What this test measures
Sodium is the main positively-charged ion in extracellular fluid, accounting for most of serum osmolality. Its concentration is tightly defended (135–145 mEq/L) by thirst, antidiuretic hormone (ADH / vasopressin) and the kidneys. Changes in serum sodium almost always reflect changes in body water, not body sodium content — hyponatraemia means there is too much water relative to sodium (often), and hypernatraemia means there is too little water.
Sodium is part of every electrolyte panel and is interpreted alongside the patient's volume status, urine sodium, urine osmolality and any relevant drugs.
Why it matters
In India, hyponatraemia is the commonest electrolyte abnormality in hospitalised patients and a leading cause of altered mental status, falls and seizures, especially in the elderly. Causes include SIADH (drugs, malignancy, lung disease), diuretic use, heart failure, cirrhosis, CKD, and hypothyroidism / adrenal insufficiency. Many older Indians on thiazide diuretics for hypertension develop unrecognised chronic hyponatraemia.
Hypernatraemia is most often seen in elderly patients with limited access to water (after surgery, sedation, dementia), in diabetes insipidus, and in severe diarrhoea / vomiting without water replacement. Rapid correction in either direction is dangerous — chronic hyponatraemia corrected too fast can cause osmotic demyelination ("locked-in syndrome"); the speed of correction matters as much as the target.
How to prepare
No fasting required. Routine venipuncture. Mention all medications (especially thiazide diuretics, SSRI antidepressants, carbamazepine, ACE inhibitors), recent vomiting / diarrhoea, and IV fluids in the last 24 hours.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Sodium (mEq/L)[1][2] | 135 – 145 | Hyponatraemia (<135). Mild (130–134) — often asymptomatic. Moderate (125–129) — nausea, headache, confusion. Severe (<125) — seizures, coma. Causes — SIADH (most common; triggers: drugs, lung disease, malignancy, CNS disease), thiazide diuretics, heart failure, cirrhosis, CKD, hypothyroidism, adrenal insufficiency, polydipsia, severe diarrhoea / vomiting with water replacement. | Hypernatraemia (>145). Mild (146–155). Moderate (155–165). Severe (>165). Causes — dehydration with limited water intake (elderly, infants, after surgery, sedated patients), diabetes insipidus (central or nephrogenic), excessive salt intake / hypertonic IV fluids. Symptoms — thirst, lethargy, confusion, seizures. Correction must be slow (<10–12 mEq/L per 24 hours) to avoid cerebral oedema. |
Working up hyponatraemia by volume status
| Volume status | Likely cause | Treatment approach |
|---|---|---|
| Hypovolaemic (dry) | Vomiting, diarrhoea, diuretics, salt-losing nephropathy, adrenal insufficiency | Isotonic saline; treat cause |
| Euvolaemic | SIADH (drugs, lung, malignancy, CNS), hypothyroidism, primary polydipsia | Fluid restriction; treat cause; vaptans / hypertonic saline if severe |
| Hypervolaemic (oedema) | Heart failure, cirrhosis, nephrotic syndrome, CKD | Fluid + salt restriction; loop diuretic; treat underlying disease |
Frequently asked questions
My sodium is 131. Should I worry?
Mild hyponatraemia is common, especially in older people. Confirm with a repeat sample. If genuine, your doctor will review medications (thiazide, SSRI, carbamazepine), check thyroid and cortisol, and assess your volume status — treatment depends on the cause.
Do I need to fast?
No fasting required for a sodium test.
What is SIADH?
Syndrome of Inappropriate Antidiuretic Hormone — the body retains water inappropriately, diluting sodium. Common triggers in India are pneumonia and TB, drugs (SSRIs, carbamazepine, antipsychotics), CNS disease, and malignancy. Treatment is fluid restriction and addressing the cause.
Can thiazide diuretics cause low sodium?
Yes. Thiazide-induced hyponatraemia is a common, often missed cause of chronic mild hyponatraemia in older Indians on hypertension treatment. Switching to a non-thiazide agent usually corrects it.
Why does hyponatraemia have to be corrected slowly?
Rapid correction (>10–12 mEq/L in 24 hours) can cause osmotic demyelination ("central pontine myelinolysis") — a serious, often permanent neurological injury. Chronic hyponatraemia in particular needs cautious, monitored correction.
My sodium is 150. What does that mean?
Mild hypernatraemia — most often dehydration in someone who could not access water (elderly, hospitalised, post-surgery). Treatment is to gradually replace water. If accompanied by very dilute urine, diabetes insipidus needs evaluation.
Can I get tested at home if I am very sick?
For an acute presentation with confusion, seizures or severe vomiting, do not wait for a home test — go to an emergency department. Sodium can change quickly and needs prompt monitoring.
Related Kidney / Electrolytes tests
Tests commonly ordered alongside SODIUM, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Sodium Blood Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Hyponatremia · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Hyponatremia Clinical Practice Guideline · accessed 2026-05-30T00:00:00.000Z
Book with Zelnoo
Get your SODIUM test done at home — transparent prices, NABL-accredited labs.
Zelnoo lets you compare diagnostic test prices across NABL-accredited labs in Mumbai & Thane, book a free home phlebotomist visit, and receive digital reports in 24–48 hours into a consent-first report vault. No subscriptions, no membership fees — pay only for the test you book.
Book SODIUM now