Fractional electrolyte excretion (FE) values (%)

FE rate (%) = Urine electrolyte x serum creatinine/urine creatinine x serum electrolyte.

Fractional electrolyte excretion (FE) values (%) below (except for Mg) Stewart AJ (2013).

The Merck Veterinary Manual) for normal horses are based on combined references given by Harris (1988) and Higgins and Snyder (1998).

Reference intervals (adult)
Sodium	0.01-1.0%
Potassium	15-80%
Phosphate	0.0- 1.18%
Chloride	<4.0%
Calcium	2-39%
Magnesium	1-35%

Urine fractional electrolyte and mineral excretion ratios measure the clearance of electrolytes and minerals from the plasma against that of creatinine. Creatinine is selected because excretion is nearly constant with normal renal tubular function. Creatinine and electrolyte/mineral concentrations are measured in both urine and serum/plasma collected within 1 hour of each other and the results are expressed as a % clearance ratio.

Fractional Electrolyte (FE) excretion is used to determine whether there are any potential metabolic cause of myopathy in horses that show signs of exertional rhabdomyolysis.

The normal ranges can be very variable and are affected by

• Diet
• Age
• Recent exercise (and type of work the horse does)
• Water intake

Thus a single sample for FE values cannot be used for diagnosis.

Spot or single sample FE values from any horse will vary during the day. Most of this variation is due to changes in the urinary rather than the plasma electrolyte concentrations. However, plasma potassium concentrations are falsely elevated by haemolysis and decreased potassium concentrations have been found after exercise (up to several hours). Potassium concentrations also decrease while a horse is eating, increasing to a peak several hours after ingestion. Calcium status may also fluctuate.

A blood sample should be collected, therefore, before exercise and prior to feeding with the plasma/serum being separated as soon as possible.

NB Some workers consider that, due to the diurnal variations in urinary electrolyte concentrations, it would be unwise to rely on the accuracy of any result obtained from, a single spot sample; although others have suggested that the single sample may be sufficiently representative to enable certain metabolic abnormalities to be detected.

When screening for abnormalities that may result in myopathies, diet needs to be regulated for 6-8 prior to testing and then repeat FE assessed when the signs recur for comparison.

Simultaneous blood and urine values should be taken.

In horses with PuPd, high FE values (especially sodium, chloride) may be consistent with renal tubular failure. However FE values should be correlated with blood urea, creatinine and USG values.

Additional applications of FE ratios

• Excess dietary P increases the FE of P, inadequate Ca intake decreases the FE of Ca; this can indicate nutritional imbalances which may lead to developmental orthopaedic disease or nutritional secondary hyperparathyroidism
• Decreased FE of K indicates K deficiency and is a better indicator of overall whole body status of K than serum
• Low FE indicates the need for electrolyte supplementation in athletes

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