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Sodium

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Also known as: Na
Formal name: Sodium

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The Test Sample

What is being tested?

Sodium is an electrolyte present in all body fluids and is vital to normal body function, including nerve and muscle function. This test measures the level of sodium in the blood and/or urine.

Sodium, along with other electrolytes such as potassium, chloride, and bicarbonate (or total CO2), helps cells function normally and helps regulate the amount of fluid in the body. While sodium is present in all body fluids, it is found in the highest concentration in the blood and in the fluid outside of the body's cells. This extracellular sodium, as well as all body water, is regulated by the kidneys.

We get sodium in our diet, from table salt (sodium chloride or NaCl), and to some degree from most of the foods that we eat. Most people have an adequate intake of sodium. The body uses what it requires and the kidneys eliminate the rest in the urine. The body tries to keep the blood sodium within a very narrow concentration range. It does this by:

  • Producing hormones that can increase (natriuretic peptides) or decrease (aldosterone) the amount of sodium eliminated in urine
  • Producing a hormone that prevents water losses (antidiuretic hormone, ADH, sometimes called vasopressin)
  • Controlling thirst; even a 1% increase in blood sodium will make a person thirsty and cause that person to drink water, returning the sodium level to normal.

Abnormal blood sodium is usually due to some problem with one of these systems. When the level of sodium in the blood changes, the water content in the body also changes. These changes can be associated with too little fluid (dehydration) or with too much fluid (edema), often resulting in swelling in the legs.

How is the sample collected for testing?

A blood sample is taken by needle from a vein in the arm. In some cases, a random or 24-hour urine sample may be required.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.