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Background: Older people are more susceptible to dehydration owing to their physiological changes and down-regulated thirst response. In the literature, serum osmolarity remains the gold standard for diagnosing water-loss dehydration. This study aimed to develop an equation specifically for calculating osmolarity in older Thai adults. Furthermore, our study compared the accuracy of the proposed equation with that of previously published equations.
Methods: This study was a secondary analysis of a prospective cohort study. We enrolled all the patients aged ≥65 years who visited our emergency department (ED) during the period from May 15, 2017, to July 31, 2017. We used linear regression to develop a new calculated osmolarity equation from the patients’ laboratory data compared with the measured serum osmolarity. In a receiver-operating characteristic (ROC) plot, the new equation was compared with other 5 equations in terms of its ability to diagnose dehydration (calculated osmolarity > 300 mOsm/kg). The Bland-Altman method was used to assess the mean difference (MD) with each equation.
Results: A total of 322 participants were included in the study. The new equation originated from our older patients’ data was 1.75 × (Na + K) + 0.9 × Glucose + Urea + 25.7, all in mmol/L. This equation had the highest ROC-area under the curve (AUC) of 0.81 among all the equations. The MD between the calculated and measured osmolarity values was 0.49 mOsm/L (95% confidence interval [CI], −0.40 to 1.38 mOsm/L). Equations 1, 2, 3, 4, and 5 showed ROC-AUC of 0.73, 0.78, 0.75, 0.80, and 0.81, respectively. The MDs (95% CI) were 6.18 (5.13–7.23), −4.19 (−5.15 to −3.23), −5.38 (−6.37 to −4.39), −5.31 (−6.20 to −4.41), and −6.89 (−7.8 to −5.98), respectively.
Conclusion: We developed a new equation with good performance in calculating osmolarity. However, further validation and ability to predict dehydration of this equation should be assessed.
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