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In hypovolemic and hyponatraemic animals calculation of the fractional excretion of sodium (FENa) can be used to determine if the kidneys are the source of excessive sodium loss (see equation below). The FENa should be less than 1% for nonrenal sources of sodium loss and 1% or greater if sodium is being lost by the kidneys (de Morais and DiBartola, 2008a).

Calculation of the fractional excretion of sodium is:

where FENa = fractional excretion of sodium, UNa = urine concentration of sodium (mEq/l), SNa = serum concentration of sodium (mEq/l), UCr = urine concentration of creatinine (mg/dl) and SCr = serum concentration of creatinine (mg/dl).

Additional diagnostic investigations including laboratory analysis and imaging will vary depending on the suspected underlying aetiologies of hyponatraemia (see Box 4.4). The reader is referred to internal medicine textbooks for further details on diagnosis and treatment of each condition.