Abstract
Background: There are several methods for calculating serum osmolality, and their accordance with measured osmolality is the subject of controversy.
Methods: The concentrations of sodium, potassium, glucose, blood urea nitrogen (BUN) and osmolalities of 210 serum samples were measured. Two empirical equations were deduced for the calculation of serum osmolality by regression analysis of the data. To choose the best equation, chemical concentrations were also used to calculate osmolalities according to our formulas and 16 different equations were taken from the literature and compared with the measured osmolalities. Correlation and linear regression analyses were performed using Excel and SPSS software.
Results: Multiple linear regression analysis showed that serum concentrations of sodium (β=0.778, p≤0.000), BUN (β=0.315, p≤0.000), glucose (β=0.0.089, p≤0.007) and potassium (β=0.109, p≤0.008) are strong predictors of serum osmolality. The data were also analyzed by manual linear regression to yield the equations: osmolality=1.897[Na +]+glucose+BUN+13.5, and osmolality=1.90[Na ++K +]+glucose+BUN+5.0. The osmotic coefficient for sodium and potassium solutes was deduced to be 0.949 from the slope of the curves of measured osmolality vs. [Na +] and [Na ++K +], respectively. The inclusion of a BUN value in the equation for osmolality increased the correlation coefficient by approximately 450% and decreased the SD of difference by approximately 35% (p≤0.002). Inclusion of the osmotic coefficient for sodium solutes caused an underestimation of measured osmolality and positive osmolal gap unless an appropriate coefficient, constant value and/or the potassium value were included in the equation. The agreement was not improved when molal chemical concentrations were used instead of molar values. The formula presented by Dorwart and Chalmers gave inferior results to those obtained with our formulas.
Conclusions: Our data suggest use of the Worthley et al. formula Osm=2[Na +]+glucose+BUN for rapid mental calculation and the formulas of Bhagat et al. or ours for calculation of serum osmolality by equipment linked to a computer.
References
1 Kill F. Molecular mechanisms of osmosis. Am J Physiol 1989; 25: R801–8. 10.1152/ajpregu.1989.256.4.R801Search in Google Scholar PubMed
2 Wrenn KD. Osmolality. Ann Intern Med 1991; 114: 337–8. 10.7326/0003-4819-114-4-337Search in Google Scholar PubMed
3 Kaplan LA. Osmolality. In: Pesce AJ, Kaplan LA, editors. Methods in clinical chemistry. St. Louis, MO: CV Mosby, 1987:18–22. Search in Google Scholar
4 Gennari FJ. Serum osmolality, uses and limitations. N Engl J Med 1984; 310: 102–5. 10.1056/NEJM198401123100207Search in Google Scholar PubMed
5 Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations, and prognostic value of such calculation. Clin Chem 1975; 21: 190–4. 10.1093/clinchem/21.2.190Search in Google Scholar
6 Coakley JC, Tobgui S, Dennis PM. Screening for alcohol intoxication by the osmolar gap. Pathology 1983; 15: 321–3. 10.3109/00313028309083513Search in Google Scholar PubMed
7 Sklar AH, Linas SL. The osmolal gap in renal failure. Ann Intern Med 1983; 98: 481–2. 10.7326/0003-4819-98-4-481Search in Google Scholar PubMed
8 Bhagat CI, Garcia-Webb P, Fletcher E, Beilby JP. Calculated vs. measured plasma osmolality revisited. Clin Chem 1984; 30: 1703–5. 10.1093/clinchem/30.10.1703Search in Google Scholar
9 Scott MG, Heusel JW, LeGrys VA, Siggaard-Andersen O. Electrolytes and blood gases. In: Burtis CA, Ashwood ER, editors. Tietz textbook of clinical biochemistry. Philadelphia, PA: WB Saunders, 1986:1172–253. Search in Google Scholar
10 Bohnen N, Terwel D, Markernik M, Ten Haaf JA, Jolles J. Pitfalls in the measurement of plasma osmolality pertinent to research in vasopressin and water metabolism. Clin Chem 1992; 38: 2278–80. 10.1093/clinchem/38.11.2278Search in Google Scholar
11 Seifarth CC, Miertschischk J, Hahn EG, Hensen J. Measurement of serum and plasma osmolality in healthy young humans: influence of time and storage conditions. Clin Chem Lab Med 2004; 42: 927–32. 10.1515/CCLM.2004.150Search in Google Scholar PubMed
12 Winters RW. Disorders of electrolyte and acid-base metabolism. In: Barnett L, editor. Pediatrics. New York: Appleton Century Crofts, 1968:336–8. Search in Google Scholar
13 Hoffman WS, editor. The biochemistry of clinical medicine. Chicago, IL: Yearbook Publishers, 1970:228. Search in Google Scholar
14 Jetter WW. Clinical osmometry. Pa Med 1969; 75: 72–9. Search in Google Scholar
15 Edelman IS, Leibman J, O'Meara MP, Birkenfeld LW. Interrelations between sodium concentration, serum osmolarity and total exchangeable potassium and total body water. J Clin Invest 1958; 37: 1236–56. 10.1172/JCI103712Search in Google Scholar PubMed PubMed Central
16 Stevenson RE, Bowyer FP. Hyperglycemia with hyperosmolal dehydration in nondiabetic infants. J Pediatr 1970; 77: 818–23. 10.1016/S0022-3476(70)80241-4Search in Google Scholar
17 Weisberg HF. Osmolality. American Society of Clinical Pathologists, Commission on Continuing Education, vol. 71. Chicago, IL: ASCP Press, 1971:1–49. Search in Google Scholar
18 Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001; 38: 653–9. 10.1067/mem.2001.119455Search in Google Scholar
19 Worthley LI, Guerin M, Pain RW. For calculating osmolality, the simplest formula is the best. Anesth Intensive Care 1987; 15: 199–202. 10.1177/0310057X8701500214Search in Google Scholar
20 Gerich JE, Martin MM, Recant L. Clinical and metabolic characteristics of hyperosmolar non-ketotic coma. Diabetes 1971; 20: 228–35. 10.2337/diab.20.4.228Search in Google Scholar
21 Tormey WP. Are the increasing clinical demands for osmolality measurements and their associated electrolytes appropriate? Ir J Med Sci 1997; 166: 75–9. 10.1007/BF02944191Search in Google Scholar
22 Glasser L, Sternglanz PD, Comble J, Robinson A. Serum osmolality and its applicability to drug overdose. Am J Clin Pathol 1973; 60: 695–9. 10.1093/ajcp/60.5.695Search in Google Scholar
23 Boyd DR, Baker RJ. Osmometry: a new beside laboratory aid for the management of surgical patients. Surg Clin North Am 1971; 51: 241–50. 10.1016/S0039-6109(16)39345-8Search in Google Scholar
24 Weisberg HF. Osmolality: calculated, delta and more formula. Clin Chem 1975; 21: 1182–5. 10.1093/clinchem/21.8.1182Search in Google Scholar
25 Garcia-Morales EJ, Cariappa R, Parvin CA, Scott M, Diringer MN. Osmole gap in neurologic-neurosurgical intensive care unit. Its normal value, calculation and relationship with mannitol serum concentrations. Crit Care Med 2004; 32: 986–91. 10.1097/01.CCM.0000120057.04528.60Search in Google Scholar PubMed
26 Partanen JI, Minkkinen PO. Thermodynamic activity quantities in aqueous sodium and potassium chloride solutions at 298.15 K up to a molality of 2.0mol/kg −1. Acta Chem Scand 1993; 47: 768–76. Search in Google Scholar
27 Noggle JH, editor. Physical chemistry. Boston, MA: Little Brow, 1985:363–417. Search in Google Scholar
28 Langhoff E, Ladefoged J. Sodium activity, sodium concentration and osmolality in plasma in acute and chronic renal failure. Clin Chem 1985; 31: 1811–4. 10.1093/clinchem/31.11.1811Search in Google Scholar
© Walter de Gruyter Berlin New York
