blood-borne viruses (BBV) are associated with chronic ill health and mortality. Early diagnosis reduces disease transmission, delays progression, and improves outcomes. Routine opt-out testing for BBV in primary care may be viable in identifying unknown disease. Aim To assess the viability and yield of routine optout testing for BBV. Design and setting A multicentre, prospective, routine opt-out testing study of BBV in primary care between September 2014 and February 2015 across four sites in Dublin, Ireland. Method All adult patients attending for routine blood tests were offered an additional BBV test during a 6-month period. All individuals were given an information leaflet before phlebotomy and were given the choice to opt out of BBV testing. Results In total, 1188 patients were invited to participate in the study and 1063 (89.5%) opted to be tested (95% confidence interval [CI] = 87.7% to 91.2%). A total of 125 patients (10.5%) opted out. There were 10 positive results, four new diagnoses, and six previously known. There were two new HBV and two new HCV diagnoses, a yield of four per 1000 (95% CI = 0.9 to 7.5 cases per 1000). No new HIV cases were diagnosed. Conclusion This study indicates that testing for BBV in patients presenting for routine blood tests in primary care is viable. The yield of HBV and HCV suggests that opt-out testing should be considered in primary care to increase detection rates of BBV.