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Serum Chemistry in Evaluation of Renal Disease
Serum Chemistry in Evaluation of Renal Disease
32
Veterinary Medicine
Graduate
12/08/2011

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Term
What is the normal value range for urea in blood?
Definition
10-30 mg/dl
Term
What can cause an increase in urea production?
Definition
Fever, tissue necrosis, GI hemorrhage (RBCs release NH4 into gut), high protein diets.
Term
What can cause an decrease in urea production?
Definition
Chronic liver failure, portosystemic shunts, negative protein balance (anorexia/cachexia), low protein diet.
Term
What is the most important factor influencing BUN levels? Why?
Definition
Renal excretion. Urea is freely filtered through glomeruli, so decrease in GFR (whether due to pre-renal, renal, or post-renal disease)means increased serum BUN.
Term
What happens to urea in the nephron?
Definition
Freely filtered by glomerulus. In tubules, reabsorption of water results in higher concentration of urea in filtrate. Tubule is not as permeable to urea as it is to water, so only about half the urea ends up being reabsorbed back into the interstitium (although the higher the urine flow rate, the less urea is absorbed proportionally). In the interstitium urea helps create the medullary osmotic gradient which enables urine concentration.
Term
How can adult ruminants maintain relatively normal BUN in the face of reduced GFR?
Definition
Some circulating urea is excreted by the salivary glands and ends up in the rumen, where microbes degrade the urea into NH3 and form proteins.
Term
What is the normal value of creatinine in serum?
Definition
<1-2 mg/dl
Term
What is the major factor determining circulating creatinine levels?
Definition
GFR. Creatinine production is fairly constant and minimally influenced by muscle catabolism or necrosis. It is freely filtered by the glomerulus, and no tubular reabsorption occurs (some secretion in male dogs and goats!).
Term
What are causes of pre-renal azotemia?
Definition
1. Hypovolemia (due to dehydration, shock, acute hemorrhage)
2. Cardiac insufficiency
Term
Is pre-renal azotemia associated with more or less concentrated urine?
Definition
More concentrated. Hypovolemia results in ADH release, and reduced GFR due to reduced CO results in activation of renin/AT system and aldosterone release.
Term
What is considered adequate urine concentration in the face of dehydration/azotemia?
Definition
Cow: SG > 1.026
Dog: SG > 1.030
Horse: SG > 1.030
Cat: SG > 1.035
Term
Is prerenal azotemia associated with normal or abnormal urine sediment?
Definition
Usually normal. However, persistent renal ischemia can result in renal tubular damage, resulting in casts in sediment.
Term
Damage to what structures can cause renal azotemia?
Definition
Glomeruli, tubules, interstitium, renal blood vessels. (Vasculature and tubules are interdependent so damage to one is usually followed by dysfunction of another.)
Term
Is renal azotemia associated with more or less concentrated urine?
Definition
Less concentrated (isosthenuric or minimally concentrated). With renal tubular damage due to ischemia/necrosis, tubular cells may be unable to respond to ADH. Remember that concentrating ability is lost with loss of 2/3 of nephrons, but azotemia occurs with loss of 3/4 of nephrons.
Term
What constitutes minimal urine concentration in face of azotemia?
Definition
Cow: SG < 1.026
Dog: SG < 1.030
Horse: SG < 1.030
Cat: SG < 1.035
Term
Do you expect to see normal or abnormal urine sediment in the presence of renal azotemia?
Definition
Abnormal. There may be:
1. Casts (due to tubular cell sloughing; renal necrosis due to persistent ischemia or toxins).
2. Inflammatory cells (due to Ag-Ab complexes in glomeruli).
3. Abnormal crystals
Term
What causes concurrent pre-renal azotemia with renal azotemia?
Definition
Loss of ability to concentrate urine (due to renal tubular damage in renal azotemia) results in excess water loss (polyuria) and possible dehydration...resulting in pre-renal azotemia!
Term
What are some exceptions to renal azotemia being associated with minimally-concentrated urine?
Definition
1. Cats can sometimes retain concentrating ability in face of renal azotemia.
2. When glomerular lesions haven't resulted in severe enough tubular lesions to impair concentrating ability ("glomerulotubular imbalance").
Term
If azotemia is not present, can you rule out kidney disease?
Definition
No. It takes the loss of 3/4 of nephrons to result in (renal) azotemia.
Term
What situations can result in post-renal azotemia?
Definition
"Plumbing" problems, including:
1. Urolithiasis
2. Neoplasm of bladder/urethra
3. Enlarged prostate
4. Uterine stump granuloma (impinging on UT)
OR
5. Neurologic deficit
Also, urinary tract rupture.
Term
Do you typically see more or less concentrated urine with post-renal azotemia?
Definition
Neither, necessarily - urine SG can be widely variable with post-renal azotemia.
Term
Is azotemia usually seen with a unilateral ureteric or pelvic obstruction?
Definition
No - the unobstructed kidney can increase urine output to maintain normal BUN/creatinine levels.
Term
What other serum chemistry changes are you likely to see with azotemia if it's post-renal?
Definition
-Increased serum K+ and P- due to decreased filtration (hyperkalemia -> bradycardia)...mitigated in ruminants by excretion of K+ and P- in saliva.
- Hypocalcemia can be seen: possibly resulting from increased P-, which complexes with Ca++ (reducing free Ca++) and eventually precipitates into soft tissues (reducing total Ca++).
Term
What serum abnormalities are you likely to see with uroabdomen?
Definition
1. Azotemia
2. Hyponatremia
3. Hypochloremia
4. Hyperphosphatemia
5. Hyperkalemia

Azotemia, hyperkalemia, and hyperphoshatemia are due to the fact that urea, K+, and P- diffuse from area of high concentration (in urine in peritoneal cavity) across peritoneum into blood. Hyponatremia and hypochloremia are due to the fact that Na+ and Cl- are usually reabsorbed from urine to a great extent, so are at low concentration in urine...so diffuse from blood INTO the peritoneal cavity.
Term
In the case of uroabdomen, does BUN or creatinine increase faster?
Definition
BUN, since urea is a smaller molecule and diffuses across peritoneum faster. Creatinine diffuses more slowly, taking about 4 hrs to reach equilibration (but continuous leak of urine means full equilibration is not likely to be reached).
Term
What ratio of creatinine in abdominocentesis effusion to serum is diagnostic of uroabdomen?
Definition
2:1
Term
What serum chemistry and urinalysis findings are consistent with extravascular hemolysis?
Definition
1. Anemia
2. Bilirubinemia and bilirubinuria
3. Plasma often icteric (yellow)
Term
What serum chemistry and urinalysis findings are consistent with intravascular hemolysis?
Definition
1. Anemia
2. Bilirubinemia & bilirubinuria
3. Pink-red plasma/serum
4. red-brown urine
5. positive urine heme reaction
6. no RBCs in urine sediment
Term
What serum chemistry and urinalysis findings are consistent with myoglobinuria (muscle necrosis)?
Definition
1. NO anemia
2. Normal plasma/serum appearance (usually)
3. red-brown urine
4. positive heme reaction
5. no RBCs in urine sediment
Term
What is nephrotic syndrome?
Definition
Collection of lab and clinical signs indicating presence of protein-losing nephropathy:
1. Proteinuria (renal)
2. Hypoalbuminemia
3. Hypercholesterolemia
4. Edema and/or ascites
Term
Why might serum creatinine be a less valuable measure of GFR in a very thin, geriatric cat?
Definition
In animals with severe muscle wasting, less creatine may be produced.
Term
What are the unique serum abnormalities with horses with CKD?
Definition
Hypercalcemia, normo- to hypophosphatemia (opposite from most species).
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