Page 952 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Urinary system 927

VetBooks.ir quantity of 10% acetic acid solution is added to the primary renal disease. The serum urea:creatinine
To assess the urine sediment accurately, a small
ratio has been used to distinguish between acute
sample to dissolve crystals. Crystals are usually abun-
dant in equine urine and may interfere with urine kidney injury (AKI) and chronic kidney disease
(CKD); however, results are highly variable and
sediment evaluation. Calcium carbonate crystals are have not proved useful in a clinical context.
the most common crystals of equine urine, followed Alterations in plasma and serum electrolyte lev-
by triple phosphate and, rarely, calcium oxalate. els may be encountered with certain types of uri-
Gamma-glutamyltransferase (GGT) is found nary tract disease. Sodium is typically lost with
in high concentrations in epithelial cells lining the polyuric renal failure, resulting in varying degrees
proximal renal tubules. Physiologically, its activity of hyponatraemia. Urinary tract disruption and/or
in urine arises from cell turnover. Any damage to uroperitoneum produce hyponatraemia through
the renal tubular epithelium will increase its activity resorption of urine, which is lower in sodium than
in urine. Alkaline phosphatase (ALP) is also abun- serum. Disrupted body electrolyte homeostasis also
dant in the renal tubules. Their activity in urine is affects serum concentration of chloride, which is
variable; therefore, their activity is expressed as an heavily excreted in polyuric renal failure in horses.
enzyme:creatinine ratio (urine enzyme activity:urine Serum potassium can be normal or elevated in renal
creatinine × 0.01). Ratios greater than 25 can be con- failure, and markedly elevated in cases of uroperito-
sidered suspicious, and values greater than 100 can neum. With AKI the excretion of phosphorous in
be considered abnormal, indicating tubular damage. urine is disrupted, causing an increase in its serum
Enzyme:creatinine ratios are also variable, have been concentration. AKI may also result in hypocalcae-
found to have a poor specificity, and values must be mia. However, hypercalcaemia and hypophospha-
interpreted in concert with other clinical and labora- taemia are often found in CKD. Serum albumin
tory findings. and globulin concentrations variably decrease in
chronic renal diseases. Albumin tends to be lost
Haematology and serum to a greater extent than globulin because of its low
chemical analysis molecular weight. In cases of neoplasia, glomeru-
Azotaemia, an increase in serum urea, may occur lonephritis, pyelonephritis or amyloidosis, serum
with pre-renal, renal or post-renal disease. Pre-renal globulin concentration may increase as a result of
azotaemia is the most common form and is typically chronic antigen stimulation. Serum enzyme activ-
associated with dehydration or other disturbances ity should be examined to assess the metabolism of
causing decreased renal perfusion. Renal azotaemia other organs and to differentiate pigmenturia.
is associated with intrinsic renal failure and does not Mild to moderate anaemia may be associated with
develop until there is a functional loss of approxi- CKD consequent to decreased erythropoietin pro-
mately 70% of nephrons. Post-renal azotaemia is duction and a shortened erythrocyte lifespan.
associated with urinary tract obstruction or rupture.
Identification of the cause of azotaemia is critical to Fractional excretion of electrolytes
the management of urinary tract disease. To define Repeated collection of urine samples obtained by
the origin of azotaemia, clinical signs and laboratory catheterisation on consecutive days at the same time
analysis findings need to be assessed simultaneously. of day and the same stage of daily routine is the most
Urine SG should be >1.018 in pre-renal azotae- practical way to employ calculations of fractional
mia, with no evidence of proteinuria, enzymuria urinary excretion of electrolytes. Volumetric urine
(increased urinary GGT and ALP) or cylindriuria collection during a 24-hour period is more labori-
(presence of renal casts – cylindrical, cigar-shaped ous to perform, requires confinement of animals, an
structures). The urine creatinine:serum creatinine adaptation period during which the animal adapts to
ratio may also provide useful information in this the urine collection device and a reliable and animal-
context. A ratio of >50:1 is associated with pre-renal friendly urine collection device. In most instances,
azotaemia. A ratio of <37:1 is usually associated with however, a single determination is performed.