To detect a problem with the body's electrolyte balance.
As part of routine blood testing, or when your doctor suspects that you have an imbalance of one of the electrolytes (usually sodium or potassium), or if your doctor suspects an acid-base imbalance. Electrolytes may also be checked if you are prescribed certain drugs, particularly diuretics or ACE inhibitors.
A blood sample taken from a vein in the arm
Electrolytes are minerals that are found in body tissues and blood in the form of dissolved salts. As electrically charged particles, electrolytes help move nutrients into and wastes out of the body's cells, maintain a healthy water balance, and help stabilise the body's acid/base (pH) level. Electrolytes are usually measured as part of a renal profile which measures the main electrolytes in the body, sodium (Na+), potassium (K+), together with creatinine and/or urea, and may occasionally include chloride (Cl-) and/or bicarbonate (HCO3-).
Most of the body's sodium is found in the extracellular fluid (ECF), outside of the body’s cells, where it helps to regulate the amount of water in your body. Potassium is found mainly inside the body’s cells. A small but vital amount of potassium is found in the plasma, the liquid portion of the blood. Monitoring potassium is important as small changes in the plasma K+ concentration can affect the heart’s rhythm and ability to contract. Chloride travels in and out of the cells to help maintain electrical neutrality, and its level usually mirrors that of sodium. The primary role of bicarbonate, which is excreted and reabsorbed by the kidneys, is to help maintain a stable pH level and, secondarily, to help maintain electrical neutrality.
Your diet provides sodium, potassium, and chloride; your kidneys excrete them. Your lungs provide oxygen and regulate CO2 which is in balance with the bicarbonate level in plasma. The balance of these chemicals is an indication of the functional well-being of several basic body functions, including those performed by the kidneys and heart.
A related "test" is the anion gap, which is actually a calculated value. It reflects the difference between the positively charged ions (called cations) and the negatively charged ions (called anions). There is more than one formula: one is sodium minus (chloride plus bicarbonate) and the other is (sodium plus potassium) minus (chloride plus bicarbonate). The occurrence of an abnormal anion gap reflects an unusual presence of some kind of charged particle in the blood. It is non-specific but can suggest certain kinds of metabolic abnormalities, such as starvation or diabetes, or the presence of a toxic substance, such as oxalate, glycolate, or aspirin.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Electrolyte measurements are used to screen for an electrolyte or pH imbalance and to monitor the effect of treatment on a known imbalance that is affecting bodily organ function. Since electrolytes are often abnormal in a variety of acute (short term) and chronic (long term) illnesses, they are frequently requested in hospitalised patients.
When is it requested?
What does the test result mean?
Electrolyte levels are affected by how much is taken in through your diet, the amount of water in your body, and the quantity of electrolytes excreted by your kidneys. They are also affected by hormones, especially aldosterone, a hormone that retains sodium in the body but increases the loss of potassium.
In specific disorders, one or more electrolytes may be abnormal. Your doctor will look at the overall balance but is likely to be especially concerned with your sodium and potassium concentration. People whose kidneys are not functioning properly, for example, may retain excess fluid in the body, diluting the sodium and chloride so that they fall below normal concentrations. Those who experience severe fluid loss may show an increase in K+, Na+, and Cl- (Cl- tends to mirror the Na+). Some forms of heart disease, muscle and nerve problems, and diabetes may also have one or more abnormal electrolytes. Electrolyte abnormalities may also be a consequence of drug treatment.
Knowing which electrolytes are out of balance can help your doctor determine the cause and treatment to restore proper balance. If left untreated, electrolyte imbalance can lead to dizziness, cramps, irregular heartbeat, and possibly death.
Test results are reported as a numerical value and must be compared with an appropriate reference range in order to determine the significance of the result. Reference ranges may vary for a variety of reasons including the patient's age and sex, as well as the instrumentation or kit used to perform the test. To learn more about reference ranges, please see the article, Reference Ranges and What They Mean. Your local laboratory will advise your doctor of the appropriate reference range for your particular test.
Is there anything else I should know?
Depending on which electrolyte(s) is out of balance and the extent of that change, treatment may involve changing your diet, for example to lower salt intake, increasing or reducing fluid intake, or taking or stopping medication such as diuretics. Once treatment has begun, you may be asked to have regular testing to determine how well the treatment has worked and to make sure the imbalance does not reoccur.
What is anion gap?
Anion gap (AG) is a value calculated using the results of an electrolyte panel. It is used to help distinguish between anion-gap and non-anion-gap metabolic acidosis. Acidosis refers to an excess of acid in the body; this can disturb many cell functions and should be recognised as quickly as possible, when present. AG is frequently used in the hospital and/or accident and emergency room setting to help diagnose and monitor acutely ill patients. If anion-gap metabolic acidosis is identified, the AG may be used to help monitor the effectiveness of treatment and the underlying condition.
Specifically, the anion gap evaluates the difference between measured and unmeasured electrical particles (ions or electrolytes) in the fluid portion of the blood. According to the principle of electrical neutrality, the number of positive ions (cations) and negative ions (anions) should be equal. However, not all ions are routinely measured. The calculated AG result represents the unmeasured ions and primarily consists of anions, hence the name “anion gap.” The most commonly used formula is:
Anion Gap (AG) = Sodium - (Chloride + Carbon Dioxide)
However, there are other AG formulas, so reference ranges are not interchangeable. Each laboratory formula will have an established normal range that should be referenced.
The anion gap is non-specific. It is increased when the number of unmeasured anions increases, indicating a state of anion-gap metabolic acidosis, but it does not tell the doctor what is causing the imbalance. The metabolic acidosis must be treated to restore the acid/base balance, but the underlying condition must also be identified and treated. Causes can include uncontrolled diabetes, starvation, kidney damage, and ingestion of potentially toxic substances such as antifreeze, excessive amounts of aspirin, or methanol. A low anion gap can also occur; this is most commonly seen when albumin (an anion as well as a protein) is low, while immunoglobulins (cations as well as proteins) are increased.