Electrolytes and Anion Gap

The electrolyte panel is used to identify an electrolyte, fluid, or pH imbalance (acidosis or alkalosis). It is frequently ordered as part of a routine health exam. It may be ordered by itself or as a component of a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP). These panels can include other tests such as BUN, creatinine, and glucose.

Electrolyte measurements may be used to help investigate conditions that cause electrolyte imbalances such as dehydration, kidney disease, lung diseases, or heart conditions. A series of electrolyte panels may also be used to monitor treatment of the condition causing the imbalance.

Since electrolyte and acid-base imbalances can be present with a wide variety of acute and chronic illnesses, the electrolyte panel is frequently used to evaluate patients who seek medical care in the emergency room as well as hospitalized patients.

The results for an electrolyte panel may also include a calculation for anion gap that can be used to help detect disorders or the presence of toxic substances (see below).

If you have an imbalance of a single electrolyte, such as sodium or potassium, your healthcare practitioner may order repeat testing of that individual electrolyte, monitoring the imbalance until it resolves. If you have an acid-base imbalance, your healthcare practitioner may also order tests for blood gases, which measure the pH and oxygen and carbon dioxide levels in an arterial blood sample, to help evaluate the severity of the imbalance and monitor its response to treatment.

An electrolyte panel may be ordered as part of a routine health exam, as recommended by your healthcare practitioner. It may also be ordered as a diagnostic aid when you have signs and symptoms, such as:

• Fluid accumulation (edema)
• Nausea or vomiting
• Weakness
• Confusion
• Irregular heart beat (cardiac arrhythmias)

It is frequently ordered as part of an evaluation when you have an acute or chronic illness and at regular intervals when you have a disease or condition or is taking a medication that can cause an electrolyte imbalance. Electrolyte tests are commonly ordered at regular intervals to monitor treatment of certain conditions, including high blood pressure (hypertension), heart failure, lung diseases, liver disease and kidney disease.

High or low electrolyte levels can be caused by several conditions and diseases. Generally, they are affected by how much is consumed in the diet and absorbed by the body, the amount of water in your body, and the amount eliminated by the kidneys. Electrolyte levels are also affected by some hormones such as aldosterone, which conserves sodium and promotes the elimination of potassium, and natriuretic peptides, which increase elimination of sodium by the kidneys.

Electrolyte levels can be affected by changes in the amount of water in your body. For example:

• If your kidneys are not functioning properly (i.e., kidney disease), you may retain excess fluid. This results in a dilution effect on sodium and chloride so that they fall below normal levels.
• On the other hand, if you experience severe fluid loss (dehydration), you may show an increase in potassium, sodium, and chloride levels.

Some conditions such as heart disease and diabetes may also affect the fluid and electrolytes balance in your body and cause abnormal levels of electrolytes. Several other conditions can cause electrolyte and pH imbalances. (See the condition article on Acidosis and Alkalosis for more information.)

Knowing which electrolytes are out of balance can help your healthcare practitioner determine the underlying cause and make decisions about treatment to restore proper balance. Left untreated, an electrolyte imbalance can lead to various problems, including dizziness, cramps, irregular heartbeat, and possibly death.

See the individual test articles for additional information on what results might mean:

• Sodium
• Potassium
• Chloride
• Bicarbonate

The results of your electrolyte panel are interpreted by your healthcare practitioner within the context of other tests that you have had done as well as other factors, such as your medical history. A single result that is slightly high or low may or may not have medical significance. There are several reasons why a test result may differ on different days and why it may fall outside a designated reference range.

• Biological variability (different results in the same person at different times): If a healthcare practitioner runs the same test on you on several different occasions, there is a good chance that one result will fall outside a reference range even though you are in good health. For biological reasons, your values can vary from day to day.
• Individual variability (differences in results between different people): References ranges are usually established by collecting results from a large population and determining from the data an expected average (mean) result and expected differences from that average (standard deviation). There are individuals who are healthy but whose tests results, which are normal for them, do not always fall within the expected range of the overall population.

So, a test value that falls outside of the established reference range may mean nothing significant. Generally, this is the case when the test value is only slightly higher or lower than the reference range and is why a healthcare practitioner may repeat a test on you and why the practitioner may look at results from prior times when you had the same test performed.

However, a result outside the range may indicate a problem and warrant further investigation. Your healthcare practitioner will evaluate your test results in the context of your medical history, physical exam, and other relevant factors to determine whether a result that falls outside of the reference range means something significant for you. For more, read the article on Reference Ranges and What They Mean.

Anion gap (AG or AGAP) 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 recognized as quickly as possible, when present. The anion gap is frequently used in the hospital and/or 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 + Bicarbonate [total CO2])

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 healthcare practitioner 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 (salicylates), 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.

Treatment depends on which electrolyte(s) is out of balance and the extent of that change. Treatment usually involves managing the imbalance while identifying and addressing the underlying cause(s) of the imbalance and providing whatever support is necessary to the affected person.

Certain drugs, such as anabolic steroids, corticosteroids, laxatives, cough medicines, and oral contraceptives, may cause increased levels of sodium. Other drugs, such as diuretics, carbamazepine, and tricyclic antidepressants, may cause decreased levels of sodium.

Drugs that affect sodium blood levels will also cause changes in chloride. Taking substantially more than the recommended dosage of antacids can also cause low chloride levels.

Some drugs may increase bicarbonate (total CO₂) levels. These include, among others, fludrocortisone, barbiturates, hydrocortisone, loop diuretics, and steroids.

Drugs that may decrease bicarbonate (total CO₂) levels include methicillin, nitrofurantoin, tetracycline, thiazide diuretics, and triamterene.

Some diuretics, which may be used for people with disorders such as high blood pressure or kidney disease, tend to lower potassium levels to dangerously low levels. To avoid this problem, a healthcare practitioner may prescribe a potassium-sparing diuretic.

A non-physiological cause of a high potassium level in a patient sample is leakage from the blood cells if the sample is not handled carefully or is delayed in transport to the lab. If a significant amount of potassium leaks from cells, it can contribute to a falsely high test result. When a healthcare practitioner suspects that a potassium result is not consistent with a patient’s condition, the test for potassium may be repeated.