Biomarker: Sodium, Blood

Overview

Blood sodium measures the concentration of sodium ions in serum or plasma. Sodium is the principal extracellular electrolyte and helps regulate fluid distribution, blood volume, nerve signaling, and muscle function. Low levels, called hyponatremia, or high levels, called hypernatremia, may reflect disorders of water balance, dehydration, kidney or adrenal disease, medication effects, or other medical conditions. The test is clinically useful for screening, evaluating, and monitoring fluid and electrolyte disturbances, often as part of an electrolyte, basic metabolic, or comprehensive metabolic panel.

Clinical Use Cases

Screening for electrolyte and fluid-balance abnormalities.
Evaluation of suspected hyponatremia or hypernatremia.
Assessment of dehydration, vomiting, diarrhea, excessive sweating, or burns.
Evaluation of kidney, adrenal, cardiac, hepatic, or endocrine disorders that affect water or sodium balance.
Assessment of symptoms such as confusion, weakness, headache, nausea, seizures, or excessive thirst.
Monitoring patients receiving intravenous fluids or treatment for sodium disorders.
Monitoring medications that may affect fluid or electrolyte balance, including diuretics, lithium, some antidepressants, and selected blood-pressure medicines.
Supporting evaluation of disorders such as syndrome of inappropriate antidiuretic hormone secretion, diabetes insipidus, Addison disease, and hyperaldosteronism.

Specimen Types

Serum from venous blood.
Plasma from venous blood.
Whole blood in selected point-of-care or blood-gas analyzers.
Urine sodium is a separate measurement that may be used with blood sodium to investigate the cause of an abnormal result.

Measurement Methods

Direct ion-selective electrode measurement using an undiluted specimen.
Indirect ion-selective electrode measurement using a diluted specimen, commonly used in automated chemistry analyzers.
Flame atomic-emission photometry, an older method that has largely been replaced by ion-selective electrodes.
Point-of-care electrolyte analysis using direct ion-selective electrodes.

Test Preparation and Influencing Factors

Fasting is not always required for an isolated sodium test, but several hours of fasting may be requested when sodium is measured as part of another panel.
Patients should report all prescription medicines, over-the-counter medicines, and supplements. Diuretics, lithium, some antidepressants, nonsteroidal anti-inflammatory drugs, corticosteroids, and certain blood-pressure medicines may affect sodium results.
Medicines should not be stopped unless specifically instructed by a healthcare professional.
Recent vomiting, diarrhea, sweating, burns, inadequate fluid intake, or excessive water intake can alter the result.
Kidney, heart, liver, adrenal, and thyroid disorders can affect sodium concentration.
Hyperglycemia and marked abnormalities in blood lipids or proteins can affect interpretation, particularly with indirect ion-selective electrode methods.
Collection and processing should follow laboratory requirements because prolonged storage or improper handling can affect specimen quality.
Results should be interpreted with clinical volume assessment and, when indicated, serum osmolality, urine osmolality, urine sodium, glucose, and other electrolytes.