Several blood chemistry biomarkers help evaluate dehydration, including BUN, sodium, potassium, chloride, RBCs, hemoglobin, and hematocrit. These should be assessed in conjunction with an individual’s history and clinical presentation.
Assessing dehydration should incorporate serum biomarkers, clinical signs and symptoms, individual history, fluid intake, and fluid losses. Many individuals do not consume enough water or fluids and fail to meet general daily fluid recommendations for adults of 2.7-3.7 liters (approximately 11.25 – 15.5 cups), though some of that fluid can be obtained from foods such as fruits and vegetables. Many don’t realize how much fluid can be lost from activities and heat exposure, an amount that can exceed 6 liters per day (25 cups/day) (Popkin 2010).
The following biomarkers and clinical conditions should be taken into account (Ekman 2020, Lacey 2019, Pagana 2021, Popkin 2011, Raymond 2021, Smith 2013, Taylor 2022, Zhang 2019):
Biomarkers of dehydration
- Elevated
- Serum sodium, potassium, chloride, BUN, creatinine, albumin, RBC count, hemoglobin, hematocrit
- Bun/creatinine ratio above 18
- Plasma hyperosmolality, i.e., elevated serum osmolality of above the normal 285-295 mOsm/kg H2O (285-295 mmol/kg)
- Concentrated urine, increased urine osmolality (3-4 times plasma osmolality), elevated specific gravity, acidic urine
- Decreased
- Glomerular filtration rate
Clinical signs and causes of dehydration
- Decreased total blood volume, hypovolemia
- Low blood pressure, orthostasis
- Reduced renal blood flow
- Weakness, dizziness, fatigue
- Poor skin turgor
- Kidney stones
- Acute weight loss of greater than 3% over 7 days
- Total body water below 69%
- Delirium, compromised cognitive function, including impaired memory and attention
- Sudden cardiac death following acute coronary artery syndrome
Causes of dehydration
- Inadequate fluid intake
- Excess fluid loss via the kidneys, sweating, vomiting, and diarrhea
- Excessively hot weather
- Diuretics, laxatives, polypharmacy
Plasma Osmolality (pOsm)
Plasma osmolality (number of solute particles per kg of solvent) increases with dehydration and decreases with overhydration, though other causes can alter it as well. For example, increased pOsm can occur with hypernatremia, hyperglycemia, hypercalcemia, renal dysfunction, and ketosis. Decreased pOsm can occur with hyponatremia and the syndrome of inappropriate anti-diuretic hormone (SIADH) secretion. Normal plasma osmolality in adults is 285-295 mOsm/kg H20. Values above 385 are associated with stupor in those with hyperglycemia, 400-420 increases the risk of grand mal seizures, and values above 420 can be fatal. The pOsm can be calculated from blood biomarkers (Pagana 2021):
Plasma Osmolality (mOsm/kg) = 2 x sodium + glucose/18 + BUN/2.8
References
Busher, Janice T. “Serum Albumin and Globulin.” Clinical Methods: The History, Physical, and Laboratory Examinations, edited by H Kenneth Walker et. al., 3rd ed., Butterworths, 1990.
Ekman, Louise et al. “Signs of Dehydration after Hip Fracture Surgery: An Observational Descriptive Study.” Medicina (Kaunas, Lithuania) vol. 56,7 361. 18 Jul. 2020, doi:10.3390/medicina56070361
Noland, Diana, Jeanne A. Drisko, and Leigh Wagner, eds. Integrative and functional medical nutrition therapy: principles and practices. Springer Nature, 2020.
Pagana, Kathleen Deska, et al. Mosby's Diagnostic and Laboratory Test Reference. 15th ed., Mosby, 2021.
Popkin, Barry M et al. “Water, hydration, and health.” Nutrition reviews vol. 68,8 (2010): 439-58. doi:10.1111/j.1753-4887.2010.00304.x
Raymond, Janice L., et al. Krause and Mahan's Food & the Nutrition Care Process. Elsevier, 2021.
Smith, Graham S., Gail L. Walter, and Robin M. Walker. "Clinical pathology in non-clinical toxicology testing." Haschek and Rousseaux's Handbook of Toxicologic Pathology. Academic Press, 2013. 565-594.
Shah, Maulik M. and Pujyitha Mandiga. “Physiology, Plasma Osmolality and Oncotic Pressure.” StatPearls, StatPearls Publishing, 3 October 2022.
Taylor, Kory. and Elizabeth B. Jones. “Adult Dehydration.” StatPearls, StatPearls Publishing, 3 October 2022.
Zhang, Na et al. “Effects of Dehydration and Rehydration on Cognitive Performance and Mood among Male College Students in Cangzhou, China