measuring renal function

Question 1

what are the 2 reasons for measuring renal function?

Answer 1

1) identification of renal impairment in your patient

2) modification of dosages of drugs which are cleared by the kidneys

Question 2

what are the 7 different types of patients at risk of renal failure?

Answer 2

1) extremes of age
2) polypharmacy
3) specific disease states
4) patients receiving long-term analgesia
5) transplant patients
6) drug therapy
7) patients undergoing imagine procedures

Question 3

what are the 3 main ways to monitor a patients renal function?

Answer 3

1) patients clinical condition
2) modern imaging techniques
3) biochemical data

Question 4

what is used for patient clinical condition?

Answer 4

• weight charts
• fluid balance charts
• degree of oedema
• results of urine dipstick testing

Question 5

how are modern imaging techniques used?

Answer 5

• include macroscopic views of renal blood flow, filtration and excretory function
• some of these are used clinically but some ca only be currently used experimentally in the lab

Question 6

what 3 types of scan fit into renography?

Answer 6

• gamma camera planar scintigraphy
• positron emission tomography (PET)
• single photon emission computerised tomography (SPECT)

Question 7

what is biochemical data useful for?

Answer 7

identifying renal impairment

Question 8

what types of blood markers are used for renal function?

Answer 8

• plasma or serum creatine (SCr)

- plasma or serum urea or blood urea nitrogen (BUN)

Question 9

what are the key characteristics of creatine?

Answer 9

• breakdown of product of creatine phosphate in muscle
• generally produced at constant rate
• filtered at the glomerulus with some secretion into proximal tubule
• normal range in plasma: 40-120 micro mol/L

Question 10

what is plasma creatine increased by?

Answer 10

• large muscle mass, dietary intake
• drugs which interfere with analysis
• drugs which inhibit tubular secretion
• ketoacidosis
• ethnicity

Question 11

what is plasma creatine decreased by?

Answer 11

• reduced muscle mass
• cachexia/starvation
• immobility
• pregnancy
• severe liver disease

Question 12

what are the key characteristics of urea?

Answer 12

• liver produces urea in the urea cycle as a waste product of protein digestion
• filtered at the glomerulus, secreted and reabsorbed in the tubule
• plasma urea described as BUN

Question 13

what does BUN stand for?

Answer 13

blood urea nitrogen

Question 14

what is the normal range of BUN?

Answer 14

2.5-7.5mmol/L

Question 15

what level of BUN indicates moderate to severe renal failure?

Answer 15

> 20mmol/L

Question 16

what is BUN increased by?

Answer 16

• high protein diet
• hyper catabolic conditions
• GI bleeding
• muscle injury
• drugs
• hypovolaemia

Question 17

what is BUN decreased by?

Answer 17

• malnutrition
• liver disease
• sickle cell anaemia
• SIADH

Question 18

what are the key features of an ideal marker of kidney function?

Answer 18

• naturally occurring molecule
• not metabolised
• only excreted by the kidney
• filtered but not secreted or reabsorbed by the kidney

Question 19

what is an example of a substance that is freely filtered but not reabsorbed or secreted by the kidney?

Answer 19

inulin

Question 20

what is an example of a substance that is freely filtered and partly or mostly absorbed by the kidney?

Answer 20

electrolytes

Question 21

what is an example of a substance that is freely filtered but fully reabsorbed by the kidney?

Answer 21

glucose and amino acids

Question 22

what is an example of a substance that is freely filtered, not reabsorbed but fully secreted by the kidney?

Answer 22

PAH

Question 23

what is clearance?

Answer 23

the volume of plasma completely cleared of a given substance in unit time

Question 24

what 3 basic functions does renal clearance provide information on?

Answer 24

• glomerular filtration (F)
• tubular reabsorption (R)
• tubular secretion (S)

Question 25

what is the rate at which kidneys excrete solute into urine equal to?

Answer 25

the rate at which solute disappears from blood plasma

Question 26

what is the equation for renal clearance?

Answer 26

Cx = Ux x V / P

Question 27

what are some drawbacks of renal clearance?

Answer 27

- measuring clearance means measurement of overall nephron function - this gives the sum of all transport processes occurring along nephrons - no information is given about precise tubular sites or mechanisms of transport

Question 28

what should the glomerular filtration rate be?

Answer 28

125mL/min

Question 29

how is glomerular filtration rate measured?

Answer 29

- GFR can be measured clinically and used as an indication of renal function - GFR can be estimated by measurement of clearance of creatine - creatine is filtered and secreted into tubule

Question 30

what is a more accurate estimation of renal function?

Answer 30

inulin clearance

Question 31

how is inulin clearance used for measurement of GFR?

Answer 31

- it is freely filtered but it is not secreted and it is not absorbed - therefore rate of excretion in urine equals the rate of filtration by the kidneys - inulin clearance = GFR - if a substance has clearance greater that inulin then it must also be being secreted; less means that it must be being reabsorbed or not filtered freely at the glomerulus

Question 32

what is inulin?

Answer 32

inulin is a plant polysaccharide Mw 5200Da

Question 33

what are the drawbacks of inulin clearance to measure GFR?

Answer 33

- inulin must be administered by IV to get relatively constant plasma or serum levels - chemical analysis of inulin in technically demanding - could use radio labelled compound instead - however, these may also bind to proteins and distort results - problems of IV infusion of GFR marker are avoided by using an endogenous substance with inulin-like properties

Question 34

how does creatinine clearance work?

Answer 34

- creatinine is filtered at the glomerulus but some is secreted into the proximal tubule - therefore using the equation above would actually over-estimate GFR by about 20% in humans - however the colorimetry methods used to measure creatinine, under-estimate creatinine concentrations by about 20% - luckily these 2 errors cancel each other out, and calculated creatinine clearance is approximately inulin clearance

Question 35

what are the advantages to creatinine clearance?

Answer 35

- cheap, easy, reliable, used clinically | - avoids IV infusion, just requires venous blood and urine samples

Question 36

how is creatinine usually produced?

Answer 36

by creatinine phosphate metabolism in muscle

Question 37

how do you use PAH clearance to measure renal blood flow?

Answer 37

- if a substance is completely cleared from the plasma, its clearance rate will be equal to the renal plasma flow - clearance of PAH - para-aminohippuric acid can be used to estimate this - PAH another normally present in the blood - when given, almost all cleared from kidney in one passage, some filtered in glomerulus and remainder secreted by proximal tubules - around 10% by-passes tubule, travels from efferent arteries into peritubular capillaries an then venous renal blood, and is not secreted - uncorrected value for PAH clearance often used: known as effective renal plasma flow

Question 38

what are the biomarkers of renal disease?

Answer 38

- indicators of renal function such as plasma creatinine or BUN increase only after there is a significant loss of renal function: typically a 60% loss in renal function before plasma or creatinine or BUN increases - urinary albumin/protein excretion can also be used as an indicator of chronic kidney disease - currently a lot of research interest in identifying blood and urinary markers which increase in the early stages of renal failure and can be measured

Question 39

what are the main proteins released into the plasma and/or urine to show renal disease?

Answer 39

- kidney injury molecule - IL-18 - fatty acid binding proteins - neutrophil gelatinise associated lipocalin - cystatin C