Specimen requirement

Plasma

Urine

Fluid

Adult patient:

 Heparin gel tube

Minimum blood volume: 5 mL

Plain bottle

Minimum volume: 10 mL

24 Hr urine:

U1 bottle / U2 bottle

Plain bottle

Minimum volume: 10 mL

Paediatric patient:

Paediatric heparin tube

Number of vials: 1

Minimum blood volume: 1.3 mL

Authorisation code required

No

24 Hr available service

Yes

(Call extension: 3353 for express result)

Spot urine: Yes

24 Hr urine: No

No

Method

Ion selective electrode

Reference interval

Plasma:

(Source: from local population study in 2008)

24 Hr urine: 40 - 220 mmol/d
 
The urinary excretion of sodium varies significantly with dietary intake.  The values given are typical of subjects on average diet.

(Source: from manufacturer’s kit insert)

Clinical indication

Plasma:

1. Assessment and monitoring of fluid and electrolyte status, particularly in patients with renal or cardiac disease, possible sodium losing states, and in those receiving intravenous fluids

Urine:

1. Investigation of hyponatraemia
    

2. Assessment of renal function in hypovolaemic shock
    

3. Investigation of compliance with a low sodium diet
    

4. Investigation of predisposing factors for hypercalciuria in patients with renal calculi

Result interpretation

Plasma:

Sodium concentration is dependent on the state of hydration, body sodium content and water shifts between plasma and other body fluid compartments.

Intravenous therapy with isotonic saline may cause hypernatraemia and volume replacement with dextrose may cause hyponatraemia.

Hyponatraemia occurs in a small percentage of patients on diuretic therapy, particularly the elderly. Severe hyper­lipidaemia or hyperproteinaemia may cause 'pseudohyponatraemia'.

Sodium is retained with mineralocorticoid excess and lost with mineralocorticoid deficiency, gastrointestinal and renal loss, or excessive sweating.

Hyponatraemia as a result of fluid retention (dilutional hyponatraemia) is seen in renal and cardiac disease and with SIADH. Urine sodium estimation may assist in interpretation.

Urine:

Urine sodium is only useful when performed at the same time with a plasma sodium.

Urinary sodium excretion exceeds 20 mmol/L in hyponatraemia due to SIADH, diuretic therapy, or Addison’s disease.

In a patient with shock and oliguria a urinary sodium > 20 mmol/L or a fractional excretion of sodium > 1.5% suggests acute tubular necrosis.

High urine sodium increases urine calcium and predisposes to calculi containing calcium.

Measurement of uncertainty

See table.

Frequency of measurement

Daily