Renal Function tests

Question 1

What substance can we measure?

Answer 1

• Need to measure rate of filtration of a substance
  – Stable plasma concentration
  – Not protein bound
  – Freely filtered by glomerulus
  – Doesn’t itself influence GFR
  – Is not secreted, reabsorbed or metabolised by tubules

• Inulin (inert fructose polysaccharide/sugar)
- All inulin filtered is excreted in the urine
- Amount of inulin filtered = volume of plasma filtered X plasma concentration of inulin
- BUT – need constant infusion to maintain stable plasma
• Creatinine

Question 2

Clearance Rates - formula:

Answer 2

GFR (ml/min) = UCR X vol of urine/unit time
————————————–
PCR

Question 3

Clearance Rates - formula:

Answer 3

• Can use similar calculation for all solutes excreted in urine/unit time
  • But some will have been reabsorbed along the tubules.
  – Eg. Glucose is completely absorbed Clearance rate = 0
  – Eg. Substances secreted by the tubules have clearan

Question 4

Estimated GFR (eGFR)

Answer 4

• Estimated GFR (eGFR) reported since 2009
• eGFR reported with blood
[creatinine]
• Based on blood [creatinine] & calculation of a formula using age, ethnicity & gender – does not require weight
• Result: range of variation ± 30%
• Influenced by recent cooked meat meal
(raises creatinine, lowers eGFR)

Question 5

Interpretation of eGFR:

Answer 5

• Normal eGFR (M) = 140 mL/min/1.73m2
• Test does not perform well for normal kidneys or mild disease
• Report actual values below 60mL/min/
1.73m2 & between 60-­‐90 mL/min/1.73m2
• Higher values reported as > 90mL/min/ 1.73m2

Question 6

Tests of renal function

Normal Urine

Answer 6

Normal
• Urine volume (1-­‐2L/24h)
• Urine specific gravity 1.005–1.025
• Urine protein (absent), Glucose (absent)
• Blood (absent)
• RBC, WBC and casts
(accumulations of cell precipitates) (absent)
• Crystals (absent)
• Creatinine clearance 90-­‐120ml/min, eGFR>60 ml/min/1.73m2 & no other indicators of disease

Question 7

Tests of renal function (Abnormal Urine)

Answer 7

In disease, ie, abnormal
• 24 hr volume <400 ml
• Urine specific gravity ~ 1.01
• Creatinine clearance (<60 ml/min); eGFR <60ml/min
• Protein -­‐level in 24 h urine a significant indicator of
severity
•Microalbuminuria = 30-­300 mg /24 h (trace) –Overt
nephropathy = > 300 mg/24 h

Question 8

Test of renal function (Blood)

Answer 8

Tests of renal function(blood)        
• In renal disease    
• Serum creatinine: ↑
• Blood urea nitrogen    (BUN):  ↑
• Blood urea nitrogen:    
creatinine ↑
•  Serum albumin:    ↓
• Electrolytes    (K+,    HPO42-­‐,    H2PO4-­‐,    Mg2+)  ↑  Ca2+,pH↓