Sodium is the major extracellular cation and exerts a major influence on plasma osmolality. It plays a central role in maintaining the normal distribution of water and osmotic pressure. Changes in serum sodium most often reflect changes in water balance rather than sodium balance. It is adjusted by antidiuretic hormone (ADH) secretion and the thirst receptors to maintain plasma osmolality and volume. Aldosterone causes tubular reabsorption of sodium. Atrial natriuretic peptide hormone decreases sodium reabsorption.
Use
Diagnosis and treatment of dehydration and overhydration. If a patient has not received large load of sodium, hypernatremia suggests need for water, and values <130 mEq/L suggest overhydration.
Electrolyte, acid " “base balance; water balance; water intoxication.
Normal range: 135 " “145 mmol/L.
Critical values: <121 or >158 mmol/L.
Increased In
Conditions associated with water loss in excess of salt loss through the skin, lungs, GI tract, and kidneys
Dehydration " ”inadequate fluid intake to replace dermal, respiratory, or GI loss of fluid
Posttraumatic: caused by tumors, cysts, histiocytosis, TB, sarcoidosis
Idiopathic: caused by aneurysms, meningitis, encephalitis, Guillain " “Barre syndrome
Renal failure and other renal causes: loop diuretics, osmotic diuresis (glucose, urea, mannitol), postobstructive diuresis, polyuric, phase of acute tubular necrosis, intrinsic renal disease
Decreased In
Hyponatremia (defined as serum sodium <135 mmol/L after the exclusion pseudohyponatremia). This can be classified as three types depending upon extracellular fluid (ECF) status.
Hypovolemic hyponatremia (reduced ECF)
Renal loss of Na and water: caused by diuretic use, salt-wasting nephropathy, cerebral salt wasting, adrenal insufficiency, renal tubular acidosis
Extrarenal loss of Na and water with renal conservation: caused by burns, GI loss, pancreatitis, bowel obstruction, blood loss
Hypervolemic hyponatremia (expanded ECF and ICF but reduced effective arterial blood volume): caused by CHF, cirrhosis, nephrotic syndrome
Euvolemic hyponatremia (expanded ECF and ICF without edema: caused by thiazide diuretic use, hypothyroidism, adrenal insufficiency, SIADH secretion)
Limitations and Interferences
Plasma Na levels depend greatly upon the intake and excretion of water and to a somewhat lesser degree the renal regulation of Na.
Determinations of blood sodium and potassium levels are not useful in diagnosis or in estimating net ion losses but are performed to monitor changes in sodium and potassium during therapy.
Hyperglycemia " ”serum sodium decreases 1.7 mEq/L for every increase of serum glucose of 100 mg/dL.
Hyperlipidemia and hyperproteinemia, which cause spurious results only with flame photometric but not with specific ion electrode techniques for measuring sodium.
Pseudohyponatremia caused by "water exclusion effect " ¯ observed on indirect ISE (ion-selective electrode) measurements due to the dilution of samples and transfusion of blood products and due to infusion of IV immunoglobulins.