Evaluating Renal Function

What is urinalysis? What is an abnormal constituents?

• Examines pee

• Colour and transparency:

  • Clear to deep yellow

  • Cloudy urine = UTI

  • Viatamins, drugs and diet can alter

• Odour

  • Slightly aromatic when fresh

  • Altered by drugs and vegetables

• pH

  • Slightly acidic (~6)

  • Diet, prolonged vomiting or UTI may alter pH

• Specific gravity

  • Higher levels mean more solutes

  • Dehydrated: 1.040

  • Overhydrated: close to 1

  • Renal dysfunction: <1.01

  • Normal: 1.01 - 1.025

What is glomerular filtration rate? What is an abnormal constituents?

• Substances in the blood plasma pass from the glomerulus into the Bowman’s capsule.

• 20% of plasma is filtered

• RBF = 1200 mL/min

• ERPF = 700 mL/min

• GFR = 125 mL/min

• Filtration fraction 15-25%

What is the Renal Clearance Test? What is an abnormal constituents?

• Volume of plasma cleared of a particular substance each minute as it is excreted into urine.

• Determines GFR, detects glomerular damage and follows progress of renal disease

• Cs < GFR = S undergone net reabsorption

• Cs > GFR = S undergone net secretion

What is blood urea nitrogen (BUN)? What is an abnormal constituents?

• Urea from protein metabolism

  • Removed solely by kidneys

  • 10-20 mg/dL (2.5-7.1 mmol/L) normal

• BUN: creatinine (normally 10-15)

  • >15 GI bleeding, dehydration, CHF

  • <7 liver disease, starvation

What is renal clearance of inulin? What is an abnormal constituents?

• For any substance freely filtered and neither reabsorbed nor secreted by kidneys

• C inuline = GFR = 125 mL/min

• Cs < 125 mL/min = reabsorbed

• Cs = 0 = subtance is completely reabsorbed

• Cs > 125 mL/min = secreted

What is the purpose of measuring GFR with Creatinine?

• By product of muscle metabolism (endogenous)

• Released into blood at relatively constant rate

• Only require one blood sample

• Creatinine is not secreted and not reabsorbed

• Often overestimates GFR by 10-20%, giving ~145 mL/min

• Amount filtered = amount excreted

Whats the physiology underlying the relationship between plasma creatinine and GFR?

• Non-linear, inverse correlation

• When kidney filtration drops, the kidneys excrete less creatinine causing it to rapidly accumulate in blood

• Creatinine is produced and released at a constant rate into the blood stream

• Once GFR dorps below 60 mL/min, nephrons lose ability to compensate, thus plasma creatinine rises.

• Insensitive marker for early kidney disease