Urinalysis
When catheterised urine samples are obtained, consideration of the
effects of sedation is important. Alpha-2 agonists have both acute
diuretic and hyperglycaemic effects. The diuretic effect is unlikely to
alter the urine sample if collected promptly as a few mLs of most
recently produced diuresed urine should not significantly affect the
litres of previously accumulated urine in the bladder. However, even
a small amount of glycosuria might confuse diagnostic interpretation.
Hence acepromazine (5-10 mg/100kg IV) perhaps supplemented by a
nose twitch or a small dose of detomidine (0.5 mg/100 kg IV) should
be used as necessary in geldings. In mares the use of stocks or a stable
door may avoid the need for sedation in amenable horses.
SPECIFIC GRAVITY
SG <1.008 (hyposthenuria)
o suggests that the kidney is actively excreting water and is
typical of primary (psychogenic) polydipsia or diabetes
insipidus.
SG between 1.008-1.014 (isosthenuria)
o suggests that the kidney is neither actively concentrating
nor diluting the filtrate and is consistent with (but not
diagnostic for) chronic renal failure (CRF). Although
isosthenuria is typical of CRF, other causes of PU/PD might
coincidentally happen to fall in the isosthenuric range.
Therefore, when isosthenuric samples are obtained, serum
urea and creatinine should be checked to rule in or rule out
possible CRF.
SG >1.014 (hypersthenuria)
o indicates that the kidney is actively concentrating urine
although SG >1.020 is usually regarded as more convincing
of good concentrating ability (and therefore absence of
chronic renal failure or diabetes insipidus – both of which
lack concentrating ability).
GLYCOSURIA – if persistent then this indicates diabetes mellitus and
usually PPID although primary diabetes mellitus is a rare possibility.
Remember the effects of alpha-2 sedatives and stress on
hyperglycaemia and glycosuria.
URINE CREATININE : SERUM CREATININE RATIO is useful in cases with
equivocal mild increases in serum creatinine concentration (e.g. 180-
250 mol/L). Mild to moderate increases in serum creatinine due to
dehydration will be expected to be matched by high urine creatinine
concentrations (>50 x serum concentration) whereas CRF cases will
have lower urine creatinine (e.g. <40 x serum creatinine).
CRYSTALS – are rarely, if ever, relevant in equine urinalysis
BACTERIOLOGY – urine should be sterile but false positive growth iscommon in catheterised or free-catch samples. Bacterial growth,
where relevant, should be accompanied by leucocytes and/or blood
in urine smears.
NITRITE – presence in urine might support bacterial infection but very
prone to false positive findings.
ENZYMURIA – urinary GGT, AP and LDH may all be raised in renal
failure due to tubular damage (more so in acute renal failure). Urinary
GGT is the most commonly assayed urinary enzyme and is usually
similar in concentration to serum levels (i.e. <40 iu/L urine).
Adjustments should be made for urine concentration and this is done
by comparison with urine creatinine concentration (expressed in
mmol/L, not μmol/L) :
Urinary GGT (iu/L)/Urinary creatinine (mmol/L) < 5.0 iu/mmol in normal horses