The serum sodium may be low in situations of excess body water. The amount of excess water can be calculated if the plasma osmolality is known.
Estimating Total Body Water in the Adult from Body Weight
total body water in liters =
= (factor) * (body weight in kilograms)
Factor to Use
elderly or cachectic
total body solute (mOsm) =
= (total body water in liters) * (plasma osmolality in mOsm/L)
normal volume of body water in liters =
= (total body solute in mOsm) / (desired plasma osmolality in mOsm/L)
body water excess in liters =
= (total body water in liters) - (normal volume of body water in liters)
= (total body water in liters) * (1 - ((serum sodium in mEq/L)/140))
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Purpose: To estimate the excesses or deficits in states of sodium abnormalities.
Specialty: Endocrinology, Clinical Laboratory
Objective: clinical diagnosis, including family history for genetics, laboratory tests
ICD-10: E87.1, E87.7,