6.4. Appendix D Adding phosphate to dialysate

Cleen Ready To Use enema comes in a 133ml pack costing 68p (March 2017). The pack is designed to deliver an 118ml dose containing 21.4g of sodium acid phosphate (sodium dihydrogen phosphate dihydrate NaH₂PO₄.2H₂O) and 9.4g sodium phosphate (disodium phosphate dodecahydrate Na₂HPO₄.12H₂O). This gives a concentration of 1.38mmol/ml phosphate.

The acid concentrate, together with the added enema, will be diluted according to the proportioning ratio of the dialysis machine. The volume of enema (V_E) that needs to be added to the canister is given by:

V_E (ml) = C_PO4 * V_AC * DF / 1.38

where C_PO4 = target dialysate phosphate (0.4mmol/l is commonly used)
V_AC = volume of the acid concentrate (e.g. 6L)
DF = dilution factor for the dialysate (e.g. 35 for machine proportioning at 1:34)

Using these numbers, for example, gives an enema volume of 61ml to be added to the concentrate. Note that this formula depends on the formulation of the enema and is specific to Cleen.

This simple formula ignores the small change in concentrate volume, as the effect is below the 5% tolerance allowed for electrolyte concentrations for haemodialysis and related therapies (ISO 23500 Part 4). Albalate used the above formula to prepare dialysates with C_PO4 between 0.48 to 1.12mmol/l, reporting good agreement between measured and target C_PO4 using Gambro and Fresenius machines and a range of acid concentrates [1].

Sodium content in the phosphate enriched acid concentrate will be reduced (since the enema sodium is 37g/l, compared to around 83g/l for an acid concentrate that proportions at 1:34, and 107g/l for one that proportions at 1:44). This will lead to a lower dialysate sodium (by 0.7 to 0.9mmol/l) with volumetric proportioning, and a higher concentrate pump rate with conductivity feedback. It is possible that this effect could trigger machine alarms, requiring machine re programming.

Reference

Albalate M, Ruiz Alvarez MJ, de Sequera P, Perez Garcia R, Arribas P, Corchete E, Ruiz Caro C, Talaván Zanón T, Alcazar R, Ortega M, Puerta M. Follow a recipe to prescribe phosphate during haemodialysis. Nefrologia. 2017 Jan Feb;37(1):34 38.