Changes in plasma osmolality and anion gap: potential predictors of impending mortality during maintenance dialysis

  • I.O. Oluwatowoju
  • R.W. Braimoh
  • O.K. Ale
  • J.O. Awobusuyi
  • A.F.B. Mabadeje


Background: Chronic kidney disease (CKD) is the 4 commonest cause of death in Nigeria. It affects the most economically productive age group. Many patients in stage 5 CKD are treated with haemodialysis which involves the movement of electrolytes and osmotic molecules from one body compartment to another, resulting in osmotic and anion-gap changes. These changes may be related to mortality in patients on haemodialysis.

Objective: To determine the relationship of mortality to plasma osmolality and anion gap inpatients on haemodialysis.

Methods: Fifty consecutive dialysis naive stable stage 5 chronic kidney disease subjects aged between 18 and 70 years, attending a Lagos Teaching Hospital were recruited. They were followed up from the salvage phase of 5 sessions carried out on alternate days through to the maintenance sessions of haemodialysis at a frequency of 2-3 times per week for six (6) months. Pre-and post-dialysis serum osmolality and anion gap were obtained for salvage and maintenance phases of haemodialysis.

Results: Out of the 50 patients that started the study, 11 (22%) patients (9 males and 2 females) died while10 (20%) subjects (9 males and 1 female) were lost to follow-up before the commencement of the maintenance haemodialysis. Six more subjects (3 males and 3 females) withdrew from the study while 23(46%) entered the maintenance phase of the study. During the maintenance phase, 8(16%) (5 male and 3 female) subjects died while 15 subjects (30%) (9 males and 6 females) completed the study. The mean age of the subjects who completed the study was 46.2± 10.8years (46.9±9.4 years for the males and 45.4±12.1 years for the females).The mean age of the patients that died during maintenance haemodialysis was 46.6±10.9 years(46.1±12.0 years and 45.0±9.8 years for male and female respectively).The mean pre dialysis osmolality and anion gap during salvage haemodialysis were higher than the post dialysis values (298.15 ± 15 mOsm/kg vs. 276.0 ± 13.5 mOsm/kg, p=0.04 and 15.1 ± 3.3 vs. 13.6 ± 3.1, p=0.049).However, the mean pre and post dialysis osmolality and anion gap during maintenance haemodialysis sessions were similar [273.35±14.6
mOsm/kg vs. 270.60±13.0 mOsm/kg (p=0.065)and 12.7±2.9 vs. 12.2±3.2(p=0.059)].There was no significant difference in the pre- and post- dialysis osmolality and  anion gap of those that died and those that survived to the end of the study (p>0.10).

Conclusion: Changes in plasma osmolality and anion gapis not associated with increased mortality and hence may not be useful in predicting mortality during maintenance dialysis.


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