Urinary fractional electrolyte excretion rate

Cortisol is rarely useful in equine medicine. Equine hyper- and hypo-adrenocorticism are both extremely rare. Common endocrinopathies such as Equine Metabolic Syndrome and PPID have been shown to have similar total cortisol concentrations to those of normal horses. There is some evidence that the free (non-protein bound) fraction of serum cortisol might be increased in PPID or EMS cases, although this requires further investigation and currently access to testing for the free fraction is difficult. Low cortisol levels might be associated with some critical care cases (CIRCI) although replacement therapy is controversial.

Intended to estimate total body status of electrolytes by comparing serum and urine
concentrations.

• Inevitably an approximation as many factors affect results.
• Simultaneously collected serum and urine samples.
• Serum must be separated from the cellular fraction promptly (within 2 hours)

Typical normal values

Na+	0.03-0.5%
K+	15-70%
Cl-	0.2-1.7%
Ca2+	<7%
PO4 3-	<0.5%
Mg2+	<15%

• High values (signifying decreased reabsorption) across several electrolytes are consistent with renal tubular failure (especially Na+ and Cl-) although excessive electrolyte consumption or fluid therapy should also be considered.
• Low Ca2+ values (signifying reduced excretion) and high PO4 3- values (increased excretion) is typical parathyroid hormone effect indicating Ca2+ deficiency (or PO4 3- excess).
• Low Na+ values (signifying reduced excretion) and high K+ values (increased excretion) is a typical aldosterone hormone effect indicating Na+ deficiency.

Assessment of the overall body electrolyte status is difficult due to the fact that some electrolytes are primarily intracellular and also that there are several effective homeostatic processes that maintain normal serum concentrations whatever the total body status is. Urinary fractional excretion ratios make use of the fact that many homeostatic mechanisms work by altering renal excretion/reabsorption and this can be detected by comparing serum and urine concentrations. Calculation of electrolyte fractional excretion rates require simultaneously collected serum and urine samples. It is vital that serum is separated from the cellular fraction promptly (within 2 hours) as leakage of potassium and phosphate from dying cells will significantly alter serum electrolyte concentrations. The basis of the test is the assumption (slightly incorrect!) that the sole route of serum creatinine loss is the urine and that it is neither secreted nor absorbed by the kidney. Thus, each electrolyte is compared with creatinine, that is supposedly neither secreted nor absorbed, to give an impression of the renal handling of each electrolyte. Hence, the clearance of any substance (e.g. an electrolyte) can be compared to creatinine as a fractional excretion rate (or creatinine clearance ratio) thus:

Fractional electrolyte excretion rate = urine output x [UELEC] ÷ urine output x [UCREAT]
[SELEC] [SCREAT]
= [UELEC] x [SCREAT] where [Ux] is urinary concentration and [Sx] is serum concentration
[UCREAT] x [SELEC]

A value of 1 (or 100%) suggests similar handling to creatinine – i.e. supposedly neither secreted nor absorbed. Values <1 (or <100%) signify active reabsorption of the electrolyte whereas values >1 (or 100%) signify active excretion. Typical normal values comprise:

Na+	0.03-0.5%	(i.e. 99.5 – 99.97% filtered Na+ is reabsorbed)
K+	15-70%	(i.e. 30 – 85% filtered K+ is reabsorbed)
Cl-	0.2-1.7%	(i.e. 99.8 – 98.3% filtered Cl- is reabsorbed)
Ca2+	<7%	(i.e.>93% filtered Ca2+ is reabsorbed)
PO4 3-	<0.5%	(i.e.>99.5% filtered PO4 3- is reabsorbed)
Mg2+	<15%	(i.e.>85% filtered Mg2+ is reabsorbed)

High values (signifying decreased reabsorption) across several electrolytes are consistent with renal tubular failure (especially Na+ and Cl-) although excessive electrolyte consumption or fluid therapy should also be considered.
Low Ca2+ values (signifying reduced excretion) and high PO4 3- values (increased excretion) is a typical parathyroid hormone effect indicating Ca2+ deficiency (or PO4 3- excess).
Low Na+ values (signifying reduced excretion) and high K+ values (increased excretion) is a typical aldosterone hormone effect indicating Na+ deficiency.