Method, apparatus, and device for calculating or approximating one or more values representing parameters of a patient

A method of operating a blood treatment device for performing a blood treatment on a patient's blood, the method comprising:

• interpolating or extrapolating, by an interpolation or extrapolation device, at least one subsequent value of a first parameter indicative of an overhydration, a relative overhydration, or a normohydrated weight of the patient, taking into account:
• at least one earlier value of the first parameter obtained from a bioimpedance measurement device or from an external database during or before an earlier treatment session,
• at least one earlier value of a second parameter indicative of a preweight, a haemoglobin concentration in blood, or an extracellular water content and obtained from a bioimpedance measurement device or from an external database during or before the earlier treatment session,
• at least one subsequent value of the second parameter obtained after the at least one earlier value of the second parameter is obtained and before the treatment is performed, and
• a mathematical relation between the at least one earlier value of the first parameter, the at least one earlier value of the second parameter, and the at least one subsequent value of the second parameter;
• and
• performing, by the blood treatment device, the treatment in accordance to or based on the at least one interpolated or extrapolated subsequent value of the first parameter, the treatment comprising at least one of a dialysis treatment, a hemofiltration treatment, an ultrafiltration treatment, or a hemodialysis treatment.