Acid base metabolism and kidney tests

Question 1

What are the pros and cons of using GFR to measure kidney function?

What are the alternatives?

Answer 1

PROS: based on SCr, good sensitivity for severe damage
CON: only raised after 50% glomeruli loss, expensive, invasive

eGFR- , sCRi, 24hr creatinine clearance, s-Urea
Decrease in accuracy —–>

Question 2

How might glomerular disease manifest?

Answer 2

• asymptomatic
• AK
• ESKD

Glomerular basement membrane dysfunction:

• haematuria, proteinuria, albuminuria
• HTN
• oedema
• kidney insufficiency

Question 3

How is albuminuria measured?

Answer 3

• urine dipstick

- expressed as albumin: creatinine ratio (ACR) to allow for varied [urea]

Question 4

What is a major clinical manifestation of diabetic nephropathy?

How does the presence of this feature provide prognostic information?

Answer 4

• hypoalbuminaemia
• predicts high risk for future nephropathy
• other diabetic microvascular disease: retinopathy, neuropathy
• risk factors: HTN

Question 5

What is urea?

How is it produced?

Answer 5

Degradation product of amino acid (from protein)

Alanine —> urea + pyruvate (by alanine transaminase-ALT)
Asparate –> urea + oxaloacetate (by asparate transaminase -AST) –>/

Question 6

What is creatinine?

How is it produced?

Answer 6

Product of muscle turnover, produced from constant decay of creatine

Question 7

What are kidney stones made from?

Answer 7

• 80% composed of calcium salts (oxalate, phosphate)

Others:

• uric acid
• ammonium as in infection
• cysteine inborn error
• xanthine (RARE)

Question 8

What are the causes of kidney stones?

Answer 8

Calcium stones
- primary hyperthyroidism; distal RTA; high sodium intake

Oxalate (hyperoxaluria)
- low sodium/high oxalate diet; primary hyperoxaluria

Low urine citrate

High purine diet

Alcohol, obesity, drugs

Question 9

What investigations would you do in suspected renal stones?

Answer 9

SERUM & URINE INVESTIGATIONS

• fluid intake
• Sodium
• Calcium
• Phosphate
• albumin

Serum
- ALP, Cl-, HCO3-, SCr, uric acid, vit-D, PTH

Radiology- residual stones, nephrocalcinosis

Urine culture & pH
- oxalate, citrate, uric acid, xanthine, cysteine,

Question 10

What is renal tubular necrosis?

Answer 10

• systemic acidosis caused by impairment of renal tubules to maintain acid-base balance
  —> hypercalcemia
  which in turn can cause kidney stones, bone disease (in elders)

Question 11

How is renal tubular necrosis diagnosed?

How is it treated?

Answer 11

Diagnosis

• hypokalaemia
• metabolic acidosis with normal anion gap
• hyperchloraemia
• inappropriately alkaline urine
• clinical context
• urine pH, citrate and bicarbonate
• urine acidification test

Treatment
- Potassium citrate which corrects acidosis and K+