This test measures the amount of sodium (salt) in blood. Sodium testing is a part of the routine lab evaluation of most patients. It is one of the blood electrolytes, which are often ordered as a group. The most common group of blood electrolytes is sodium, potassium, chloride, and bicarbonate (total CO2). These electrolytes are also included in the basic metabolic panel, a larger group of tests widely used when someone has non-specific health complaints. High serum sodium levels (hypernatremia) occur at sodium concentrations over 145 mM, with severe hypernatremia over 152 mM. Hypernatremia is usually caused by diseases that cause excessive urination. In these cases, water is lost, but sodium is still retained in the body. The symptoms include confusion and can lead to convulsions and coma. Low serum sodium levels (hyponatremia) are below 130 mM, with severe hyponatremia at or below 125 mM. Hyponatremia often occurs with severe diarrhea, with losses of both water and sodium, but with sodium loss exceeding water loss. Hyponatremia provokes clinical problems only if serum sodium falls below 125 mM, especially if this has occurred rapidly. The symptoms can be as mild as tiredness but may lead to convulsions and coma.