A fractional dialysate collection method to estimate solute removal in continuous venovenous hemodialysis

Abstract

Background. Fractional direct dialysis quantification (fDDQ), whereby a known proportion of dialysate effluent is sampled, can reliably estimate total solute removal in intermittent hemodialysis (IHD). Our study aimed to develop and test the technique in continuous venovenous hemodialysis (CVVHD). Methods. Twenty dialysate collections (mean duration 23.5 hours, range 17.25 to 26.6) were performed in 12 patients on CVVHD. An infusion pump diverted 10% of the total effluent volume to the fractional collection (fc), the remainder being channeled into the bulk collection (bc). Both fc and bc were collected on ice and assayed for urea nitrogen (UN) and creatinine (Cr). Actual solute removal (ASR) was calculated from the measured effluent volume and solute concentrations of the fc and bc. Estimated solute removal (ESR) was calculated from the product of the fc solute concentration and effluent volume. All fc/bc samples in 15 out of 20 collections underwent gram stain and aerobic/anaerobic culture. Results. Bland-Altman analyses suggested good agreement between ASR and ESR [absolute values of percentage differences: 95% CI = 1.73, 5.17% (UN); 1.88, 4.31% (Cr)]. Favorable concordance correlation coefficients confirmed this [r(c) = 0.995 (UN), 0.997 (Cr)] and were apparently unaffected by heavy pseudomonal growths in 4 out of 7 culture positive collections [r(c) = 0.997 (UN), 0.997 (Cr); culture negative (N = 8), r(c) = 0.996 (UN), 0.997 Cr)]. Conclusion. fDDQ, using 24-hour, pump-assisted, cooled fractional dialysate sampling reliably estimates total solute removal and provides a practical alternative to total dialysate collection in assessing delivered dialysis dose.