An extracorporeal therapy machine removes blood from a patient in order to treat the blood and then return the blood to the patient. If the removal needle or the return needle is dislodged and blood is not returned, the patient may suffer a great blood loss. A new technique for detecting dislodgement uses the normal, initial values of a plurality of parameters of the patient and the therapy machine to set criteria or limits for an alarm, and then adjusts those criteria based on the later values of those parameters as they change during one therapy or over several therapies experienced by the patient.