Difference between true functional haemoglobin and pre-dialysis haemoglobin is associated with plasma volume variation: a multicentre study

Due to the well-known intradialytic haemoglobin (Hb) change, it has been suggested that true functional Hb differs from pre-dialysis Hb. However, mechanisms of this change are not understood properly. We aimed at studying the role of plasma volume variation. This is a multicentre study with two consecutive phases. First, true functional Hb was predicted by Krisper's formula that estimates time-averaged haemoglobin concentration (TAC-Hb) and by the average (Avg-Hb) of Pre-Hb and Post-Hb (n = 346). Erythropoiesis-stimulating agent (ESA) doses were estimated according to the FDA instructions. Second, we investigated the correlations between Hb changes (Delta Hb), body weight loss (Delta BW) and relative plasma volume changes (%Delta PV) in subjects with a weight gain higher than 1 kg (n = 208). First: Avg-Hb and TAC-Hb were similar to each other and higher than midweek Pre-Hb (p < 0. 002). Odds of having an Hb change > 1 g/dL were 0.415. Odds were similar in both women and men [odds ratio (OR) 0.923; 95 % confidence interval (CI) 0.591-1.441]. If ESA doses were estimated by TAC-Hb or Avg-Hb instead of Pre-Hb, they should be lower in 40 % of the 346 patients. Second: Mean %Delta PV was -12.1 % (95 % CI -10.72/-13.48), and mean Delta Hb was 1.1 g/dL (95 % CI 0.97/1.25). Correlation between Delta Hb and %Delta PV was -0.92 (r (2) 0.84) whereas that of Delta Hb/Delta BW was just -0.45 (r (2) 0.21; p < 0.000). Prevalence of Delta Hb > 1 g/dL was independent of Delta BW from 1.4 to 6.3 kg. By contrast, prevalence of Delta Hb > 1 g/dL in %Delta PV quartiles was clearly linear: 0, 52, 90 and 100 %, respectively. Plasma volume variations play a major role in intradialytic Hb change. True functional Hb should be estimated in all patients with a weight gain higher than 1 kg.