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Evaluation of 36 formulas for calculating plasma osmolality
Andreas S Fazekas1, Georg-Christian Funk, Daniela S Klobassa
1Department of Respiratory and Critical Care Medicine, Otto Wagner Hospital, Sanatoriumstrasse 2, 1145 Vienna, Austria. andreas.fazekas@wienkav.at
Intensive Care Medicine
|October 20, 2012
Summary
A new formula for calculating plasma osmolality, incorporating electrolytes and metabolites, accurately predicts measured osmolality. This advancement in critical care medicine offers a more precise diagnostic tool, potentially replacing separate osmolality measurements.
Area of Science:
- Critical care medicine
- Clinical chemistry
- Biomedical engineering
Background:
- Plasma osmolality is crucial in critical care for diagnosing metabolic acidosis.
- Existing formulas for osmolality calculation often rely on limited parameters like sodium, urea, and glucose.
- The osmolal gap, derived from measured and calculated osmolality, aids in differential diagnosis.
Purpose of the Study:
- To evaluate the accuracy of 36 published formulas for calculating plasma osmolality.
- To compare these formulas against measured osmolality in a diverse patient cohort.
- To assess a novel formula by Zander that includes potassium, chloride, lactate, and bicarbonate.
Main Methods:
- Blood samples from 236 subjects (outpatients and inpatients) were analyzed.
- Measured osmolality was compared with calculated osmolality from 36 distinct formulas.
- Statistical performance evaluation was conducted using the Bland and Altman method.
Main Results:
- Traditional formulas exhibited significant mean differences (up to 35 mosmol/kg H2O) compared to measured osmolality.
- Zander's novel formula demonstrated a negligible mean difference (0.5 mosmol/kg H2O).
- The novel formula achieved the narrowest 95% limits of agreement (-6.5 to 7.5 mosmol/kg H2O).
Conclusions:
- Only a minority of existing formulas (4 out of 36) provided acceptable accuracy (<1 mosmol/kg H2O mean difference).
- Zander's formula shows excellent concordance with measured osmolality, enhancing diagnostic precision.
- This new equation may obviate the need for separate osmolality measurements in many clinical scenarios.
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