Development of a Calculated Serum Osmolality Equation for Older Adults

Hypertonic Dehydration (HD) is the term used for the loss of bodily water in older adults and is linked to a number of morbidities, such as urolithiasis, ischemic stroke, urinary tract infections, venous thromboembolism, morbidity, and death.

This study aimed to develop and validate a new calculated serum osmolality equation for older adults and to compare its accuracy with five previously published equations.

This was a prospective diagnosis accuracy study conducted between September 1, 2019, and August 31, 2020, including older adults aged 60 years and above living in Bangkok, Thailand. We measured serum osmolarity by automatic osmometer using the freezing point depression method. For the training group, we developed a new calculated serum osmolality equation from blood glucose, Blood Urea Nitrogen (BUN), and serum electrolytes. In the validation group, we validated the accuracy of the calculated serum osmolarity equation (index test) compared to the measured serum osmolality (reference standard test). We evaluated the agreement between the reference standard test and index test with the Bland-Altman method and concordance correlation coefficient. We determined the mean difference, accuracy, and precision between the index test and five previously published equations with reference standard test.

We included 586 participants in this study (410 in the training group and 176 in the validation group). Multiple linear regression analysis revealed a significant association among blood glucose, sodium, BUN, and serum osmole. Our new equation corresponded to osmolality estimate = 1.3x(Na+ + BUN) + 0.9xglucose + 86 mOsm/kg. We used the dataset from 176 participants and calculated the mean difference for the index test and compared it with five previously published equations. The index test exhibited a mean difference of -0.5 (95% CI; -1.2-0.2) mOsm/kg, which was similar to that of the previous eq. 2 and eq. 3. and had great accuracy (0.98) and good precision (0.76), while equation 3 exhibited the highest accuracy (0.99) and precision (0.77). Additionally, the new equation's Area Under the Receiving Operating Characteristics (AUROC) curve was 0.87 (95% CI 0.80-0.94), indicating a very good ability to forecast HD (serum osmolality > 300 mOsm/kg as the gold standard). The AUROC curve of the index test did not differ from that of the others.

In this work, we have developed a new serum osmolality equation able to predict hypertonic dehydration in Thai community-dwelling older adults aged 60 years and above, but not in general. An external validation study should be conducted.