Comparison of optical, electrical, and centrifugation techniques for haematocrit monitoring of dialysed patients

Haematocrits were measured as a function of ultrafiltration in a simulated haemodialysis circuit using bovine blood (plasma conductivity 12 mS cm-1) and hypotonic (8.6 mS cm-1) or hypertonic (16 mS cm-1) dialysates as well as in the absence of dialysate. A comparison was made between measurements by light absorption due to haemoglobin, by impedance in the blood line at 5 kHz using Hanai's model of blood conductivity, by conductivity measurements of blood samples at 1.2 kHz using a conductimeter, by centrifugation of blood samples and by calculations using fluid conservation. The validity of Hanai's model was verified to be satisfactory by direct blood and plasma conductivity measurements. In the absence of ionic transfer the impedance device underestimated the haematocrit by 5 to 7%. This underestimation reached 18% in the case of hypertonic dialysate, but this effect can be minimised if the haematocrit necessary for calibration is measured by centrifugation after 15 min of dialysate circulation when ionic balance is achieved. It was found that the optical method monitors haemoglobin concentration rather than red cell volume changes and is not affected by osmotic red cell swelling in the case of hypotonic dialysate. It can be concluded that the light absorption technique is both more accurate and more convenient to use than impedance.