Comparison of measuring serum osmolality and equations estimating osmolality in peritoneal dialysis patients

Abstract

The osmolar gap increases with kidney failure. A number of equations have been proposed to calculate serum osmolality, allowing determination of the osmolar gap by comparison with measured osmolality. As glucose and icodextrin absorption can potentially interfere with the laboratory measurement of serum sodium, a key component in equations calculating osmolality, we reviewed the performance of 14 equations used to calculate serum osmolality compared to the measurement of serum osmolality in 144 patients with peritoneal dialysis (PD); 81 (56.3%) males, 76 (52.5%) diabetics, mean age of 64.4 ± 16.3 years, 115 (79.9%) prescribed icodextrin and 38 (26.4%) 22.7 g/L glucose dialysates. Measured serum osmolality was 311 (304–320) mosmo/kg (mmol/kg), whereas calculated osmolality for the 14 equations ranged from a median of 274 (269–284) mosmo/kg to 307 (300–316) mosmo/kg. Bland–Altman mean bias showed that measured serum osmolality was greater than the calculated osmolality ranging from 4.0 mosmo/kg to 36.2 mosmo/kg between the 14 equations, with wide 95% limits of agreement (LoA) ranging from −27.1 mosmo/kg to 19.4 mosmo/kg and from −58.5 mosmo/kg to −13.8 mosmo/kg. Only 2 of the 14 equations gave a mean osmolar gap of <10 mosmo/kg and showed no systematic bias, median serum osmolality of 307 (300–316) and 303 (298–312) mosmo/kg, Spearman ρ of 0.57, 0.62, both p < 0.001, respectively. Our study would suggest that only 2 of the 14 equations we compared with measured serum osmolality showed no systematic bias, but still had too great a bias to be useful in clinical practice. As such we propose a new equation to calculate serum osmolality in patients with PD.

Mind the gap: Measured and Calculated Osmolarity are Not Interchangeable in Diabetic Hyperglycemic Emergencies

“…While we acknowledge that our findings should be further confirmed in a prospective study, multiple previous authors have cautioned that formulas for the calculation of osmolarity may show substantial discrepancies with measured osmolality due to a variety of measurement issues 19 24 25 26 27 28 or when applied to patient groups outside the population in which they were originally validated 4 6 29 30 31 . We suggest that clarification may be needed in clinical guidelines for the management of DKA and HHS regarding the use of measured and calculated osmolarity.…”

“…While we acknowledge that our findings should be further confirmed in a prospective study, multiple previous authors have cautioned that formulas for calculation of osmolarity may show substantial discrepancies with measured osmolality due to a variety of measurement issues [19,[24][25][26][27][28] or when applied to patient groups outside the population in which they were originally validated [4,6,[29][30][31].…”

The accuracy of serum osmolarity calculation in small children

“…2; 1.89; 1.86, are used in different formulas to compensate the fact, that sodium solution is not completely dissociated in aqueous medium, moreover minor particles also contribute to osmolality and plasma contains only 93% water. Most formulas for osmolarity calculation were developed and validated for the adult population [22] [23] [24] . It is still too few proofs that in newborns and small children measured osmolality may be replaced by calculated value and it is still unknown which formula might be used.…”

“…The results of the published studies also do not clearly indicate which formula gives the most accurate results. Usually, the conclusions are strongly dependent on the statistical methods using in the study and their interpretation, as well as on both age and health status of the examined population [24] [26] [28] . Additionally, sodium shows dynamics changes during all periods of childhood [32] .…”

High Osmol Gap Hyponatremia Caused by Icodextrin: A Case Series Report

“…Hyperosmolality might be a greater threat since dialysis patients have increased concentrations of small organic osmolytes which increase the osmolar gap [16]. In PD patients on icodextrin, a systemic bias was shown in most equations used to calculate serum osmolality due to interference of glucose and icodextrin absorption with routine laboratory measurement of serum sodium [17]. We used the direct ion selective method to measure serum sodium and the most frequently used equation to calculate serum osmolality; a systematic bias however would have influenced absolute values rather than changes.…”