Electrolyte Tests Health Article

Measurement of electrolytes

Electrolytes are measured by a process known as potentiometry. This method measures the voltage that develops between the inner and outer surfaces of an ion selective electrode. The electrode (membrane) is made of a material that is selectively permeable to the ion being measured. This potential is measured by comparing it to the potential of a reference electrode. Since the potential of the reference electrode is held constant, the difference in voltage between the two electrodes is attributed to the concentration of ion in the sample.

Precautions

Electrolyte tests are performed on whole blood, plasma, or serum, usually collected from a vein or capillary.

Special procedures are followed when collecting a sweat sample for electrolyte analysis. This procedure, called pilocarpine iontophoresis, uses electric current applied to the arm of the patient (usually an infant) in order to convey the pilocarpine to the sweat glands where it will stimulate sweating. Care must be taken to ensure that the collection device (macroduct tubing or gauze) does not become contaminated and that the patient's parent or guardian understands the need for the electrical equipment employed.

Preparation

Usually no special preparation is necessary by the patient. Samples for calcium and phosphorus and for magnesium should be collected following an eight-hour fast.

Aftercare

Discomfort or bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure to the puncture site until the bleeding stops reduces bruising. Applying warm packs to the puncture site relieves discomfort.

Risks

Minor temporary discomfort may occur with any blood test, but there are no complications specific to electrolyte testing.

Normal results

Electrolyte concentrations are similar whether measured in serum or plasma. Values are expressed as mmol/L for sodium, potassium, chloride, and bicarbonate. Magnesium results are often reported as milliequivalents per liter (meq/L) or in mg/dL. Total calcium is usually reported in mg/dL and ionized calcium in mmol/L. Since severe electrolyte disturbances can be associated with life-threatening consequences such as heart failure, shock, coma, or tetany, alert values are used to warn physicians of impending crisis. Typical reference ranges and alert values are cited below:

• serum or plasma sodium: 135–145 mmol/l; alert levels: less than 120 mmol/l and greater than 160 mmol/l
• serum potassium: 3.6–5.4 mmol/l (plasma, 3.6–5.0 mmol/l); alert levels: less than 3.0 mmol/l and greater than 6.0 mmol/l
• serum or plasma chloride: 98–108 mmol/l
• sweat chloride: 4–60 mmol/l
• serum or plasma bicarbonate: 18–24 mmol/l (as total carbon dioxide, 22–26 mmol/l); alert levels: less than 10 mmol/l and greater than 40 mmol/l
• serum calcium: 8.5–10.5 mg/dl (2.0–2.5 mmol/l); alert levels: less than 6.0 mg/dl and greater than 13.0 mg/dl
• ionized calcium: 1.0–1.3 mmol/l
• serum inorganic phosphorus: 2.3–4.7 mg/dl (children, 4.0–7.0 mg/dl); alert level: less than 1.0 mg/dl
• serum magnesium: 1.8–3.0 mg/dl (1.2–2.0 meq/l or 0.5–1.0 mmol/l)
• ionized magnesium: 0.53–0.67 mmol/l
• osmolality (calculated) 280–300 mosm/kg

BOOKS

Henry, J. B. Clinical Diagnosis and Management of Laboratory Methods. 20th ed. Philadelphia: W. B. Saunders Company, 2001.

Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2001. 40th ed. New York: Lange Medical Books/McGraw-Hill, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests. 7th ed. Philadelphia: Lippincott Williams & Wilkins, 2000.

OTHER

MedLine Plus. "Electrolytes." October 18, 2001 [cited April 4, 2003]. <http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm>.

National Institutes of Health. [cited April 5, 2003]. <http://www.nlm.nih.gov/medlineplus/encyclopedia.html>.

Erika J. Norris Mark A. Best, M.D.