27. Renal function testing

28/11/2025

What are the key functions of the kidneys?

• Regulates extracellular fluid volume

• Homeostasis of hydrogen ions

• Excretion of urea

• Erythropoetin production

• Metabolism of vitamin D

Where is aldosterone secreted from?

Adrenal cortex

What is the role of aldosterone?

Increases sodium reabsorption from the kidney by acting on principle cells in the distal tubules and the collecting ducts

Where is vasoporessin secreted from?

Posterior pituitary gland

What is the role of vasopressin?

• Acts on the collecting duct cells to promote insertion of aquaporin-2 water channels into the luminal membrane

• Increases permeability of collecting ducts

• Increases water reabsorption

What does glomerular filtration rate assess?

Glomerular function

When can low creatinine plasma concentration be observed?

• Pregnancy

• Wasting diseases

• Children

When can high creatinine plasma concentration be observed?

• High meat intake

• Vigorous exercise

What is creatinine?

A waste product removed from blood by the kidneys

Pros of using GFR as a biomarker of renal damage

• Cheap

• Easy to do

• Not invasive

• Sensitive indicator of kidney damage before a change in the plasma biomarkers

Cons of using GFR as a biomarker of renal damage

• Only an estimation not an accurate reading

• High risk of ‘performance’ error because the patient may forget to collect urine at specific times or drink too much water, this interferes with the result so not a reliable method

What is plasma creatine dependent on?

Muscle mass

Pros of using plasma creatinine as a biomarker of renal damage

• More precise than GFR

• Easier to measure than GFR- blood sample taken and run through analyser

• Decreases once normal kidney function is restored in acute kidney injury

Cons of using plasma creatinine as a biomarker of renal damage?

• GFR changes before plasma creatinine- may be too much damage done by the time plasma creatinine is measured

• Elevated plasma creatinine does not alway mean there is kidney damage- high meat diet or vigorous exercise

What causes elevated creatinine in plasma?

• Impaired renal perfusion

• Loss of functioning nephrons

• Increased pressure on the tubular side of the nephron

Why does impaired renal perfusion cause elevated creatinine in plasma?

Due to decreased blood pressure, reduced blood volume or narrowing of the renal artery

Why does increased pressure on the tubular side of the nephron cause elevated creatinine in plasma?

• Prostatic enlargement can push against kidney nephrons

• Kidney stones can block nephrons

What is glomerulonephritis?

Inflammation of the nephrons, often associated with antibody mediated damage or deposition of immune complexes in the glomeruli

What causes elevated plasma urea without kidney involvement?

• High protein diet

• Increased protein catabolism after trauma

What causes decreased plasma urea without kidney involvement?

• Liver failure

• Low protein diet

• Water retention

What is urea?

The final breakdown product of amino acids from proteins

• Nitrogen in the form of ammonia produced in liver when protein broken down

• Nitrogen combines with other chemicals in liver to form urea

• Urea released into blood and carried to kidneys where filtered out and excreted

Pros of using plasma urea as a biomarker of renal damage

• Very low concentration in the serum of normal individuals because 90% of urea produced is removed

Cons of using plasma urea as a biomarker of renal damage

• High plasma urea can be caused by a range of factors without the involvement of the kidney

• Not tissue specific enough because has close associations with liver (liver failure causes high urea in plasma)

What is glucosuria?

Excretion of glucose in the urine

What can cause glucosuria?

• Malabsorption of glucose in proximal tubule

• Diabetes

Why does diabetes mellitus cause glucosuria?

• Unable to take up glucose into cells

• excess glucose in the blood causes the blood to become viscous/ glucose ends up in the urine

• Viscous blood damages kidney glomerulus

• So glucose gets into the urine

Test for proximal tubule function

Glucosuria

How is distal tubular function assessed?

Urine concentration tests

• Specific gravity

• Osmolality

What does specific gravity measure?

ratio of the density of a substance to the density of a reference substance

What is osmolality?

The number of solute particles

Pros of using specific gravity as a test for renal damage

Measures distal tubular function

Cons of using specific gravity as a biomarker for distal tubular function?

It is not a good biomarker if there is protein in the urine, because this heavily influences the density of urine

State 3 ways distal tubular function can be assessed.

• Specific gravity

• Osmolality

• Injecting synthetic analogue of vasopressin and collecting urine samples

  • Osmolality of urine should increase

What is albumin?

Large protein

What is microalbuminuria?

Small amounts of albumin in the urine

• 30-300mg/24h

Which disease would you find microalbuminuria?

Diabetic nephropathy

What is macroalbuminuria?

Large amounts of albumin in the urine

• >300mg/24h

What is diabetic nephorpathy?

Prolonged elevated blood glucose causes damage to the glomeruli in the kidney nephrons

Pros of using micro/macroalbuminuria as a biomarker for renal damage?

• Low in the urine of normal individuals because albumin is a large protein

• Indicative of renal damage

• Easily measured

Cons of using micro/macroalbuminuria as a biomakrer of renal damage

• Indicates that there is renal damage after the damage has already happened

How are urine dipsticks semi quantitative?

Intensity of the colour can be an indication of the concentration of a particular molecule in the urine

Pros of using urine dipsticks as a test for renal damage?

• Quick

• Easy

• Cheap

• Good starter test that can be used to identify potential renal problems that may need further testing

Cons of using urine dipsticks as an indicator for renal disease

• Qualitative so does not give a precise number or concentration

• Colour changes on the stick can be subjective/ difficult to tell which colour matches which concentration shown on the bottle

Protein in the urine indicates

Glomerular disease

Altered pH of urine indicates

Renal tubular disease

Erythrocytes in the urine indicates

Glomerular disease

Elevated specific gravity of urine indicates

Distal tubular function

Ketones in the urine indicates

Diabetes mellitus

Glucose in the urine indicates

Diabetes mellitus

What is AKI?

• Acute kidney injury

• A sudden episode of kidney failure or kidney damage causing a buildup of waster products in the blood

How is AKI diagnosed?

• Oligouria (low urine output<40ml/24h)

• Hyperkaleamia (High potassium in blood)

Who is AKI most problematic for?

• Elderly patients

• Critically ill patients

How is serious AKI managed?

Dialysis

Name a good biomarker for AKI and why it is good

• Neutrophil gelatinase-associated lipocalin

• Released into the urine within 2 hours of AKI

  • This is important because AKI has a high mortality rate

Summarise the 3 phases of AKI

• Oligouric phase

  • Low urine output

• Diuretic phase

  • Urine is being produced because treatment is working

• Recovery phase

  • Kidney function has returned to normal

What is chronic kidney disease?

The gradual loss of kidney function over a large period of time, usually no symptoms early on

What is chronic kidney disease characterised by?

• Decreased number of functioning nephrons

• Reduced GFR

  • Because of this there is an increase in plasma:

    • urea, creatinine, phosphate and potassium concentrations

Which disorders does chronic kidney disease occur in?

• Glomerulonephritis

• Diabetes mellitus

• Pyelonephritis (subclinical repeated infection)

• Polycystic renal disease (genetic)

What does AKI always cause retention of?

• Urea

• Creatinine

• H+