CENTRAL  SOUTH  DAKOTA  MEDICAL NEWS
THE CLINICAL VIEW
 By: Phillip E. Hoffsten, MD
12  JULY  2001

What are Serum Electrolytes?

One of the most common blood tests done in the clinic or the hospital is called "serum electrolytes". To start this test, blood is drawn from a vein and allowed to clot. It is then separated into the red cell part of the blood and the serum part of the blood which is a clear amber fluid. The test is then done on the serum to determine its concentration of sodium, potassium, chloride and bicarbonate.

The concentration of sodium is normally 135 mEq/liter to as high as 145 mEq/liter. When things go wrong, the sodium concentration can either be too high or too low. Serum sodium concentrations that are too high are relatively unusual but most commonly occur in elderly individuals whose thirst mechanism is not working. An example is an elderly individual maybe with a stroke or Parkinson's disease or Alzheimer's disease whose thirst mechanism does not work and they simply do not get enough water. Then, as they perspire and lose more water that is not replaced, the amount of salt left behind is concentrated in the body and becomes too high. Serum sodium concentrations in excess of 160 mEq/liter are seen relatively commonly in our nursing homes or in elderly individuals living alone in apartments where they may lose their ability to take care of themselves. Correction of a high serum sodium must be done very slowly to prevent further damage to the brain. Giving this individual too much water too quickly can result in swelling of the brain and a fatal outcome. Thus, surprisingly, the most common problem in people who have a high serum sodium, called hypernatremia, is not that they have too much salt but that they don't have enough water.

The problem of a low serum sodium is most often an effect of too much water as opposed to not enough salt. It might be a surprise to most people to know that on a daily basis a person generally drinks twice as much water as compared to the amount of salt that they eat. The average American diet has about 9 grams of salt per day but generally about 2 quarts of water or more. In normal serum, there are about 9 grams of salt per quart of water, and it is the kidney's job to get rid of the excess water and also to balance out the salt and water excretion so that your body has the same salt concentration day after day. In a large number of illnesses, including heart failure with ankle swelling or kidney failure or liver failure, the ability to get rid of water is impaired. This results in the body keeping more water than is needed and dilutes out the salt concentration of the blood. For those individuals who remember their health class in the eighth grade telling them to drink eight glasses of water per day, this can be a major problem.

When a person's sodium concentration drops too low, weakness, fatigue, sometimes seizures and eventually a coma can result. To correct this, the person needs to restrict the amount of water that they take in. Rarely the person has to add more salt to their diet. There is an incredible array of diseases in which either high serum sodium concentrations or low serum sodium concentrations occur and proper medical treatment of these problems is very complex.

The next component of the serum electrolytes is called the serum potassium concentration. This is normally between 3.6 and 5.4 mEq/liter. A normal individual generally takes in between 60 and 100 mEq of potassium daily and excretes that amount in the urine so that balance is maintained. Low serum potassium most commonly occurs in individuals using water pills (diuretics). Generally, a diuretic will result in an additional loss of potassium from the body of about 40 mEq per day necessitating an extra 40 mEq of potassium intake to stay in balance. Most commonly, this supplement is provided as a pill. Often people will try to compensate for potassium losses by eating potassium-rich foods such as bananas, tomatoes or oranges but this is relatively inefficient. There are only 6 mEq of potassium in a banana. In order to get the extra potassium needed, you would have to eat six bananas per day in addition to your normal diet. That is the reason that potassium supplements as pills are provided.

The problem of high serum potassium almost always relates to kidney disease. Since the kidney is the primary organ for potassium excretion, potassium tends to accumulate in individuals who have kidney failure or a disease of the kidney that results in potassium imbalance.

The third component of the serum electrolytes is called the chloride concentration. This is rarely an independent manifestation of illness and understanding abnormalities is very complex.

The last component of the serum electrolytes is called the bicarbonate concentration. Generally, it is between 21 and 29 mEq/liter. Bicarbonate is the same chemical that comes in Arm & Hammer baking soda that you buy at the store on the grocery shelf. That baking soda is very pure sodium bicarbonate. It is often used as a supplement for people who need extra bicarbonate. By the same token, Turns or calcium carbonate pills are turned into bicarbonate in the body when you eat them.

Bicarbonate is the chemical the body uses most commonly to neutralize acid. Everyday the body produces about 60 mEq of acid that needs to be neutralized by at least 60 mEq of bicarbonate. Normally, the body manufactures its own bicarbonate for this purpose. In a host of kidney diseases, this ability to manufacture bicarbonate is lost and then the body slowly develop: excess acid. This is most commonly manifest as muscle cramps, weakness and in extreme cases very rapid breathing. These individuals are usually very ill and require medical attention.  Sometimes, the person who is not in good physical condition who exercises or works harder than they normally would and becomes very fatigued will develop muscle cramps. These can be made less a problem by taking extra Tums or calcium carbonate or sodium bicarbonate to alleviate the cramps.

The problem of too much bicarbonate in the body is very complex and beyond the scope of this discussion.

In summary, the serum electrolytes are made up of a concentration of sodium, potassium, chloride and bicarbonate and are a standard laboratory test in your clinics and hospitals. The topic is much more complex than this short column can describe. The healthcare professionals at your clinics and hospitals are extensively trained in the understanding of the effects, the causes and the treatment of abnormalities in your serum electrolytes.