STUDY OBJECTIVE: Inadvertent arterial placement of a femoral venous catheter may result in serious morbidity, including limb necrosis. The Flush the Line and Ultrasound the Heart (FLUSH) test is visualization of the heart by a subxiphoid ultrasonic view while the central catheter is flushed with agitated saline solution. We wish to determine whether the FLUSH test can verify proper femoral venous line placement. METHODS: We prospectively studied a convenience sample of children undergoing cardiac catheterization, for whom both femoral venous and arterial access were part of their standard care. The cardiologist flushed manually agitated saline solution through each catheter in randomized sequence while the blinded physician sonographer recorded the presence or absence of right atrial opacification. We calculated the sensitivity and specificity of the FLUSH test relative to our reference standard, the cardiologist's fluoroscopic visualization of catheter wire placement. RESULTS: Of the 51 subjects enrolled, the FLUSH test was 100% sensitive (95% confidence interval 95% to 100%) and 90.3% specific (95% confidence interval 81% to 96%) in confirming femoral catheter placement. In no case was an arterial flush misidentified as a femoral flush. The interrater reliability of the test was strong: kappa 0.82 for all images and 0.9 for those of good quality. CONCLUSION: The FLUSH test is simple and reliable, and appears to accurately confirm femoral venous line placement.