Assessing kidney function

Question 1

GFR

Answer 1

Volume of plasma filtered per unit of time

Conc urine x urine flow rate
/
conc plasma

Question 2

Creatinine

Answer 2

Metabolic product of creatine and phosphocreatine
Does not bind plasma proteins
Freely filtered and almost never reabsorbed

But it is secreted by tubules so overestimates by about 10%
Increases error at lower GFR
Related to muscle mass (malnutrition/elderly?)
Trimethoprim/cimetidine compete for excretion

Question 3

Urea

Answer 3

Metabolic product of amino acids
Exogenously acquired from protein intake
Freely filtered

But…50% reabsorbed by PCT - depends on water/Na reabsorption
Liver disease reduces plasma urea levels
Protein degradation in intestines increases urea

Question 4

Why may estimates for clearance levels based on serum creatinine be inaccurate?

Answer 4

Extremes of age and body size
Severe malnutrition/obesity
Disease of skeletal muscle
Veg diet
Rapidly changing kidney function
Pregnancy

Question 5

Cystatin C

Answer 5

Cysteine protease inhibitor
produced in nearly all nucleated human cells
Independent of body mass, sex, age, inflamm or malignancy
Freely filtered
Reabsorbed and metabolised by proximal tubule cells
Serum levels correlate with GFRs
Better for elderly populations
But not yet validated

Question 6

How do you calculate eGFR?

Answer 6

Cockcroft-gault

140-age x weight / 72 x serum cr x constant (M/F)
Weight surrogate for muscle mass

Question 7

How does renal blood flow affect eGFR?

Answer 7

Due to the hydraulic pressure in glomerulus based on afferent and efferent arterioles dilation/constriction

Question 8

How do ACE-I and ARBs cause drop in GFR?

Answer 8

Dilate efferent > afferent arterioles, reducing intraglomerular pressure, dropping GFR

Question 9

How do NSAIDs cause drop in GFR?

Answer 9

Inhibit PG synthesis, no vasodilation potential, combining with low RBF means reduction in intraglomerular pressure and therefore lower GFR

Question 10

What scan do we use for assessing perfusion, GFR and ERPF?

Answer 10

Perfusion – MAG3, DTPA
GFR – Cr-51 EDTA, DTPA
ERPF – MAG3

Question 11

Cr-EDTA/DTPA

Answer 11

Both excreted by GF
Low radiation dose
Smaller fraction of DTPA bound to proteins than EDTA
Not useful if impaired RF – GFR <30ml/min

Question 12

DMSA

Answer 12

Concentrates in renal cortex and binds to plasma proteins (retained for longer) – 6hrs
Relative kidney function
Areas of scarring/non-functioning

Question 13

MAG3

Answer 13

Highly protein bound
Cleared by proximal tubules (89%)
Extraction fraction is 40-50% (better than DTPA)
Independent indicator of ERPF and RF

Question 14

Urinalysis

Answer 14

Urine consists of 95% water and >3000 chemicals

Metabolic breakdown products, drugs, anions/cations, environmental chemicals, bacterial breakdown products

Question 15

What do urinalysis products show?

Answer 15

Blood – red cells/free haem
Ketones – DKA/fasting
Glucose – diabetes
Protein – specific for albumin
Nitrates – bacterial product
Leucocytes – UTI’s/allergies
pH – not accurate on dipstick
Specific gravity

Question 16

Maintaining Hb

Answer 16

EPO increases reticulocyte production and release from bone marrow
Made in cortex and outer medulla
Hypoxia stimulates EPO mRNA synthesis
Acidosis reduces o2 affinity of Hb so inc tissue oxygenation, reducing EPO
CKD causes fibroblastoid cells to become myofibroblastoid and so less EPO production

Question 17

Calcium haemostasis

Answer 17

Calcium is freely filtered and reabsorbed along nephron, actively entering cells via PTH stimulation
Low calcium increases PTH
PTH cases bone resorption and inc vit D synthesis in kidney