From the Guidelines
To assess dehydration, healthcare providers should prioritize measuring serum osmolality, with an action threshold of >300 mOsm/kg, as the primary indicator of hydration status, particularly in older adults, as recommended by the US Panel on Dietary Reference Intakes for Electrolytes and Water 1. When evaluating dehydration, a comprehensive approach is necessary, incorporating both physical examination and laboratory tests.
- Begin with vital signs assessment, including heart rate, blood pressure, and respiratory rate, to identify signs of volume depletion such as tachycardia and orthostatic hypotension.
- Perform a physical examination to assess skin turgor, mucous membranes for dryness, capillary refill time, and evaluate for sunken eyes or fontanelles in infants.
- Laboratory tests should include serum electrolytes (sodium, potassium, chloride, bicarbonate), blood urea nitrogen (BUN), creatinine, and calculation of the BUN/creatinine ratio (>20:1 suggests dehydration) 1.
- Additionally, urine specific gravity and urine osmolality provide valuable information about kidney concentration ability, while complete blood count may show hemoconcentration with elevated hemoglobin and hematocrit.
- In severe cases, arterial blood gases help assess acid-base status, guiding appropriate fluid replacement therapy. The most recent and highest quality study 1 emphasizes the importance of serum osmolality in assessing dehydration, particularly in older adults, where renal function may be impaired, making clinical judgment and renal parameters less reliable.
From the Research
Dehydration Assessment Tests
To assess dehydration, several tests can be completed, including:
- Clinical biomarkers in blood, such as osmolality and blood urea nitrogen/creatinine 2
- Clinical biomarkers in urine, such as osmolality and specific gravity 2
- Blood pressure assessment 2, 3
- Clinical symptoms in the eye, such as tear production and palpitating pressure 2
- Clinical symptoms in the mouth, such as thirst and mucous wetness 2
- Acute loss of weight, which is considered the most reliable measure of dehydration 4
- Saliva osmolality, which has been shown to have moderate diagnostic accuracy for detecting dehydration 3
- Urine color and urine specific gravity, although these have been found to have limited utility in determining hydration status 3, 5
- Hematologic indexes, such as hematocrit and serum osmolality, which can be affected by dehydration 5
Physical Diagnostic Examination
A physical diagnostic examination can also provide important information for diagnosing dehydration, including:
- Tachycardia (>100 bpm) 3
- Low systolic blood pressure (<100 mm Hg) 3
- Dry mucous membrane 3
- Dry axilla 3
- Poor skin turgor 3
- Sunken eyes 3
- Long capillary refill time (>2 seconds) 3
Laboratory Tests
Laboratory tests, such as plasma osmolality and the blood urea nitrogen to creatinine ratio, can be used as reference standards of hydration 3.