Intro to urinary system medicine & clinical approach

Recall the functions of the kidney

• Excretion of waste —> azotaemia

• Control of body fluid balance (vol and composition)

  • Electrolytes

  • Acid-base

  • Blood pressure

• Endocrine organ

  • Renin

  • Erythropoietin (EPO)

  • 1,25-dihydroxyvitamin D (calcitriol)

Define azotaemia and recall the types

Azotaemia = increase urea and creatinine

• Pre-renal azotaemia —> decreased GFR due to decreased renal perfusion

  • Dehydration, hypovolaemia, heart failure

• Renal azotaemia (Acute or chronic) —> renal parenchymal disease

  • When enough nephrons aren't functional

• Post renal azotaemia —> interference with urine excretion

  • Urethral obstruction, UT rupture

What concentrations of urine are expected in pre-renal and renal azotaemia?

Pre-renal —> Concentrated urine (USG: >1.030)

Renal —> Dilute urine (USG: 1.008-1.029)

Define CKD

Structural or functional abnormalities of one or both kidneys that have been there for 3 months or longer i.e. presence of kidney damage or reduced kidney function

(irreversible dx)

Define AKI, what can cause it?

• Rapid loss of kidney function (varied from mild to severe e.g. leptospirosis, ethylene glycol, NSAID toxicity

Describe the pathogenesis of AKI

• Results in abnormal GFR, tubular function and urine production

• A sudden inability to maintain fluid, acid-base and electrolyte balance

• May result in azotaemia

What are the most common consequences of reduced kidney function?

PUPD —> increased filtered load per surviving nephron, disruption of normal countercurrent system, impaired response to ADH

Anorexia —> can result from oral pain

Anaemia —> EPO deficiency)

Retinopathy → blindness

What is the difference between azotaemia and uraemia?

Azotaemia —> abnormal conc. of urea, creatinine and other nitrogenous compounds (LAB CHANGE)

Uraemia —> clinical syndrome that results from loss of kindey function, clinical signs (CLINICAL PICTURE)

What are the "target organs" affected by high blood pressure?

• Kidney

• Heart

• CNS (brain)

• Eyes/Retina

Which acid/base disturbance is most common in CKD?

Metabolic acidosis

(inability to excrete acid)

What may an owner report in history taking when the animal has kidney disease

Drinking and urination changes

• PUPD, Pollarkiuria (increased frquency|), Stanguria (strain) , Dysuria (diffulcty passing urine)

Haematuria

• Throughout urination without pollakuria, stranguria (upper UT)

• Associated with pollakiuria, stranguria if bladder problem

• Bleeding between urinations probably urethral/prostatic/genital tract

What value defines polydipsia?

If more than 100ml/kg/day

Define:

• Pollakiuria

• Stranguria

• Pollakiuria —> increased frequency

• Stranguria —> straining

What must be distinguished from PUPD during history taking?

• Urinary incontinence

• Cystitis

• Submissive urination

• Marking behaviour

What are the recommended core tests for kidney disease?

• Haematology

• Biochemistry

• Urinalysis

• +/- urine culture

What haemotology results may you expect in kidney disease?

Raised WBC —> inflm such as pyelonephritis

Ht, Hb, RBC: Normocytic, normochromic non regen anemia can occur

What biochem results would you expect in kidney disease?

• Urea, creatinine increase

• Increase phosphorus

What extent of damage does renal azotaemia imply?

Loss of at least 75% of functioning nephron

• NB modest increase can represent significant damage

How do urea and creatinine differ as markers?

Urea

• Made in liver from ammonia from catabolised proteins

• Correlates well to kidney size

• Urea can be affected by diet

Creatinine

• More accurate

• Produced in muscle in constant process, can be affected by muscle mass

• Correlated well to GFR

Other than urea and creatinine what markers should be looked at in biochem results for suspected kidney disease

• Albumin

  • decrease in PLN (protein losing nephropathy)

  • also important in Ca interpretation

• Potassium

• Phosphorus

  • Excreted by the kidneys

  • Hence iP increases when GFR reduced

• Calcium

  • Total Ca may be low, normal or high in renal disease

  • Hypercalcaemia can cause kidney damage

• SMDA

How can you perform urinalysis?

• Free catch midstream (non-sterile)

• Cystocentesis (sterile)

• Catheterisation (not sterile, transfer of bacteria from LUT)

What gross examination points should you look at in a urine sample?

• Colour

  • pale yellow to amber = normal

  • Red to brown = haematuria or haemoglobinuria

  • Dark yellow / brown = bilirubinuria

• Turbidity

  • normal urine is clear

  • cloudy with increased cells, crystals, bacteria, mucus, lipids, casts, sperm

What bilirubin values would you expect on a dipstick? When does it increase?

• Small amount = normal in dog

• Always abnormal in cat

• Increased in hepatic disease / haemolysis

When would blood/Hb be positive on dipstick and what should you do in positive reaction?

• Positive if blood, Hb or Mb present

• Centrifuge to see if RBC settle out

  • Will in case of RBC, wont with haemoglobinuria

Which values of a dipstick should be ignored?

• Leukocytes

• Nitrites

• USG

What is the most accurate quantification of proteinuria?

Urine protein to creatinine ratio (UPCR)

• more accurate than dipstick

Adequately concentrated USG in dogs and cats?

Dog- 1.030 or higher

Cat- 1.035 or higher

What is isosthenuria? What USG values reflects this?

• Same specific gravity as plasma

• May be normal if drinks a lot of water

• Definitely abnormal if dehydrated

• 1.008-1.012

Rule of thumb:

Inadequate conc + azotaemia = renal azotaemia

What does a moderately/sub-optimally concentrated USG suggest?

• Grey area

• May be normal if patient is hydrated

• Is abnormal in dehydration

• If azotaemia present suggests renal azotaemia

What is hyposthenuria?

• Lower Specific Gravity than Plasma

• Requires functioning nephrons = clinically useful

• Typically ADH problems

• Maybe problem is drinks lots of water

• Definitely abnormal in dehydration

What crystals are these?

Struvite

What crystals are these

Calcium oxalate dihydrate

By how much does GFR need to reduce before serum creatinine becomes abnormal?

75%

Define dysuria

Difficulty passing urine

vs anuria = no outflow of urine