Renal Function Tests

Assessing the Kidney The Kidney The StructureThe Function Structure and function are not completely independent Some tests give information on both aspects

Assessing the Kidney The Kidney The Structure Radiography Plain Contrast Excretory urography (assesses function too) Retrograde urography Computed tomography Magnetic resonance imaging Ultrasonograph y Biopsy (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 disease Poor 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 creatinine Mostly an indicator of glomerular function. Why? Also poor indicator of renal function – non linear relationship with GFR (normal serum creatinine does not indicate normal kidney function) Useful for monitoring progression of disease BUT Little change with dehydration, Diet Mostly related to muscle mass

Analysis of Blood / Plasma 3. Serum electrolytes Serum Na +, K +, HCO3 -, Ca 2+ and Phosphate may be abnormal in kidney disease Very non specific test Many other factors cause changes in serum electrolytes

Analysis of Blood / Plasma 4. Plasma proteins Renal disease can cause urinary loss of proteins and lead to low plasma protein levels Non specific test Other 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 rate Needs 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 creatinine Age Body weight Height

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 Urinanalysis 1.Physical 2.Chemical 3.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