1) I much prefer creatinine over BUN
a) The BUN is influenced by many non-renal factors
i) High protein diet: increase BUN
ii) GI bleeding: increase BUN
iii) Increased protein catabolism: increase BUN
(1) Corticosteroids, burns, fever, tetracycline
iv) Dehydration: increase BUN
v) Prerenal renal failure: increase BUN
vi) Polyuria: decrease BUN
vii) Severe liver disease: decrease BUN
b) Creatinine is lowered when there is extreme weight loss to the point of emaciation. This can be very significant in geriatric cats with chronic renal disease.
2) Urine specific gravity
a) The cat has a much greater ability to concentrate urine than the dog.
b) As a rule, the USG becomes abnormally low before the creatinine rises, but many cats in renal failure have USG above 1.030.
c) Do not rule out renal disease because the USG is normal.
3) J Veterinary Internal Medicine, Sept/Oct 2008: Survival times of cats with renal disease
a) IRIS IIb (creatinine 2.2-2.8): 1,151 days (mean)
b) IRIS III: (creatinine 2.9-5.0): 778 days (mean)
c) IRIS IV: (creatinine > 5.0): 103 days (mean)
4) Stages of renal disease
a) Renal insults
i) Bacterial infections, toxins, trauma, aminoglycosides, urethral obstruction, etc.
ii) Generally, these result in loss of some renal function but it is less than 75% so the creatinine and USG remain normal.
iii) There may be a few days of lethargy and anorexia but often PU/PD does not occur.
b) Renal insufficiency
i) Loss of 75-85% of renal function
ii) Creatinine typically = 2.5-5.5 (when the normal range is up to 2.4)
iii) No or mild clinical signs (which may be missed by the owners)
(1) PU/PD, reduced appetite, slight weight loss.
c) Renal failure
i) Loss of >85% of renal function
ii) Lab findings
(1) Creatinine = 5.5 – 20
(3) Acidosis (+/-)
(4) Anemia of chronic disease
iii) Clinical findings
(1) Anorexia, dehydration, very sick cat
(2) The clinical signs increase in severity as the creatinine rises.
5) Further diagnostics
a) All of these cats should have a urine culture
i) 22% positive: J Fel Med Surg April 07, p.124.
b) Ultrasound and possibly biopsy should be performed if:
i) The cat is less than 10 years of age.
ii) One or both kidneys are enlarged.
iii) One or both kidneys are painful.
c) Sizing kidneys
ii) Radiographs: Normal is 2.0-2.5 X the length of the body of L2.
iii) Ultrasound: 38-42 mm in a young cat.
d) Urine culture – by cystocentesis.
i) Indicated when the cat is < 10 years of age or the kidneys are enlarged.
(1) Confine the biopsy to the renal cortex.
(2) Direct the needle in a longitudinal plane.
(3) Consider a fine needle biopsy using a 22 gauge disposable needle.