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The diagnosis and treatment of fluid and electrolyte disorders are based on (1) careful history, (2) physical examination and assessment of total body water and its distribution, (3) serum osmolality and electrolyte concentrations, and (4) urine osmolality and electrolyte concentrations.

A. Body Water and Fluid Distribution

Total body water is different in men than in women, and it decreases with age (Table 21–1). Among adults, approximately 40–60% of body weight is water. Fat has a lower water content than muscle; individuals with more fat have a lower ratio of total body water to body weight. Two-thirds of total body water is intracellular, while one-third is extracellular. Only one-fourth of extracellular fluid (ECF) (or approximately 5% of body weight) is intravascular. Total body water content fluctuates and acute changes can be evaluated clinically by measuring changes in body weight.

Table 21–1.Approximate total body water (as percentage of body weight) in relation to age and sex.

B. Serum Electrolytes

The cause of electrolyte disorders may be determined by reviewing the history, underlying diseases, and medications, though further diagnostic tests, most often blood or urine laboratory tests, are often required.

C. Evaluation of Urine

The urine concentration of an electrolyte can be helpful in determining whether the kidney is appropriately (or inappropriately) excreting or retaining an electrolyte in response to high or low serum levels. A 24-hour urine collection for daily electrolyte excretion is the gold standard for renal electrolyte handling, but it is slow and can be challenging to perform correctly. A more convenient method is the fractional excretion (Fe) of an electrolyte X (Fex) calculated from a spot urine sample:

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A low fractional excretion indicates renal reabsorption (high avidity or electrolyte retention), while a high fractional excretion indicates renal wasting (low avidity or electrolyte excretion). Thus, the fractional excretion can often help the clinician determine whether the kidney’s response is appropriate for a specific electrolyte disorder.

D. Serum Osmolality

Solute concentration is measured by osmolality in millimoles per kilogram. Osmolarity is measured in millimoles of solute per liter of solution. At physiologic solute concentrations (normally 285–295 mmol/kg), the two measurements are clinically interchangeable. Tonicity refers to osmolytes that are impermeable to cell membranes. Differences in tonicity across cell membranes lead to osmosis (shift of water across the cell membrane from an area of higher tonicity to one of lower tonicity). Substances that easily permeate cell membranes (eg, urea and ethanol) do not contribute to tonicity and thus do not cause water shifts across fluid ...

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