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 measurem ents by light absorption due to haemoglobin, by impedance in the blood line at 5 kHz using Hanai's model of blood conductivity, by conductivity measur ements of blood samples at 1.2 kHz using a conductimeter, by centrifugation of blood samples and by calculations using fluid conservation. The validit y of Hanai's model was verified to be satisfactory by direct blood and plas ma conductivity measurements. In the absence of ionic transfer the impedanc e device underestimated the haematocrit by 5 to 7%. This underestimation re ached 18% in the case of hypertonic dialysate, but this effect can be minim ised if the haematocrit necessary for calibration is measured by centrifuga tion after 15 min of dialysate circulation when ionic balance is achieved. It was found that the optical method monitors haemoglobin concentration rat her than red cell volume changes and is not affected by osmotic red cell sw elling in the case of hypotonic dialysate. It can be concluded that the lig ht absorption technique is both more accurate and more convenient to use th an impedance.