Renal Function Tests

Assessing the Kidney

The Kidney

The Structure The Function

Structure and function are not completely independentSome tests give information on both aspects

Assessing the Kidney

The Kidney

The Structure

Radiography PlainContrast

Excretory urography (assesses function too)Retrograde urographyComputed tomographyMagnetic resonance imagingUltrasonographyBiopsy (microscopic structure)

Assessing the Kidney

The Kidney

The Function

Extracellular fluid

Glomerulus

Tubule

Urine

Renal Function Tests

The tests may be on Extracellular fluid (blood/plasma as representative of ECF) Glomerular function Tubular function Urine

Analysis of Blood / Plasma

Abnormalities, if due to renal dysfunction may be caused by glomerular or tubular problems

There may be compensation for any abnormalities caused by renal dysfunction

Changes may be due to problems other than renal dysfunction

Analysis of Blood / Plasma

1. Blood urea – excreted by kidney, expected to increase in renal diseasePoor indicator of renal function

May be normal until renal function is severely compromised

Dehydration raises blood urea when kidney function is normal

Depends on the diet (protein)

Analysis of Blood / Plasma

2. Serum creatinineMostly an indicator of glomerular function. Why?Also poor indicator of renal function – non linear relationship with GFR (normal serum creatininedoes not indicate normal kidneyfunction)Useful for monitoring progression of diseaseBUTLittle change with dehydration,DietMostly related to muscle mass

Analysis of Blood / Plasma

3. Serum electrolytesSerum Na+, K+, HCO3-, Ca2+ and Phosphate may be abnormal in kidney disease

Very non specific testMany other factors cause changes in serum electrolytes

Analysis of Blood / Plasma

4. Plasma proteinsRenal disease can cause urinary loss of proteins and lead to low plasma protein levels

Non specific testOther factors cause changes in plasma proteins

Glomerular Function

1. Glomerular filtration rate Creatinine clearance

Most often used indicator of GFR Inulin clearance

Not used clinically, as infusion is needed Urea clearance

A poor indicator of GFR as much reabsorption occurs and at a variable rate

Radionuclide excretion rateNeeds more sophisticated instruments, not routinely used

Glomerular Function

1. Glomerular filtration rate Creatinine clearance

Creatinine clearance may be calculated using a formula that takes into account

Serum creatinineAgeBody weightHeight

Glomerular Function

2. Examination of urine Presence of abnormal constituents

e.g. proteins, if present in significant quantities could have come only from the glomerulus – abnormality of filtration barrier

Red blood cells – abnormal appearance of red cells in urine may indicate they have leaked from the glomerulus

Tubular Function

1. Examination of urine Substances not usually excreted may appear in significant

amounts Renal glycosuria Aminoacidurias

Estimation of 24 hour excretion of normally excreted substances Non specific, as handling of many is influenced by other

factors Changes in plasma levels cause changes in excretion rate

Tubular Function

2. Specialised tests Urine concentration and dilution ability

Water deprivation test Water loading test Plasma and urine osmolality

Urine acidification tests

Examination of Urine

Urinanalysis1. Physical2. Chemical3. Microscopic

Examination of Urine

Physical examination Appearance

haematuria Volume

Polyuria, oliguria, anuria Osmolality /specific gravity

Meaningful only with plasma osmolality or if consistently high/low pH

Meaningful only in combination with plasma pH

Examination of Urine

Chemical examination Blood Protein Glucose Nitrites Bilirubin Urobilinogen

Examination of Urine

Microscopic examination Bacteria Blood cells – RBC, WBC Casts