The invention relates to a dialysis machine having the capability of determining a predialytic property in the blood of a dialysis patient which has an extracorporeal blood circuit, a dialyzate circuit, a dialyzer and a processing unit, wherein at least one sensor for determining a property of the dialyzate is arranged in the dialyzate circuit. The processing unit is configured such that temporal evaluation ranges are fixed during an initial phase of the dialysis treatment, in which temporal evaluation ranges all stability criteria from a predefined group are satisfied； and in that only measured values determined by the at least one sensor within these temporal evaluation ranges are used for determining a predialytic property of the patient's blood.
