Electrolytes and Anion Gap
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
An electrolytes and anion gap test is performed on a blood sample to measure key electrolytes such as sodium, potassium, chloride, and bicarbonate, and to calculate the anion gap. It is used to help detect and evaluate electrolyte imbalances and acid–base disorders such as metabolic acidosis or alkalosis, often in conditions affecting the kidneys, lungs, or metabolism.
Why get tested?
To detect a problem with the body’s electrolyte balance.
When to get tested?
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.
Sample required?
A blood sample taken from a vein in the arm or in some cases, a urine sample.
Test preparation needed?
None
What is being tested?
In body tissues and blood, electrolytes are found in the form of dissolved salts and minerals. The main electrolytes include sodium, potassium, bicarbonate, and chloride. Calcium, magnesium, phosphate, sulphate, organic acids and proteins are other electrolytes found in body fluids.
Electrolytes help move nutrients into body’s cells and help move waste out of the body’s cells. Electrolytes 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-). Calcium, magnesium and phosphate are measured when imbalances in their concentrations are suspected.
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 potassium concentration can affect the heart’s rhythm and ability to contract (pump). 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 and other electrolytes. In order to maintain a “normal” electrolyte concentration your kidneys excrete them in urine. Your lungs provide oxygen and regulate carbon dioxide 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 (see below ‘What is 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).
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm. In some cases, a urine sample may also be collected. This allows comparison of the concentration of electrolytes in the blood to the concentration of the electrolytes in the urine. This can identify inappropriate loss of the electrolyte when blood concentration is low or inappropriate retention of the electrolyte when blood concentration is high.
Common questions
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.
It may be requested as part of routine testing or as a diagnostic aid for a specific disease, such as oedema (too much water in tissues) or cardiac arrhythmias. If one or more of the electrolytes is imbalanced, your doctor may want to monitor that individual electrolyte closely while treating the cause of the imbalance.
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 via the kidney.
In specific disorders, one or more electrolytes may be abnormal. Your healthcare professional 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 potassium, sodium, and chloride concentration (chloride tends to mirror the sodium concentration). 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 healthcare professional 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. Your local laboratory will advise your healthcare professional of the appropriate reference range for your particular test.
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.
The Anion gap (AG) is a value calculated using the results of an electrolyte panel. It estimates the difference (gap) between the measured positively charged ions (called cations) and the measured negatively charged ions (called anions) in the fluid portion of blood (serum or plasma).
The most commonly used formula is:
Anion Gap = Sodium – (Chloride + bicarbonate)
Note sodium (Na+) is the main cation measured in blood and chloride (Cl-) and bicarbonate (HCO3-) are the main anions measured.
The number of positive ions (cations) and negative ions (anions) must be equal (cancel each other out) to maintain electrical neutrality of the blood. 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”.
If the anion gap is higher than expected (i.e. the number of unmeasured anions is increased) it indicates the unusual presence of an excess of an anion in the blood. Examples are ketones (in uncontrolled diabetes or starvation), sulphates, phosphates and organic acids (in kidney disease or damage), and other organic acids such as oxalate or glycolate (resulting from the ingestion of potentially toxic substances such as antifreeze or methanol or excessive amounts of aspirin).
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.
The metabolic acidosis must be treated to restore the acid/base balance, but the underlying condition must also be identified and treated. If anion-gap metabolic acidosis is identified, the AG may be used to help monitor the effectiveness of treatment and the underlying condition.
AG is frequently used in the hospital and/or accident and emergency room setting to help diagnose and monitor acutely ill patients. 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.
Note, there are other AG formulas (some include the potassium (K+) concentration to estimate cation concentration), so reference ranges are not interchangeable. Each laboratory formula will have an established normal range that should be referenced.