From the Guidelines
Dehydration is characterized by serum or plasma osmolality >300 mOsm/kg, which is a key laboratory finding. This threshold value has been rigorously developed by Cheuvront et al. 1 and is supported by cohort studies assessing effects of raised serum osmolality in older people. Laboratory findings of dehydration may also include other abnormalities such as elevated blood urea nitrogen (BUN) and creatinine, serum electrolyte abnormalities, and increased hematocrit.
Key Laboratory Findings
- Elevated serum osmolality (>300 mOsm/kg) 1
- Elevated blood urea nitrogen (BUN) and creatinine
- Serum electrolyte abnormalities, including hypernatremia or hyponatremia
- Increased hematocrit and hemoglobin concentration due to hemoconcentration
- Urine studies revealing concentrated urine with high osmolality and low urine sodium
Clinical Implications
Dehydration can lead to serious complications, including metabolic acidosis, hyperkalemia, and hypokalemia. The severity of laboratory abnormalities generally correlates with the degree of dehydration. Therefore, prompt recognition and treatment of dehydration are crucial to prevent morbidity and mortality. The laboratory findings of dehydration should be interpreted in the context of the patient's clinical presentation and medical history.
From the Research
Laboratory Findings of Dehydration
The laboratory findings of dehydration can be identified through various tests, including:
- Plasma osmolality: raised plasma osmolality (pOsm) usually indicates dehydration, with pOsm >300 and ≤280 mOsm/kg classifying a person as hyper or hypo-osmolar 2
- Urine osmolality and specific gravity: these can be used to assess hydration status, with significant positive associations existing between urine color and both urine specific gravity and urine osmolality 3
- Serum sodium level: extreme hypernatremia (sodium levels >190 mmol/l) can occur in dehydration, with a case report describing a patient with sodium levels of 226 mmol/l 4
- Blood urea nitrogen (BUN) and creatinine: these can be used to assess hydration status, with a study finding that dehydrated patients with central diabetes insipidus had lower serum urea levels compared to those without central diabetes insipidus 5
- Hematocrit: this can be used to assess hydration status, with a study measuring hematocrit as part of a range of laboratory tests to assess hydration status in elderly veterans 3
Biochemical Markers
Biochemical markers such as osmolality and blood urea nitrogen/creatinine can be used to diagnose dehydration, with a review concluding that a combination of history, physical examination, laboratory values, and clinician experience remains the best approach to the diagnosis of dehydration 6
- Plasma osmolality is considered a valuable, objective surrogate marker of hypertonic dehydration, although it is underutilized in clinical practice 2
- Urine osmolality and specific gravity can provide important information for diagnosing dehydration, with a study finding significant positive associations between urine color and both urine specific gravity and urine osmolality 3