The most frequent electrolyte abnormalities, both clinically and as reflected in abnormal laboratory values, involve sodium. This is true because sodium is the most important cation of the body, both from a quantitative standpoint and because of its influence in maintaining electric neutrality. The most common causes of low or high serum sodium values are enumerated in the box. Some of these conditions and the mechanisms involved require further explanation.

Technical problems in sodium measurement may affect results. For many years the primary assay technique for sodium and potassium was flame photometry. Since 1980, instrumentation has been changing to ion-selective electrodes (ISEs). ISEs generate sodium results that are about 2% higher than those obtained by flame photometry (in patient blood specimens this difference is equivalent to 2-3 mEq/L [2-3 mmol/L]). Potassium values are about the same with both techniques. Many, but not all, laboratories automatically adjust their ISE sodium results to make them correspond to flame photometer values. Sodium concentration can be decreased in blood by large amounts of glucose (which attracts intracellular fluid, creating a dilutional effect). Each 62 mg of glucose/100 ml (3.4 mmol/L) above the serum glucose upper reference limit results in a decrease in serum sodium concentration of 1.0 mEq/L. Large amounts of serum protein (usually in patients with myeloma) or lipids (triglyceride concentration >1,500 mg/100 ml [17 mmol/L]) can artifactually decrease the serum sodium level when sodium is measured by flame photometry (values obtained by the ISE method are not affected). One report suggests a formula whose result can be added to flame photometry values to correct for severe

Clinical Situations Frequently Associated With Serum Sodium Abnormalities

I. Hyponatremia
A. Sodium and water depletion (deficit hyponatremia)
1. Loss of gastrointestinal (GI) secretions with replacement of fluid but not electrolytes
a. Vomiting
b. Diarrhea
c. Tube drainage
2. Loss from skin with replacement of fluids but not electrolytes
a. Excessive sweating
b. Extensive burns
3. Loss from kidney
a. Diuretics
b. Chronic renal insufficiency (uremia) with acidosis
4. Metabolic loss
a. Starvation with acidosis
b. Diabetic acidosis
5. Endocrine loss
a. Addison’s disease
b. Sudden withdrawal of long-term steroid therapy
6. Iatrogenic loss from serous cavities
a. Paracentesis or thoracentesis
B. Excessive water (dilution hyponatremia)
1. Excessive water administration
2. Congestive heart failure
3. Cirrhosis
4. Nephrotic syndrome
5. Hypoalbuminemia (severe)
6. Acute renal failure with oliguria
C. Inappropriate antidiuretic hormone (IADH) syndrome
D. Intracellular loss (reset osmostat syndrome)
E. False hyponatremia (actually a dilutional effect)
1. Marked hypertriglyceridemia*
2. Marked hyperproteinemia*
3. Severe hyperglycemia
II. Hypernatremia
Dehydration is the most frequent overall clinical finding in hypernatremia.
1. Deficient water intake (either orally or intravenously)
2. Excess kidney water output (diabetes insipidus, osmotic diuresis)
3. Excess skin water output (excess sweating, loss from burns)
4. Excess gastrointestinal tract output (severe protracted vomiting or diarrhea without fluid therapy)
5. Accidental sodium overdose
6. High-protein tube feedings

*Artifact in flame photometry, not in ISE.

hyperlipidemia (triglyceride >1,500 mg/100 ml): % that Na value should increase = 2.1 Ч [triglyceride (gm/100 ml) – 0.6]. There is an interesting and somewhat inexplicable variance in reference range values for sodium in the literature, especially for the upper end of the range. This makes it highly desirable for each laboratory to determine its own reference range. Another problem is a specimen drawn from the same arm that already has an intravenous (IV) line; this usually happens when the phlebotomist cannot find a vein in the opposite arm. However, this may lead to interference by the contents of the IV system.