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Also known as: Na
Formal name: Sodium

At a Glance

Why Get Tested?

To determine whether your sodium concentration is within normal limits and to help evaluate electrolyte balance and kidney function; to monitor chronic or acute hypernatremia or hyponatremia

When to Get Tested?

If you are experiencing dehydration or edema; also to monitor certain chronic conditions, like high or low blood pressure

Sample Required?

A blood sample drawn from a vein in the arm or, in some cases, a 24-hour urine sample

Test Preparation Needed?


The Test Sample

What is being tested?

This test measures the level of sodium in the blood. Sodium is an electrolyte that is vital to normal body processes, including nerve and muscle function. 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. Sodium is present in all body fluids but 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 kidney.

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 excrete the rest in the urine to maintain the sodium concentration in blood within a very narrow range. It does this by:

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

Abnormal blood sodium is usually due to some problem with one of these control systems. When the level of sodium in the blood changes, the water content in the body also changes. These changes can be associated with dehydration or edema, especially in the legs.

How is the sample collected for testing?

A blood sample is taken by needle from the arm. In some cases, a 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.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana K, Pagana T. Mosby’s Manual of Diagnostic and Laboratory Tests. 4th edition, Pp. 479-482; 989-992.

USDA. Dietary Guidelines for Americans, 2010. Available online at through Accessed August 2011. Hyponatremia. Available online at through Accessed August 2011. 

Sources Used in Previous Reviews

Tietz. Fundamentals of Clinical Chemistry, 2001; Fifth edition. Elsevier Health Sciences.

Boh, LE. Pharmacy Practice Manual, 2001; Second edition. Lippincott Williams & Wilkins.

NewsTarget. Pass on the Salt: News Release. Friday, August 18, 2006. Available online through Accessed January 2008.

Jacobs & DeMott. Laboratory Test Handbook, 2001; 5th edition. Lexi-Comp, Inc.

Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. St. Louis: Elsevier Saunders; 2006.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC.

Food and Nutrition Board. Institute of Medicine of the National Academies, Dietary Reference Intake Tables, Water and Electrolytes. PDF available for download at

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri.

Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006.

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(1995-2004). Minerals and Electrolytes. The Merck Manual of Medical Information Second Home Edition [On-line information]. Available online through

Ben-Joseph, E., Reviewed (2004 July). Dehydration. Information for Parents [On-line information]. Available online through

A.D.A.M. editorial, Updated (2003 October 15). Electrolytes. MedlinePlus Medical Encyclopedia [On-line information]. Available online at