Apparatus, particularly with a memory and a digital signal processor, comprising: a first determination unit configured to determine a normalized change of a blood parameter (ΔRBV) of the individual during a treatment session; a second determination unit to determine a fluid status of the individual, a calibration unit to calibrate the determined normalized change of the blood parameter (ΔRBV) based on the fluid status, the apparatus being configured to perform a method for quantitative determination of the fluid status or a change in the fluid status comprising the following steps: determining a first change of the blood parameter (ΔRBV) of the individual during a first treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the first treatment session; determining a first fluid status of the individual; and calibrating the determined first normalized change of the blood parameter (ΔRBV) based on the first fluid status determining a further change of the blood parameter during at least one further treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the at least one further treatment session; and deriving the fluid status or a change in the fluid status from the further normalized change of the blood parameter. a first determination unit configured to determine a normalized change of a blood parameter (ΔRBV) of the individual during a treatment session; a second determination unit to determine a fluid status of the individual, a calibration unit to calibrate the determined normalized change of the blood parameter (ΔRBV) based on the fluid status, the apparatus being configured to perform a method for quantitative determination of the fluid status or a change in the fluid status comprising the following steps: determining a first change of the blood parameter (ΔRBV) of the individual during a first treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the first treatment session; determining a first fluid status of the individual; and calibrating the determined first normalized change of the blood parameter (ΔRBV) based on the first fluid status determining a further change of the blood parameter during at least one further treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the at least one further treatment session; and deriving the fluid status or a change in the fluid status from the further normalized change of the blood parameter. determining a first change of the blood parameter (ΔRBV) of the individual during a first treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the first treatment session; determining a first fluid status of the individual; and calibrating the determined first normalized change of the blood parameter (ΔRBV) based on the first fluid status determining a further change of the blood parameter during at least one further treatment session normalized by ultrafiltration volume and/or ultrafiltration rate of the at least one further treatment session; and deriving the fluid status or a change in the fluid status from the further normalized change of the blood parameter.