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Background and Aims It is well known that haemoglobin significantly increases after haemodialysis and it is associated with the rate of weight loss during dialysis. However, it has been shown that the increase in haemoglobin is not always proportional to the rate of body weight loss during haemodialysis (ultrafiltration). The purpose of the study was to investigate the correlation between haemoglobin changes, body weight loss and plasma volume changes during haemodialysis Method A prospective study was performed on 92 patients in mid-week treatments. Pre-dialysis and post-dialysis haemoglobin and weight were measured. 27 patients were excluded because clinical instability, no change or a decrease in Hb and low pre-dialysis haemoglobin (< 9 g/dL). The correlation was statistically significant between %ΔPV and ΔHb in both groups (R2=0.59), whereas the correlation between %ΔBW and ΔHb was lower (R2=0.19). So, analysis show that only 19% of the variability in haemoglobin is explained by %ΔBW, and that 59% of the variability in haemoglobin is explained by %ΔPV. Results The mean age was 61.16 ± 13.11 year. The men were 52 (56%). The mean pre-dialysis Hb was 11.31 ± 1.16 g/dL, the mean post-dialysis Hb level was 12.53 ± 1.47 g/dL. The mean absolute change of haemoglobin (ΔHb g/dL) and percent of change of haemoglobin (%ΔHb) were 1.22 ± 0.76 and 9.44 ± 5.31, respectively. Average percent of weight change (%ΔBW) was - 2.44 ± 1.01. Percent of plasma volume change was - 9.22 ± 5.46. The patients were divided in two groups according post-dialysis haemoglobin level: Group A with haemoglobin Hb < 13 g/dL (64 patients) and group B with Hb ≥13 g/dL (28 patients). The mean time average haemoglobin concentration (TAC Hb) in all patients was 11.68 ± 1.11 g/dL.(Predicted Hb TAC was calculated according Krisper′s formula) In both groups there was an increase in %ΔHb, but in the group with post dialysis Hb ≥ 13 g/dL, %ΔHb was greater than in group B with post dialysis Hb < 13 g/dL (13.08 ± 5.11 versus 7.87± 4.61, P = 0.000) despite the relatively small difference of %ΔBW between the two groups (- 2.85 ± 0.95 versus - 2.23 ± 1.02; P = 0.010). However, the difference in %ΔPV between the two groups was significant (- 12.90 ± 5.63 versus - 7.61 ± 4.57; P < 0.000). Conclusion The intradialytic changes in haemoglobin levels are predominantly determined by changes in plasma volume. Changes in body weight are of little predictive value in evaluation of variation of haemoglobin levels. TAC Hb determination should be performed in patients with large variations in plasma volume, because the plasma volume has very little effect on TAC Hb.

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