Measured vs. calculated plasma osmolality in infants and head injured patients in critical care units

Introduction: The body fluid osmolality plays an important role to control the treatment of water imbalance and outcome in intensive care unit (ICU) and neonatal ICU (NICU) patients. There are two options to determine serum or plasma osmolality (Posm); direct measurement and calculation. To calculate Posm, many empirical models are suggested. The objective of this study was to calculate Posm by three most frequently used empirical models and compared with direct measured osmolality in ICU and NICU patients. Material and methods: This research was carried out on fifty head injured patients in ICU and 45 infants in NICU. The patients’ serum sodium (Na), glucose (Gl) and blood urea nitrogen (BUN) concentrations were measured to calculate Posm by three frequently used equations. The Posm also was measured directly by vapor pressure osmometer and compared with calculated Posm. Results: The results indicate that calculated Posm using equation {1}; Posm = 2 Na + Gl/18 + BUN/2.8 in head injured patients, and using equation {2}; Posm = 1.86 Na + Gl/18 + BUN/2.8 in infants have no significant differences from measured Posm. The findings also demonstrated no significant difference in calculated or measured osmolality between two groups of under and above 2.5 kg weight infants. Posm in head injured patients with low Glasgow coma scale (GCS) was greater than patients with GCS>8 (p<0.05), and there also was a correlation between measured /or calculated Posm and GCS (r=0.50, p<0.05). Conclusions: The plasma osmolality is a helpful parameter to control the body fluid in ICU and NICU patients. When equipment for direct measurement is not available, special attention is needed to choose the suitable empirical models for plasma osmolality calculation. (Clin Exp Med Lett 2008; 49(4): 211-214)

Keywords: measured osmolality, calculated osmolality, ICU, NICU