Serum sodium
Alternative names:
Na+; sodium - serum
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.
How to prepare for the test:
The health care provider may advise you to withhold drugs that may affect the test (see special considerations).
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
How the test will feel:
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed:
This test is performed when symptoms of sodium imbalance are present, or when disorders associated with abnormal sodium levels develop.
Sodium (Na+) is the major positive ion of the extracellular (outside of the cell) fluids. The concentration of sodium inside cells is only about 5 mEq/L compared to 140 outside. Sodium is the major determinant of extracellular osmolality. The sodium content of the blood is a result of a balance between dietary intake and renal excretion (only a small percent is lost through the stool or sweat).
Many factors affect sodium levels, including the steroid hormone aldosterone (for more information see the aldosterone test) which decreases loss of sodium in the urine. ANP (atrial natriuretic protein) is a hormone secreted from the heart that increases sodium loss from the body. Water and sodium are interrelated in that, for example, retention of increased sodium is followed by retention of fluid and vice versa. However, the body is able to regulate sodium and water separately if necessary, controlling sodium by means of aldosterone and ANP and water by ADH (antidiuretic hormone).
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