Hyponatremia diagnostic study of choice

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Saeedeh Kowsarnia M.D.[2]

Overview

Best diagnostic test to measure hyponatremia, serum sodium < 135 mEq/L, is direct ion-specific electrode potentiometry. Other tests are associated with false results in certain conditions.

Different etiologies of hyponatremia are differentiated based on serum osmolality, urine osmolality, and urine sodium.

To see the different caused of hyponatremia, click here.

Study of choice

Previously, there were two methods to determine serum sodium    :

• Flame emission spectrophotometry

• Ion-specific electrode (ISE) potentiometry

ISE potentiometry has two different subtypes: Direct (undiluted) and indirect (diluted).

Direct ISE measures plasma sodium directly from a whole-blood sample and it's not associated with either pseudohyponatremia or pseudonormonatremia.

FES or indirect ISE requires sample dilution before assay and both are associated with pseudohyponatremia.

Sequence of Diagnostic Studies

The most diagnostic studies which can help to diagnose and differentiate between different causes of hyponatremia are: Serum osmolality, urine osmolality, urine sodium.

Biochemical evaluation for finding the etiologies of hyponatremia :

• Serum sodium
• Serum osmolality
• Serum potassium
• Serum chloride
• Serum creatinine
• Serum other solutes
• Serum urea
• Blood Glucose
• Total protein and albumin
• Serum lipids
• Total bilirubin and direct bilirubin
• Red and white cell blood count
• Serum cortisol
• Adrenocorticotropine hormone
• ADH level
• TSH
• Urine sodium
• Urine chloride
• Urine osmolality
• Urine for other solutes
• Fraction excretion of sodium
• Calculated GFR

For differential diagnosis click here.

Diagnostic Approach to Hyponatremia

Serum sodium < 135 mEq/L

Psuedohyponatremia • Check for hyperglycemia • Check for hyperproteinemia • Check for hyperlipidemia • Check for other solutes in serum • Check for sign of Jaundice • Check for history of operation

Measure serum Osmolality

Low < 280 momol/kg

Normal or High > 280 momol/kg

Hypotonic Hyponatremia

Isoosmolar or Hyperosmolar hyponatremia

• Low GFR • History of Thiazide use

• Post TURP or hysteroscopy (Glycine, Sorbitol) • Check direct sodium by direct potentiometry if normal measure total protein and lipid

No

Yes

• Renal failure • Thiazide induce hyponatremia

Patients with edema (pulmonary, peripheral),ascites

Signs and Symptoms of hypovolemia (↓ BP, Orthostatic hypotension)

Heart failure Cirrhosis

Less < 25 mEq/L Hypovolemic Hyponatremia • Extra renal loss Gastrointestinal losses, Diuretics, Third space losses

No

Yes

Measure urine sodium and serum osmolality

Measure urine sodium

25 to 40 mEq/L • Infuse Isotonic saline 1 liter over 1 hour • Remeasure urine sodium

Urine Osm < 100

Hight > 40 mEq/L Hypovolemic Hyponatremia • Renal loss

Yes

No

Urine osmolality measured After therapy initiated

Yes

Use of diuretics

Recovery from one of the followings: • Mild hypovolemia(Patients given isotonic fluids • Hypopituitarism (Patients given glucocorticoids))

Low cortisol, Positive ACTH stimulation Primary adrenal insufficiency

Urine Na > 40 mEq/L

No

Head injury/surgery Cerebral-salt wasting

No

Yes

Patient with rapid water consumption

Ensure that sodium intake > 150 mEq/L over next 24 hours (infuse 1 liter of isotonic fluid over one or more hour)

Remeasure urine osmolality and sodium

No

Yes

High-fluid low-protein diet including: • Beer potomania • Tea and toast diet

Water intoxication: • Psychosis • Endurance activity (Marathone) • Ecstasy use

Urine Na >40 or Urine Osm > 100

Urine Na < 40 or Urine Osm < 100

Hypovolemic Hyponatremia

Check for: • Glucocorticoid deficiency with Cortisol level and ACTH stimulationand • Hypothyroidism with TSH

check morning cortisol and ACTH stimulation test

Yes

No

Glucocorticoid deficiency

Elevated TSH

No

Yes

SIAD Nephrogenic SIAD Reset Osmostat

Severe Hypothyroidism

Evaluate underlying etiology