Page 385 - The Veterinary Laboratory and Field Manual 3rd Edition

354 Susan C. Cork, Willy Schauwers and Roy Halliwell

diet. A sample of urine can be tested directly way to the bladder. In many kinds of kidney dis-
for biochemical changes using reagent strip ease these tubules become blocked with protein
sticks (for example, Multistix™) that change or the remains of dead cells. Very often the solid
colour in response to pH, the presence of glu- substance blocking the tubule becomes loose
cose, ketones, bilirubin and so on. Some of these and goes down the tubule and into the urine.
test sticks also detect the presence of whole and This material, which was blocking the tubule
haemolysed blood. The normal urinary pH for and has come loose is called a cast. Because it
an individual depends on the animal’s diet but was formed inside a tubule a cast has the shape
in general, ruminants and equines have alka- of the tubule from which it came. Casts are
line urine (pH > 7.0) whereas carnivores have seen in several kidney diseases, and there are
acid urine (pH < 7.0). For pH tests on urine several types (see Figure 7.13). Hyaline (like
some laboratories routinely use a pH meter (see glass) casts are clear or transparent, they either
Chapter 2). Urine reagent strips are readily avail- have no granules in them or only a few granules.
able and rely on a colour change on a single or Normal urine often has some of these hyaline
multiple reagent strip; the colour reaction can casts, and they do not indicate that the kidney
be compared with the standard colour codes is diseased. However, in large numbers hyaline
provided on the container or reagent strip box. casts can indicate pathology. Granular casts
The instructions that come with reagent sticks are made of coarse granules. If they are found
are usually quite simple and should be followed in the urine they usually indicate that the kid-
carefully. On most reagent strip containers, neys are damaged, and are therefore significant.
there will be a colour chart to allow the result Granular casts are found when there is protein
to be read. However, many reagent strip testing in the urine. All urine in which protein is found
kits are developed for human use and therefore (++ or more) should be centrifuged, so that
interpretation of results for animals may not be the deposit can be examined for the presence of
simple. In healthy animals, there should not be casts under a microscope. Casts are sometimes
any trace of protein, ketones, glucose or bilirubin found that are partly granular and partly hyaline.
in the urine. There may be some blood in urine Other kinds of casts are also seen – cellular, waxy
samples collected during the oestrus period in and pigment casts but these are not common.
female animals of some species, due to contami- Animals that pass red blood cells in their
nation from the reproductive tract, so this must urine are said to have haematuria (for example,
be taken into consideration when examining as seen in cystitis, neoplasia and bracken fern
free-caught samples. The presence of glucose poisoning). The urine will be turbid, ‘smokey’ or
may indicate that there is a pancreatic endo- red. Red blood cells are sometimes destroyed in
crine disorder, for example, diabetes mellitus, the circulation, and haemoglobin rather than red
but mild hyperglycaemia may also be associated cells may be passed in the urine; this is known as
with ‘physiological stress’ or metabolic disorders haemoglobinuria (for example, as seen in babe-
in ruminants. The presence of ketones is usually siosis). It is easy to distinguish between the two
a bad sign and indicates severe emaciation or the conditions if the urine sample is centrifuged or
development of serious metabolic disease such is allowed to settle overnight. In cases of hae-
as ketosis and/or toxaemia. moglobinuria the sample remains red while in
haematuria, red blood cells will be seen in the
caStS and cELLS spun deposit or settled specimen. Sometimes
In the kidney there are millions of tubules (small there may be both haemoglobin and red blood
tubes) through which the urine passes on its cells in the urine (see Chapter 10).

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