Introduction
Electrolytes are formed when a substance dissolves in water and become ionized molecules. Electrolytes carry electric charges, which may be positive or negative. Examples of positively charged electrolytes are sodium and potassium ions. These positive-charged ions (also called cations) are denoted with a “+” symbol, i.e. Na+ and K+. Similarly, the negative charged ions or anions such as chloride are written with a “minus (-)” sign (C1–).
In our bodies, electrolytes include sodium (Na+), potassium (K+), calcium (Ca2+), bicarbonate (HCO3–), magnesium (Mg2+), chloride (Cl-), and hydrogen phosphate (HPO42-).
The body has two main fluid compartments, the intracellular (inside the cell) and extracellular (outside the cell). The extracellular fluids include blood plasma, urine and body fluids. Electrolytes are found in both intracellular and extracellular fluids. However, the composition of water, electrolytes and other components in both fluid compartments differ.
Electrolytes are essential for life. The electrolytes play the regulatory role in maintaining osmotic pressure and water distribution as well as pH balance in the various body fluid compartments. They are also involved in the chemical reactions such as oxidation-reduction and catalysis within the body.
Neurons (nerve cells) and muscles, also known as the “electric tissues”, depend on electrolytes to maintain the voltage across their cell membranes and carry electrical impulses to other cells. The presence of calcium (Ca2+), sodium (Na+), and potassium (K+) ions is required for muscle contraction. Muscle weakness will occur if these key electrolytes are insufficient.
Therefore, the common symptoms involving these tissues and organs are fatigue, confusion, muscle weakness, paralysis and even coma. These symptoms are usually observed in patients when their levels of electrolytes are lower or higher than normal.
Electrolyte levels are kept constant by kidneys and several hormones such as antidiuretic hormones, aldosterone and parathyroid hormones. Our daily source of electrolytes comes from food and they are absorbed through the gastrointestinal (stomach and intestines) tract. Any excess will be filtered by the kidneys and removed from the body.
(Na+) is found mostly in extracellular fluid. Thus, it plays a central role in maintaining the amount of water distribution throughout the body and the osmotic pressure outside the cells.
On the contrary, (K+) is found mainly inside the cells. It helps move nutrients into the cells and waste products out of the cells. (K+) also affects the communication between the nerves and muscles. A small amount of (K+) can affect the contraction of heart muscles and subsequently change the heart rate. Further increase in (K+) level will cause cardiac arrest.
(C1–) is the major extracellular anion. It moves in and out of the cells to help maintain the electrical neutrality and balance of fluids in the body. Its level usually mirrors that of (Na+). Therefore, drugs that affect (Na+) blood levels will also cause changes in (C1–).
Electrolytes testing
The most common human electrolytes measured in the medical laboratory are (Na+), (K+) and (C1–). They are usually measured together in a panel test as baseline investigation and as a tool in monitoring the treatment progress of a patient.
The electrolyte of interest can also be measured individually for the purpose of monitoring the effectiveness of treatment given.
The most common sample for electrolytes testing is whole blood, collected from a vein. Plasma is the fluid derived from whole blood after being mixed with preservative (e.g.heparin). Serum is the fluid drawn from clotted whole blood. Electrolyte tests are usually performed on serum, plasma and occasionally whole blood.
Other types of sample include cerebrospinal fluid, sweat, urine, faeces or gastrointestinal fluids. Urine should be collected without addition of a preservative. Faecal and gastrointestinal fluid specimens require preparation before testing.
Sweat sample is collected in a special procedure called pilocarpine iontophoresis by applying electric current to the arm to stimulate sweating.
Methods of measuring electrolytes
Ion-selective electrode (ISE) is the most widely used method of measurement for electrolytes in medical laboratories. The ISE system contains Na+ electrodes with glass membranes and K+ electrodes with liquid ion-exchange membranes as well as a reference electrode.
This method measures the voltage that develops between the inner and outer surfaces of an ISE. It is then compared to the reference electrode. As the reference electrode is constant, the difference between the two electrodes is attributed to the concentration of electrolytes in the sample.
There are two types of ISE methods, – direct and indirect. In the indirect method, a sample is diluted with a large volume of high ionic strength solution before being measured. In the direct method, a sample is presented to the electrodes without dilution.
Other methods of determining the electrolytes are flame emission spectrometry (FES) and spectrophotometry.
References ranges
The reference range (sometimes known as normal range) given below as a theoretical guideline should not be used to interpret your test result. The reference range may differ between laboratories.
|
Electrolytes
|
Reference Range
|
|---|---|
| Sodium ion | Serum/Heparinized plasma: 136 – 145 mmol/L Urine: 40 – 220 mmol/day Cerebrospinal fluid: 146 – 150 mmol/L |
| Potassium ion | Serum: 3.5 – 5.1 mmol/L Heparinized plasma: 3.4 – 4.5 mmol/L Urine: 25 – 125 mmol/day Cerebrospinal fluid: 2.5 – 3.2 mmol/L |
| Chloride ion | Serum/Heparinized plasma: 98 – 108 mmol/L Urine: 110 – 250 mmol/L Cerebrospinal fluid: 118 – 132 mmol/L |
Clinical disorders of electrolytes
An electrolyte level lower than reference range is given a prefix “hypo” whereas level higher than normal is added a prefix “hyper”. The most common imbalances are hypernatremia, hyponatremia, hyperkalemia and hypokalemia.
Electrolyte changes with the water levels in the body. Severe electrolyte disturbances such as dehydration (lack of water) and overhydration (water in excess) may lead to cardiac and neurological complications.
Hyponatremia (decreased plasma of (Na+) level) may arise from inadequate of sodium intake in diet, excessive loss of (Na+) through sweat and urine, prolonged vomiting or persistent diarrhoea. Loss of (Na+) may be due to impaired kidney functions. Inappropriate use of diuretics drug can also disrupt kidney functions and cause hyponatremia.
The symptoms of hyponatremia include confusion, headache, irritability, loss of appetite, muscle weakness, nausea, vomiting, fatigue, decreased consciousness, hallucination and coma in severe cases.
Hypernatremia, where the level of (Na+) is high in plasma, is usually caused by excessive water loss (dehydration). This may occur in profuse sweating, prolonged hyperventilation, vomiting, diarrhoea or polyuria (production of large amount of urine). Certain drugs, such as anabolic steroid, corticosteroids, laxative, cough medicines and oral contraceptives, may also cause increased levels of (Na+).
The first symptom of hypernatremia is thirst. A person may feel weak and sluggish. If the (Na+) level persistently goes up, that person will suffer from confusion, paralysis, seizures and eventually goes into a coma. Therefore, the best treatment is to increase water intake by giving fluid intravenously.
Hypokalemia, where the (K+) level in the plasma is low, may be due to decreased intake, redistribution of extracellular K+ into intracellular fluid, and increased loss of (K+–)rich body fluids. Decreased intake can be seen in chronic starvation and postoperative therapy with (K+–)poor fluids. Diuretics make the kidneys excrete more (K+) and water in urine.
Loss of (K+) from gastrointestinal fluids is through vomiting, diarrhoea, or intestinal fistulas. (K+) loss from the kidneys may be the result of renal tubular acidosis, aldosteronism, or Cushing’s or Bartter’s syndrome.
Other conditions that may be associated with low serum (K+) levels include cirrhosis, Conn’s syndrome, and digitalis toxicity as well as the use of drugs like ticarcillin, carbenicillin, amphotericin B, and theophylline.
When the (K+) levels falls far below normal, the patient will suffer from fatigue, confusion, muscle weakness and cramps. If the level continues to drop, the person can become paralyzed and develop arrhythmias (unusual heart rhythms).
Low (K+) is treated with potassium oral supplements. Patients may increase their (K+) level by changing their diet.
Hyperkalemia (increased plasma (K+) concentration) may be due to decreased excretion of (K+) in renal failure.
A person suffering hyperkalemia may exhibit symptoms such as fatigue, numbness, nausea, vomiting, breathing problem, chest pain and palpitations. In extremely high level of (K+), it can cause paralysis and heart arrest.
References
- Burtis, C.A., Ashwood, E.R., Bruns, D.E. Tietz Fundamentals of Clinical Chemistry. 5th Edition. W.B. Saunders Co.
- Nordqvist, C. What are electrolytes? What causes electrolytes imbalance? http://www.medicalnewstoday.com/articles/153188.php
- American Association for Clinical Chemistry. Electrolytes. https://labtestsonline.org/understanding/analytes/electrolytes/tab/test/
| Last Reviewed | : | 31 October 2017 |
| Writer | : | Katheleen Ng Li Fong |
| Accreditor | : | Lau Kim Bee |
