Tests for renal function and integrity Flashcards
Which substances do the kidneys work to conserve?
- Water
- Amino acids/proteins
- Glucose
- Bicarbonate
- Sodium
- Chloride
- Calcium
- Magnesium
Which substances do the kidneys work to excrete?
Urea
Creatinine
Potassium
H+ ions
Phosphate
Ketones and lactate
What are the functions of the kidney?
Excretion of waste
Control of body fluid balance (volume and composition)
- Electrolytes
- Acid-base
- Blood pressure
Production of hormones
Which hormones are produced by the kidney?
Erythropoietin
Calcitriol (active Vit D)
Renin (part of the RAAS)
Name the functional unit of the kidney
Nephron
Which structures make up the nephron
Glomerulus
PCT
LOH
DCT
CD
Describe the process of glomerular filtration
- Water and solutes move across glomerular capillaries and into the nephron
- Glomerular filtration in health dependent upon renal plasma flow
- All blood components except: cells, proteins larger than albumin, most lipoproteins
- Restricted by size, and to a lesser extent charge
Which blood tests can be used to assess renal function?
Biochemistry
- Nitrogenous wastes (urea, creatinine)
- Electrolyte balance (Na, K, Cl, Ca, PO4)
Haematology
Blood gas analysis
Which urine tests can be used to assess renal function?
Always: USG, dipstick, sediment
Maybe: UPC ratio (if proteinuria)
Rarely: fractional excretions of electrolytes
In practice GFR is indirectly estimated using?
Urea or creatinine
Describe the main features of urea
Made in liver from ammonia
Can be reabsorbed
Urea is a good marker for severity of clinical signs
Describe the main features of creatinine
- Produced in muscle in a constant process
- Filtered by glomerulus, not reabsorbed (this is what makes it a better marker)
- Concentration depends also on muscle mass
How are the levels of urea and creatinine altered when there is a decreased GFR?
Both of these increase if there is a decreased GFR as they are not being filtered out fast enough
What is the main cause of an increase in urea?
Decreased GFR
List some possible causes of mild urea increases
Upper GI haemorrhage
High protein meal
Recent meal
Catabolism (e.g. fever, corticosteroids)
List the causes of a decreased urea
- Severe liver disease or portosystemic shunt
- Low protein diet
- Aggressive fluid therapy
- PU/PD
- Young animals
What are the 3 possible causes of increased creatinine?
Decreased GFR
High muscle mass (mild increase)
High dietary protein
Decreases creatinine is due to?
Reduced muscle mass
Define azotaemia
An abnormal concentration of urea, creatinine and other nitrogenous compounds in the blood
What is the cause of pre-renal azotaemia?
Due to dehydration or decreased cardiac output: clinical evidence of dehydration and or hypovolaemia – signs of heart failure, shock, etc
How is the USG affected by pre-renal azotaemia?
Maximally concentrated urine specific gravity
>1.030 in dogs
>1.035 in cats
How is pre-renal azotaemia treated?
Fluid therpay
What is the cause of renal azotaemia?
Kidney problem
How is the USG affected by renal azotaemia?
- Isosthenuria (1.008-1.012) when >66% of nephrons lost = same concentration as plasma
- USG inadequately concentrated (so > 1.012 but <1.030 in dogs and <1.035 in cats)
- USG falls progressively
What is the cause of post-renal azotaemia?
Failure of urine output
Easy to diagnose
Expect hyperkalaemia too
Name two other markers of GFR
SDMA = symmetrical dimethyl arginine
Iohexol clearance
Describe SDMA as a GFR marker
May be useful for earlier detection of CKD
- Increases with 40% reduction of GFR (75% needed for creatinine)
Value only when creatinine is normal or borderline
Results may be slightly higher in puppies, kittens, and greyhounds
How do the levels of phosphorus change when there is kidney disease?
Hyperphosphataemia
- Reflection of decreased GFR in species with high dietary phosphorus content
Which spp is the exception to hyperphosphataemia in kidney disease?
Equine
- tend to be HYPOphosphataemic with decreased GFR
- low dietary phosphorus along with GI losses and increased renal loss
How do the levels of calcium change when there is kidney disease?
↑ or↓ total calcium - check iCa if available
- Often increased total calcium
- with normal to low ionized calcium
How do the levels of potassium change when there is kidney disease?
Increased:
- Fluid compartment shift in acidosis
- Decreased urinary output (bladder rupture, obstruction)
Decreased:
- Increased urinary losses (esp. in CKD)
- Decreased food intake or GI losses
Does azotaemia mean there is an abnormal renal function?
Not always as this can be caused by other things
Define uraemia
- Not the same as azotaemia or increased urea.
- Literally “urine in the blood”
- Uraemia is the clinical syndrome that results from loss of kidney function, involving multiple metabolic derangements
What are the clinical signs of uraemia?
Vomiting, anorexia, weight loss, diarrhoea, anaemia, ulcerative stomatitis, muscle tremors, convulsions, coma
What is USG a measure of?
Solutes in urine
Where along the nephron is the USG of urine the same as plasma?
Glomerulus
PCT
How would you interpret a USG over 1.030 (dogs) and 1.035 (cats)?
Adequately concentrated
- Indicates functioning tubules (>2/3) *but cats can trick you
- Supports dehydration if present
How would you interpret a USG of 1.012 - 1.029 (dogs) and 1.034 (cats)?
Grey area
- May be normal if patient is hydrated
- Is definitely abnormal in dehydration
- Rule out extra-renal causes before blaming the kidney!!!
- Examples: Hypercalcaemia, Addison’s disease, Drugs
How would you interpret a USG of 1.008 - 1.012?
Same Specific Gravity as Plasma
May be normal if drinks a lot of water
Is definitely abnormal if dehydrated
Rule out extra-renal causes before blaming the kidney
How would you interpret a USG of 1.000 to 1.008?
Lower Specific Gravity than Plasma
Requires functioning nephrons = clinically useful
- Cushings
- Diabetes
- Hypercalcaemia
Is protein in urine normal?
Very small amount yes, more than 1+, no
Describe dipstick analysis for proteinuria
Must be interpreted in light of specific gravity: more significant if low USG
Which test should be used after finding high levels of protein on a dipstick test?
Urine protein to creatinine ratio test - more accurately quantifies proteinuria
How must a proteinuria be classified?
By:
- location
- persistence
- magnitude
Describe locating the proteinuria
Rule out pre- and post-renal causes of proteinuria by examining the haematology and serum biochemistry as well as performing cystocentesis.
How can you assess the persistence of a proteinuria?
The presence of proteinuria should (ideally) be confirmed by taking three samples, two weeks apart
How can you assess the magnitude of a proteinuria?
The magnitude of the proteinuria should be confirmed using the UP:C
>2.0 – Suggestive of glomerulopathy
List some possible causes of a pre-renal proteinuria
Fever
Systemic inflammation
Haemoglobinaemia
Hypoglobinaemia
List some possible causes of a post-renal proteinuria
UTI
Nephrolithiasis
Tumours of the urinary tract
What are the proteinuria treatment threshold levels in dogs and cats?
Dogs >0.5
Cats >0.4
Describe the main features of a glucosuria
- Not normally present in urine
- Glucosuria seen with serum glucose above: 10 mmol/l in dogs and 13-16 mmol/l in cats
- If serum glucose normal, it could be due to failure of resorption by tubules: Renal glucosuria (e.g. Fanconi’s syndrome)
WBCs and bacteria seen on a sediment analysis indicate?
A urinary tract infection
What are casts?
- Cylindrical moulds of tubules composed of mucoproteins +/- cells
- Occasional hyaline and granular casts may be normal
Granular, cellular and waxy casts indicate?
Tubular damage = clinically useful!
- Indicate something is going on in the kidney – acute kidney injury
When can you look for crystals in urine?
Look in fresh urine - don’t look for crystals in a sample an owner brings in
Lots of normal animals have crystalluria - not synonymous with urolithiasis!
What are struvite crystals
Coffin shape
Magnesium ammonium phosphate
Triple phosphate
Describe the significance of struvite crystals
Significance:
- UTI
- Urolithiasis
- Or normal
Form in ALKALINE urine
Also in vitro
Describe the main features of amorphous crystals
Urate
- Acidic pH
- No clinical significance
Phosphate
- Alkaline pH
- No clinical significance
Which crystals have a ‘maltese cross’ appearance?
Calcium oxalate dihydrate - can form in a standing urine sample
Which crystals have a ‘picket fence’ appearance
Calcium oxalate monohydrate
Are Calcium oxalate monohydrate crystals a significant finding?
Yes!
Ethylene glycol (EG) toxicosis
Absence does not rule it out
Describe the significance of ammonium biurate or uric acid crystals
Portosystemic shunts
Liver disease
Dalmatians
Cystine crystals are associated with?
Metabolic defect in the tubular reabsorption of cystine
- Bulldogs, other breeds
List some possible urine contaminants
Pollen
Mucus
Fat droplets
Sperm
Conidia and amorphous debris from free catch
Starch from gloves
What is the normal range of urine pH?
6-8
How can you confirm a bladder rupture from fluid obtained from the abdomen following trauma?
Measure creatinine content
