Sodium Clinical Chemistry

The distribution of sodium in the body differs from that of potassium. Sodium is the predominant extracellular ion. Renal function is the single most important homeostatic mechanism in relation to plasma concentrations of sodium and potassium.

Small animals

Marked hyponatraemia or a Na:K ratio <25:1 warrants further investigation. Marked hyponatraemia may cause fluid movement into cells with an effect on neurological function. Hypernatraemia is uncommon in small animals. It almost always indicates water intake which is inadequate to balance fluid losses. Marked hypernatraemia may lead to neurological signs due to the net movement of water out of the cells.

Causes of hypernatraemia

• Increased sodium intake (salt poisoning)
• Alimentary - vomiting, diarrhoea (without adequate fluid replacement), small intestinal obstruction
• Hyperadrenocorticism (mild changes)
• Other polyuric disorders including renal loss due to diuretics or intrinsic renal disease
• Insensible - panting
• Diabetes insipidus, central or nephrogenic
• Water deprivation
• Inability to drink
• Primary hypodipsia in Miniature Schnauzers

Causes of hyponatraemia

• Severe diarrhoea or vomiting
• Hypoadrenocorticism
• Blood loss
• Third space loss, for example peritonitis, ascites, ruptured bladder
• Psychogenic polydipsia
• Nephrotic syndrome
• Advanced renal failure
• Syndrome of inappropriate ADH secretion (rare)

Complementary tests

In small animals a Na:K ratio is useful as an aid to the diagnosis of hypoadrenocorticism; a ratio of <27:1 is suspicious, <25:1 is suggestive. An ACTH stimulation test should be considered for confirmation in dogs with appropriate clinical signs. This ratio may be affected by some gastrointestinal conditions and other diseases leading to effusions.

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Equine

Sodium levels reflect the relative amounts of water and electrolytes in the extra-cellular fluid. It is the principle determinant of ECF volume. Hyponatraemia occurs more commonly due to excessive losses than to reduced intake. Hypernatraemia indicates water loss in excess of electrolytes.

Causes of hypernatraemia

• Dehydration
• Excess saline therapy/salt poisoning
• Water deprivation

Causes of hyponatraemia

Decreased fluid volume

• Alimentary: vomiting, diarrhoea, excess salivation, oesophageal obstruction
• Blood loss
• Excessive sweating
• Fluid drainage for example, pleural fluid or gastric reflux

Sequestration of fluid in third space:

• Ascites
• Urinary tract disruption for example, ruptured bladder in neonatal foals
• Peritonitis
• Gut torsion

Complementary tests

Fractional urinary electrolyte excretion rates are more useful for assessing electrolytes than serum electrolyte assays.

Test Codes - Please visit www.nwlabs.co.uk or see our current price list for more information

Authors & References

NationWide Laboratories