One of the most significant safety concerns in the automation of extracorporeal blood treatments such as dialysis is the risk of blood leakage. Extracorporeal blood treatment systems draw blood at such a high rate that a loss of integrity in the blood circuit can be catastrophic. There are a number of mechanisms for detecting and preventing leaks and/or air infiltration, but none is perfect. According to the present invention, multiple inputs are combined to provide greater security against leakage and/or air infiltration by providing sensors for external presence of liquid (plasma, replacement fluid, blood, etc.) outside a fluid circuit and infiltration of air or bubbles into the fluid circuit.