Rubidium deficiency in dialysis patients

BACKGROUND: Since dialysis has brought long-term survival to uremic patients, we can now speculate on more subtle problems derived from imbalance or sub-optimal regulation of some elements such as trace metals. We focused on the rubidium (Rb) status in dialysis patients (HD), as concerns about its possible deficiency have been raised. METHODS: Rb in uremic patients was evaluated by: A) serum concentration (graphite furnace atomic absorption spectroscopy) from blood samples of 70 patients on chronic hemodialysis (HD) in comparison with 75 controls; B) tissue concentration (neutron activation analysis) from autopsy or biopsy samples (20) of HD patients in comparison with 21 controls; C) in vivo intradialytic mass balance during standard bicarbonate dialysis in 8 HD patients. RESULTS: A) Serum Rb concentrations in HD patients significantly were lower than in normal controls (304 +/- 81 micrograms/L versus 350 +/- 74 micrograms/L p < 0.001, log-transformed 5.68 +/- 0.28 versus 5.84 +/- 0.20, p < 0.001). Univariate logistic regression analysis found a significantly higher risk of serum Rb < 250-300 and 350 micrograms/L in uremic patients than in controls (Odd ratios or 12.6, 95% CI 2.77-57.04; 4.0, 95% CI 1.92-8.4; 2.08, 95% CI 1.02-4.25, respectively). B) Rb was significantly lower in tissues of HD patients, including brain (2250 +/- 1520 ng/g versus 5490 +/- 1250 ng/g, p = 0.0002) than normal controls. C) Rb was transferred from the patients' blood to the dialysis bath during a standard bicarbonate dialysis session, giving mean intradialytic Rb removal of 4.0 +/- 1.1 mg/session. CONCLUSIONS: These results confirm that Rb deficiency may arise in uremic patients, and indicate that diffusive dialysis treatments allow Rb removal which, however, with a standard bicarbonate schedule does not seem to be any greater than that expected with normal urine output (20 mg/week). Further studies are needed to clarify the roles of many factors in this Rb deficiency, including the effects of uremia by itself, pre-dialysis factors (diet, impaired renal function and drugs), dialysis procedures (frequency, hours, diffusive/convective components) or other biochemical/clinical parameters (hemoglobin, body mass index, age). The finding of a Rb deficiency in uremia is important as it has a role in neurobehavioural functions, mainly as an antidepressant. As Rb deficiency may be implicated in central nervous system alterations which strongly influence the quality of life, we believe that monitoring serum Rb in uremic patients and clarifying the causal mechanisms of deficiency will facilitate future therapeutic approaches.