Urinary Sodium, Potassium, and Chloride

Lithium heparinate is the anticoagulant of choice for plasma electrolyte measurements (except, of course, for plasma lithium). Falsely elevated potassium values occur when potassium sequestrenated (EDTA) samples are measured, and the anticoagulant sodium heparinate will give falsely elevated sodium values. Sodium fluoride is a suitable anticoagulant for plasma lactate measurements and is more convenient than iced perchloric acid (Evans 1987). 

The anion gap (AG) is calculated by subtracting the sum of plasma chloride and bicarbonate concentrations from the sum of the sodium and potassium it represents the balance between the plasma anions and cations (Feldman and Rosenberg 1981 Kraut and Madias 2007). The unmeasured anions that contribute to the anion gap 

The urinary concentrations of these cations are highly variable because they are dependent on diet, and samples are subject to the effects of fecal contamination. The data are also highly variable when urinary output is affected by dehydration or excessive fluid losses via the gastrointestinal tract. Urine electrolyte values vary in intravenous studies due to rate of administration, tonicity, and electrolyte concentration of the vehicle. 

The fractional excretion (FE) of ions may be calculated as a measure of the urinary concentration of an analyte versus the amount of ion absorbed by the kidney. For example, the fractional excretion of sodium may be calculated by the formula  

The use of these calculations is limited in toxicology (Lefebvre et al. 2008) but occasionally used when comparing the clearance of a xenobiotic that also affects electrolyte renal secretion and excretion rates. 

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