A blood detection system is used to detect blood in a patient undergoing an extracorporeal blood therapy, such as hemodialysis or apheresis. The blood detection system uses a digital video device, which may be a camera, such as a standard digital video camera, a low light camera, an infrared camera, or a night vision camera. The digital image is sent to a computer with image processing and pattern recognition software. The software is used to detect whether a pool of blood is forming at the access site. The system is able to detect a pool of blood atop the access site, such as when a leak or a dislodgement of a needle occurs. The system is also able to detect pooling of blood under the skin, as a result, for instance, infiltration of an arteriovenous fistula.