Sodium Blood Test

What Does This Test Measure?

Sodium is the primary extracellular cation and the main determinant of plasma osmolality and fluid balance. Sodium concentration reflects water balance more than sodium balance; low sodium usually means too much water, not too little sodium. It is tightly regulated by ADH, thirst, and kidney function.

Why Your Doctor Ordered This Test

Sodium is ordered whenever a person shows signs of disturbed fluid balance (confusion, lethargy, seizures, extreme thirst, or uncontrolled vomiting and diarrhea) because sodium concentration reflects the body's water content more than its salt content. Emergency departments draw it immediately in altered mental status to rule out severe hyponatremia (<120 mEq/L) or hypernatremia (>160 mEq/L), both of which can cause life-threatening brain swelling or shrinkage. Physicians also check sodium when starting or adjusting diuretics (especially thiazides), treating heart failure or cirrhosis, or monitoring patients on SSRIs, carbamazepine, or lithium, all of which can provoke SIADH or nephrogenic diabetes insipidus. In primary care, unexplained fatigue, muscle cramps, or falls in the elderly often prompt a sodium test to uncover chronic, mild hyponatremia that contributes to gait instability.

Beyond emergencies, sodium is a core metabolic panel component used pre-operatively and in routine health screenings to flag unsuspected water imbalance. In patients with pituitary or adrenal disorders, sodium helps evaluate ACTH and aldosterone deficiencies that impair free water clearance. For those with excessive urination (polyuria) and thirst, the combination of sodium, serum osmolality, and urine osmolality distinguishes water deprivation states, primary polydipsia versus central or nephrogenic diabetes insipidus, by revealing whether the sodium is diluted, concentrated, or inappropriate. Ultimately, the goal is to identify the underlying cause: true sodium depletion (rare), water overload (common), or a more complex endocrine or renal problem that threatens fluid homeostasis.

What High Sodium Means

High sodium (hypernatremia, >145): almost always due to water loss, not sodium gain. Causes: dehydration (inadequate water intake, elderly, infants, impaired thirst), diabetes insipidus (ADH deficiency or resistance), excessive water loss (diarrhea, vomiting, burns, diuretics), and rarely excessive sodium intake (salt tablets, hypertonic saline). Mild hypernatremia causes thirst; severe (>160) causes confusion, seizures, and coma.

What Low Sodium Means

Low sodium (hyponatremia, <135): usually reflects excess water relative to sodium. Causes: SIADH (drug-induced, SSRIs, carbamazepine, cyclophosphamide; pulmonary disease; CNS disorders), heart failure and cirrhosis (reduced effective arterial blood volume triggers ADH release), diuretics (especially thiazides), hypothyroidism, adrenal insufficiency, polydipsia (excess water intake), and beer potomania (very low solute intake). Severity and speed of onset determine symptoms: mild/chronic may be asymptomatic; acute/severe (<120) causes confusion, seizures, and coma. Correct chronic hyponatremia slowly to avoid osmotic demyelination syndrome.

How to Prepare and What Affects the Result

Most sodium measurements are performed as part of a basic or comprehensive metabolic panel that may require an 8–12 hour fast, but fasting is not a direct requirement for sodium itself; it simply prevents lipemic interference and addresses glucose and lipid panels drawn simultaneously. However, recent water intake can transiently lower sodium, so avoid drinking large volumes of water immediately before the blood draw if hyponatremia is being investigated. Exercise and heavy sweating concentrate sodium, so the test should be drawn in a rested, euhydrated state. Certain medications profoundly alter sodium: thiazide diuretics (hydrochlorothiazide) are a leading cause of drug-induced hyponatremia; SSRIs (fluoxetine, sertraline), carbamazepine, and cyclophosphamide can stimulate SIADH; lithium and demeclocycline may cause nephrogenic diabetes insipidus raising sodium. If possible, the ordering physician will consider whether to hold these medications temporarily, but such decisions must be made on clinical grounds.

The blood sample is collected in a serum separator or plasma tube (lithium heparin), and sodium is measured by ion-selective electrode. Pseudohyponatremia, a falsely low result, occurs when serum contains extremely high lipid or protein levels (e.g., severe hypertriglyceridemia >1,500 mg/dL, multiple myeloma) analyzed by an indirect ion-selective electrode, because the instrument assumes a normal water fraction. A direct ion-selective electrode (blood gas analyzer) gives the true, physiologically active sodium. Therefore, if the sodium result conflicts with the clinical picture, repeating the test using a direct method or checking serum osmolality helps confirm true hyponatremia versus lab artifact. Prolonged tourniquet time can cause hemoconcentration and falsely elevate sodium, so the draw should be performed with minimal stasis.

Common Misinterpretations

The most common mistake is assuming that a high sodium result reflects dietary salt overload. In reality, hypernatremia almost always signals a water deficit, not sodium excess, so focusing on salt restriction while missing inadequate water intake (elderly, debilitated) or polyuria from diabetes insipidus delays correct treatment. Conversely, low sodium is frequently blamed on 'low salt' intake when the true problem is water retention from heart failure, cirrhosis, or SIADH; recommending salt tablets without addressing the water imbalance can worsen hyponatremia. Another critical error is failing to correct sodium for severe hyperglycemia: each 100 mg/dL rise in glucose above normal pulls water into the extracellular compartment, diluting sodium by roughly 1.6–2.4 mEq/L. A sodium of 130 mEq/L during diabetic ketoacidosis may be pseudohyponatremia that resolves with insulin, not true sodium depletion requiring hypertonic saline.

Pseudohyponatremia from hypertriglyceridemia or paraproteinemia is often overlooked when indirect ion-selective electrode methods are used, leading to unnecessary fluid restriction or demeclocycline therapy. Additionally, clinicians sometimes misinterpret mild chronic hyponatremia (128–134 mEq/L) as harmless; in the elderly, even mild hyponatremia increases the risk of falls, cognitive decline, and bone fractures, and it may unmask underlying SIADH or diuretic use. On the high-sodium side, mistaking essential hypernatremia (reset osmostat) for diabetes insipidus leads to dangerous water forcing that the patient's thirst mechanism resists. Finally, acute hyponatremia after surgery (due to hypotonic fluids and high ADH) is frequently misdiagnosed as overhydration alone, but the real driver is non-osmotic ADH secretion, requiring rapid recognition to prevent osmotic demyelination if corrected too hastily.

Frequently Asked Questions

What causes high sodium levels?

Hypernatremia (sodium >145 mEq/L) is most often due to water loss, not excess sodium intake. Common causes include inadequate water intake (elderly, infants, impaired thirst), diabetes insipidus where the kidneys lose large amounts of dilute urine, vomiting, diarrhea, burns, and excessive sweating. Certain diuretics and lithium can also cause water wasting. A rare cause is accidental ingestion of salt tablets or hypertonic saline. The hallmark is thirst, progressing to confusion and seizures at levels above 160 mEq/L.

Can low sodium cause confusion or seizures?

Yes. Acute hyponatremia (especially <125 mEq/L) disrupts brain cell volume regulation, causing water to move into brain cells and leading to cerebral edema. This manifests as headache, nausea, confusion, lethargy, and can progress to seizures and coma if untreated. Chronic mild hyponatremia often causes subtle cognitive slowing and unsteady gait, but a rapid drop, such as after surgery or polydipsia, can precipitate severe neurological symptoms. Emergency treatment focuses on controlled sodium correction to avoid permanent brain damage.

What medications can lower sodium levels?

Thiazide diuretics (hydrochlorothiazide) are a leading drug cause of hyponatremia. SSRIs (fluoxetine, sertraline), carbamazepine, and cyclophosphamide stimulate antidiuretic hormone release (SIADH). NSAIDs can potentiate water retention. Vincristine and certain antipsychotics have also been implicated. These drugs impair the kidney's ability to excrete free water. If hyponatremia develops, the medication's role is evaluated first; stopping it often resolves the sodium disturbance over days.

What does a sodium blood test examine?

A sodium blood test measures the concentration of sodium in the liquid portion (plasma or serum). Sodium is the major positive ion outside cells, so its level reflects the body's water balance. The test helps diagnose dehydration, overhydration, and disorders of antidiuretic hormone (ADH), such as SIADH or diabetes insipidus. It is part of the basic metabolic panel and is used to evaluate kidney function, fluid status, and the effect of medications on electrolyte balance.

How is low sodium treated?

Treatment depends on the cause and severity. Mild asymptomatic hyponatremia (127–134 mEq/L) is often managed by restricting fluid intake to less than 1–1.5 liters per day and addressing the underlying condition (diuretic cessation, treating SIADH). Severe or symptomatic hyponatremia (<120 mEq/L with neurological signs) requires cautious infusion of hypertonic saline (3% sodium chloride) in a hospital setting, with sodium correction not exceeding 8–10 mEq/L per 24 hours to avoid osmotic demyelination syndrome. Loop diuretics may be added to promote free water excretion.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Sodium result: