BackgroundTransradial access (TRA) has been associated with reduced access site related bleeding complications and mortality after percutaneous coronary intervention (PCI), although it is unclear whether these observed benefits are affected by baseline bleeding risk. We have investigated this association by quantifying baseline bleeding risk, TRA use, and procedure-related outcomes. MethodsWe studied patients undergoing PCI who were enrolled in the British Cardiovascular Intervention Society (BCIS) database of PCI procedures done in the UK. We calculated baseline bleeding risk using a modification of the Mehran bleeding risk scores in 348 689 PCI procedures, undertaken in patients between 2006 and 2011. Four categories for bleeding risk were defined for the modified Mehran risk score (MMRS): low (<10), moderate (10–14), high (15–19), and very high (≥20). We assessed the effect of baseline bleeding risk on 30-day mortality and its association with access site. FindingsTRA was independently associated with a 35% reduction in 30-day mortality (odds ratio [OR] 0·65, 95% CI 0·59–0·72; p<0·0001), with the magnitude of mortality reduction related to baseline bleeding risk (OR for MMRS<10 = 0·73, 95% CI 0·62–0·86; OR for MMRS≥20=0·53, 0·47–0·61). In patients with a MMRS of less than 10, TRA was used in 71 771 (43·2%) of 166 083 PCI procedures compared with 8655 (40·1%) of 21 559 patients with MMRS of 20 or greater, showing that TRA was used less in those at highest risk from bleeding complications (p<0·0001). InterpretationTRA is independently associated with a reduction in 30-day mortality and the magnitude of this effect is related to baseline bleeding risk, with individuals at highest risk of bleeding complications gaining the greatest benefit from TRA during PCI. Paradoxically, use of TRA was lower in patients most at risk of bleeding complications than in patients with least risk. Our data suggest that optimum access site practice guided by simple assessment of baseline bleeding risk has the potential to substantially improve PCI related patient outcomes. FundingSA is funded by a National Institute for Health Research academic clinical lectureship in cardiology.