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Author Interview: Jing Chen  

Division of Nephrology
Huashan Hospital, Fudan University
12 Wulumuqi Road
Shanghai 200040, China


Phosphate removal model: An observational study of low-flux dialyzers in conventional hemodialysis therapy

Wang, M., Li, H., Liao, H., Yu, Y., You, L., Zhu, J., Huang, B., Yuan, L., Hao, C. and Chen, J. (2012), Phosphate removal model: An observational study of low-flux dialyzers in conventional hemodialysis therapy. Hemodialysis International. doi: 10.1111/j.1542-4758.2012.00678.x

What are the main findings of the study?

We found that total amount of phosphate removal by low-flow dialyzer within 4-hour HD was mostly 15-30mmolThe average ratio of phosphate removal was 34.8±4.3%, 25.1±2.0%, 21.0±2.5%, 19.2±3.3% at the first, second, third and fourth treatment hour, respectively.

By backward process, one model (Model 1) predicting total phosphate removal was: Tpo4=79.6
´C45(mmol/L) - 0.023´Age(years) + 0.065´Weight(kg) - 0.12´TCO2(mmol/l) + 0.05´Clearance(ml/min) - 3.44, where C45 was phosphate concentration in spent dialysate measured at the 45 min of HD and Clearance was phosphate clearance of dialyser in vitro conditions offered by manufacturer’s data sheet. Since the parameter TCO2 needed serum sample for measurement, we further derived a noninvasive model (Model 2):Tpo4=80.3´C45 - 0.024´Age + 0.07´Weight + 0.06´Clearance - 8.14. Coefficient of determination (Model 1:0.94  Model 2:0.94), Root Mean Square Error(Model 1:1.82  Model 2:1.84), and residual plots (the differences were within 3mmol in both models) showed the appropriateness of two models.

Model validation further suggested good and similar predictive ability of them.

Were any of the findings unexpected?

The phosphate removal by low-flux dialyzer was much lower than that of high-flux dialyzer in previous studies.

Much to our surprise, we found that phosphate concentration in the dialysate was selected into model while predialysis serum phosphate concentration was not by backward process. It may be because that the phosphate concentration in dialysate taken at 45min into HD may not only reflects the changes of plasma phosphate concentration but also the activation of diffusion from plasma to dialysate.

Besides, the data presented here indicated that the disturbance of acid-base metabolism in HD patients was not very important for phosphate removal, since the fitness and predictive ability of the modified model without serum TCO2 (Model 2) was almost similar to the primary model (Model 1).

What should clinicians and patients take away from this study?

According to the kinetics of phosphate removal during hemodialysis, increasing the time of the dialysis session is still an important way to raise overall phosphate removal. The model also indicates that phosphate clearance of dialyzer, patient’s weight and age can affect phosphate removal. By calculating phosphate removal amount in 4-hour hemodialysis, this model can be used as a tool to prescribe the individualized treatment in MHD patients. For example, if daily phosphate intake is about 750 mg. Given a gastrointestinal absorption rate of 60%, the effective weekly phosphate intake will reach 3150 mg.

If calcium carbonate (oral 2 tablet, tid) is prescribed as phosphate binder to patient, the weekly phosphate excretion is estimated to be 550mg. Therefore, the residual phosphate (at least 2600mg) has to be removed by dialysis to reach the balance.

According to the model, it is easily to calculate phosphate removal by 4-hour low-flux hemodialysis and the HD doses the patient needs.

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