Changes in hydration following haemodialysis estimated with bioimpedance spectroscopy.

INTRODUCTION

Bioelectrical impedance devices are being more frequently used as an aid to clinical assessment in helping to determine normo-hydrated weight, or target weight for haemodialysis patients. Pre-dialysis measurements are more convenient in clinical practice for patients and staff. We wished to determine the value of pre-dialysis measurements of over-hydration by comparing post-dialysis values with actual weight loss.

METHODS

We measured bioimpedance using bioimpedance spectroscopy pre-dialysis and then post-dialysis.

RESULTS

Bioimpedance spectroscopy was measured pre-dialysis and post-dialysis in 49 haemodialysis patients, 70% male; mean age 63.9 ± 17.9 years. Weight fell from 69.5 ± 17.3 to 67.7 ± 16.9 kg, P < 0.001, with a fall in mean over-hydration (OH) from 2.1 ± 3.4 to 1.3 ± 2.8 L, P < 0.017, and extracellular water (ECW) from 17.0 ± 5.2 to 16.5 ± 5.0 L, P = 0.03. The change in OH underestimated weight change (mean bias -1. 1). Change in OH did not correlate with measured weight loss on univariate analysis (r = 0.26, P = 0.07) but positively correlated with the change in ECW (r = 0.29, P = 0.046) and negatively with intracellular water (r = -0.58, P < 0.001).

SUMMARY

Although more convenient to make bioimpedance measurements of hydration status pre-dialysis, we did not find that changes in OH post-dialysis mirrored the changes in measured weight. As such, single pre-dialysis bioimpedance measurements of hydration status may not accurately predict post-dialysis hydration status.