Potassium-based dilutional method to measure hemodialysis access recirculation

Assessment of access recirculation (AR) is crucial to dialysis efficiency and there is thus a need for a method yielding a highly accurate, fast, easy and economical measurement that can be applied in any busy dialysis clinic. Non-urea based dilutional methods are more accurate than urea based methods and avoid problems with cardiopulmonary recirculation, but they require expensive specialized devices, which limit their applicability.

We developed a simple dilutional method of AR which does not require any specific device, based on the determination of serum potassium [K+] in two samples. Briefly, a basal sample is drawn at the time of needle insertion (basal [K+]); needles are connected to blood lines and blood flow rate is quickly increased to 300 ml/mm; a second sample (arterial [K+]) is drawn from the arterial line port within 5 to 10 seconds, to avoid errors due to cardiopulmonary recirculation of the normal saline entering the blood stream. At this time, if recirculation is present, part of the normal saline will enter the arterial line and dilute the serum [K+]. The AR formula is: AR (%) = 100 x [1 - arterial K+/basal K+]. We compared our method with the two-needle urea and ultrasound velocity dilution methods.

AR values by the ultrasound method > 10% were hypothesized as gold standard for AR, against which values obtained with the potassium method were compared. The potassium based method showed: sensitivity (100%,); specificity (95%); predictive value, positive (91%); predictive value, negative (100%). In addition, the potassium based method appears to be more reliable than the two-needle urea based method.

Our method, similar to other dilutional methods, is not influenced by cardiopulmonary recirculation or veno-venous disequilibrium and is fast and accurate. Moreover it is very simple, economical, and can easily be performed in any dialysis unit.