Sodium
What it measures, and what changes the number.
Sodium is the main positively charged electrolyte in the fluid outside cells. The test measures its concentration in serum or plasma and reflects the balance between sodium and water in the body, which is regulated by the kidneys and hormones.
It is part of the basic metabolic panel and is read with potassium, chloride, glucose, urea and creatinine, since these together describe fluid, electrolyte and kidney status.
Most labs use ion-selective electrodes. The indirect method, common on large analyzers, can report a falsely low value when blood fats or proteins are very high, while the direct method is not affected in the same way. Very high glucose also shifts measured sodium. Reports may list it as Sodio (Spanish) or Sodi (Catalan).
Often read alongside: Potassium, Chloride, Glucose, Urea, Creatinine