A method for calculating or estimating or approximating one or more values representing parameters of a patient includes the step of interpolating or extrapolating of at least one later value of a first parameter taking into account at least one earlier value of the first parameter, at least one earlier and at least one later value of a second parameter, and a mathematical relation between the first and the second parameter. An apparatus, a blood treatment device, a digital storage device, a computer program product, and a computer program are also described.