From the Guidelines
Dehydration is best indicated by a serum or plasma osmolality greater than 300 mOsm/kg, as this threshold value has been rigorously developed and validated in various studies, including those by Cheuvront et al. 1. This threshold is considered the key physiological trigger for protection mechanisms against dehydration, such as thirst and increased concentration of urine by the kidney. Some key laboratory signs of dehydration include:
- Elevated blood urea nitrogen (BUN) and creatinine
- Electrolyte abnormalities, including hypernatremia (sodium >145 mEq/L) or hyponatremia
- Hemoconcentration, leading to increased hemoglobin, hematocrit, and plasma protein levels
- Metabolic acidosis, reflected by decreased bicarbonate and pH
- Urine studies showing increased specific gravity (>1.020) and osmolality (>800 mOsm/kg), with decreased urine sodium (<20 mEq/L) It is essential to note that the interpretation of raised serum osmolality as a sign of dehydration depends on checking that serum glucose and urea are within normal range, as these can also affect osmolality 1. In older adults, renal function is often poor, making renal parameters less reliable for signaling low-intake dehydration, and clinical judgment can also be highly fallible 1. Therefore, serum or plasma osmolality >300 mOsm/kg should be used as the primary indicator of hydration status in older adults, as recommended by the US Panel on Dietary Reference Intakes for Electrolytes and Water 1.
From the Research
Signs of Dehydration on Labs
- Dehydration can be diagnosed through laboratory tests, including plasma osmolality, which is considered a valuable and objective surrogate marker of hypertonic dehydration 2.
- Plasma osmolality >300 and ≤280 mOsm/kg can classify a person as hyper or hypo-osmolar, indicating dehydration 2.
- Other laboratory tests that can be used to diagnose dehydration include blood urea nitrogen/creatinine and urine osmolality and specific gravity 3.
- The diagnosis of dehydration requires an appraisal of the patient and laboratory testing, clinical assessment, and knowledge of the patient's history 4.
Laboratory Variables
- Plasma osmolality is the gold standard for determining dehydration, with raised pOsm usually indicating dehydration 2.
- Serum or plasma osmolality cut-off ≥ 295 mOsm/kg can be used as a reference standard for water-loss dehydration 5.
- Urine osmolality and specific gravity can also be used to assess hydration status, but their diagnostic accuracy is limited 5.
Diagnostic Accuracy
- Few studies have investigated the diagnostic accuracy of laboratory tests for dehydration in older people, and the evidence is limited 5.
- Combining several tests may improve diagnostic accuracy, but individual tests should not be relied on to indicate dehydration 5.
- The diagnostic utility of any individual clinical symptom, sign, or test, or combination of tests, to indicate water-loss dehydration in older people is limited 5.