Absolute blood volume in dialysis patients is a useful patient attribute to know for dialysis treatment, diagnosis, adjustments, etc. In some cases, it is difficult or impossible to directly determine absolute blood volume. Estimating absolute blood volume may be used to overcome the inability to directly determine the absolute blood volume. Estimating absolute blood volume may include obtaining a series of measurements of hemoconcentration of a patient over a time period, and estimating parameters for a physiological model based on the series of measurements. The absolute blood volume of the patient may be determined using the physiological model.