Tsujimoto Y, Tsutsumi Y, Ohnishi T, Kimachi M, Yamamoto Y, Fukuhara S: Low pre-dialysis plasma calculated osmolality is associated with higher all-cause mortality: The Japanese Dialysis Outcomes and Practice Patterns Study (J-DOPPS). Nephron DOI 10.1159/000504194
The calculated plasma osmolality (Posm=2 X plasma [Na+] = serum urea nitrogen/ 2.8 + serum glucose/1.8) is higher in hemodialysis patients (averaging 307±9 mOsm/kg H20) compared to normal subjects (290±4 mOsm/kg H20). However, the predialysis calculated Posm is variable due to changes in the components of the calculation (levels of Na+, urea nitrogen and glucose).
Tsujimoto and colleagues studied the impact of pre-dialysis treatment session values of calculated Posm for all-cause mortality in 1249 patients undergoing maintenance hemodialysis and enrolled in the Japanese Dialysis Outcomes and Practice Patters Study (J-DOPPS). The Posm was stratified into 3 levels: <300, 3000–310, and >310 mOsm/kg H20. Patients in the lowest Posm group were older, had peripheral arterial disease and lower post dialysis body weight. Inter-dialytic weight gain, and pre-dialysis blood pressure were not different in the 3 groups. Importantly measured Posm and osmolal gap (calculated Posm minus measured Posm) were not evaluated in this study and almost 40% of the subjects studied had at least one missing variable.
In a multi-variate analysis each 10mOsm lower pre-dialysis calculated Posm was associated with a higher all-cause mortality (HR=1.48; CI=1.30–1.78). There was no apparent interaction between calculated Posm and the plasma sodium, urea nitrogen and diabetes status. Adjusting for urea nitrogen appearance rate and BMI also showed similar HR results.
These are novel findings which lack a clear explanation. The lack of data on measured Posm is a serious weakness as is the high rate of missing data. The predictive power of calculated Posm needs to be compared to other mortality prediction formulas and the impact of pre-dialysis calculated Posm at the facility level needs to be analyzed. The hypothesis generated by this novel study needs to be pursued further in a prospective manner where the important variables can be more carefully controlled.
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