Renal Function Tests (i) - PDF

Renal Function Tests (i) Dr Karan Rana Learning Outcomes Familiarise ourselves with the structure and function of the kidney. Importance of measuring Glomerular filtration rate and plasma creatinine as biomarkers of kidney dysfunction. Anatomy of the kidney 3 Stru...

Renal Function Tests (i) Dr Karan Rana Learning Outcomes Familiarise ourselves with the structure and function of the kidney. Importance of measuring Glomerular filtration rate and plasma creatinine as biomarkers of kidney dysfunction. Anatomy of the kidney 3 Structure of the glomerulus  Kidney functions: podocyte  Extracellular fluid volume.  H+ homeostasis. Endothelial cell  Excretion of urea.  Erythropoietin production  Vitamin D metabolism. a) Basement membrane. b) Capillary lumen. Mesangial cell c) Urinary space. contractile and phagocytic properties 4 Aldosterone  Secreted from adrenal cortex.  Acts on principal (P) cells in distal tubule and collecting duct.  Increases sodium reabsorption.  Modifies sodium channels.  Promotes expression of new channels and ATPases. 5 Vasopressin (ADH)  Released from posterior pituitary.  Acts on collecting duct cells to promote insertion of aquaporin-2 water channels into luminal membrane.  Increases water permeability.  Increased water reabsorption. 6 Glomerular filtration rate (GFR)  Used to assess glomerular function.  Creatinine formed from creatine in muscle - an end product of nitrogen metabolism.  Plasma concentration depends on muscle mass and therefore lower values may be found in wasting diseases and in children.  Decreased in pregnancy and mildly increased by high meat intake or vigorous exercise.  Units are (ml/min). 7 Glomerular filtration rate (GFR)  Urine collection:  8.00 am empty bladder and void.  Start collecting.  8.00 am next day empty bladder, collect urine and stop.  Incorrect performance a major source of error in GFR measurement.  Creatinine concentration is relatively static over time.  Not a great biomarker! 8 GFR or plasma creatinine?  GFR changes substantially before change in plasma creatinine is detectable.  Therefore GFR is a more sensitive indicator of potential renal damage.  However:  Measurement of plasma creatinine concentration is more precise than of GFR.  Plasma creatinine is easier to measure than the GFR.  Plasma creatinine is entirely satisfactory to follow the course of renal disease or to detect rejection after renal transplantation. 9 Causes of elevated creatinine in plasma  Impaired renal perfusion.  Due to decreased blood pressure, reduced blood volume or narrowing of the renal artery.  Loss of functioning nephrons.  e.g. glomerulonephritis which is associated with antibody mediated damage or deposition of immune complexes in glomeruli.  Increased pressure on tubular side of nephron  e.g. prostatic enlargement or a kidney stone (most commonly consisting of calcium oxalate or calcium oxalate and phosphate). 10 Interpretation of plasma urea measurements  Wide normal range 2.5 - 6.6 mmol/l limits use.  High protein diet, or increased protein catabolism after trauma, will increase plasma urea without kidney involvement.  Liver failure, low protein diet or water retention will decrease plasma urea without renal involvement.  As with creatinine, decreased perfusion of the kidney, renal disease and obstruction of urine outflow will elevate plasma urea. 11 Summary Kidney is vital in regulating pH balance and ridding the body of toxins. Renal function also regulates fluid balance. GFR and Creatinine in plasma function as biomarkers for kidney function, but come with pros and cons