1. Renal function- basic data
for students and residents
Department of Paediatrics
Section for Pediatric Nephrology
University Hospital Motol, Prague
2. Functions of the kidney
regulation e.g. homeostasis,
water, acid/base
excretion e.g. urea, creatinine
endocrine e.g. renin,
erythropoietin,
1,25 dihydroxycholecalciferol-
conversion only in kidney!
3. Renal function tests
® detect renal
damage
® monitor functional
damage
® help determine
etiology
4. Laboratory tests of renal function
® glomerular filtration ® urine protein
rate (GFR) ® urine glucose
® plasma creatinine
® hematuria
® plasma urea
® osmolality
® urine volume
® urine urea
® minerals in urine
5. Kidney Function
® A plumbers view Input
Arterial
Filter
Processor
Output Output
Venous Urine
6. Kidney – basic data
® Urine excreted daily in adults: cca 1.5L
® Kidney only ca 1% of total body weight, despite it
® The renal blood flow= 20% of cardiac output
® Plasma renal flow= PRF ca 600 mL/Min./1.73 M2
® Reflects two processes
® Ultrafiltration (GFR): 180 L/day
® Reabsorption: >99% of the amount filtered
7. How do you know it’s broken?
Input
Arterial
® Decreased urine
production
Filter ® Clinical
symptoms
Processor
® Tests
Output Output
Venous Urine
8. Where can it break?
Input
Arterial
® Pre-renal
® Renal
Filter
(intrarenal)
Processor
® Post-renal
Output Output
Venous Urine (obstruction)
9. Causes of kidney functional disorders
® Pre-renal e.g.
decreased
intravascular
volum
® Renal e.g. acute
tubular necrosis
® Postrenal e.g.
ureteral
obstruction
10. Tests of renal function
® glomerular filtration ® urine protein
rate=GFR ® urine glucose
® plasma creatinine= Pcr ® hematuria
® plasma urea-Purea ® osmolality
® urine volume= V
® urine urea- Uurea
® cystatin C in plasma?
11. Renal Function Tests-
Urine volumes
® Adults:
® 1.5 L/24 h
typical in health,
® oliguria < 400 mL,
® anuria < 100 mL,
® polyuria > 3000 mL
® Children: ca 1.5 ml/Kg
of b.w./1 hour!
12. Principle of of Clearance
® Some substances when filtered enter the tubules
are not reabsorbed and so 100% excreted=
GFR (inulin= gold standard for GFR,
creatinine (but this one partially reabsorbed,
particularly in uremia, then clearance <GFR
® Some substances are filtered, enter tubules, and
more of the substance is secreted enters the
tubules by excretion. Clearance>GFR
® Some substances are filtered, enter tubules, but
are completely reabsorbed, so they did not
reach the final urine (e.g. cystatin C)
13. Glomerular filtration rate
® Glomerular filtration= major physiologic
responsibility of kidney, GFR used as index of
overall excretory function
® Methods:
® clearence of inulin, creatinine, EDTA and
DTPA (=both derivates of acetic acid), cystatin
C
® GFR= Ux x V (V=volum of urine/ 1 minute or 1 second)
® P x x= clearence of substance used
14. Glomerular filtration rate
® Also service of nuclear medicine dptm.
® Follow up the inulin clearence, EDTA or
DTPA clearence labelling the substances
with chromium or Tcm99
Where will you catch the activity with
detectors?
Never in the kidney or bladder area!!
15. Glomerular filtration rate
® GFR in children, value always adapted to the
BSA!! Ideal BSA in adults is 1.73m2
®Schwartz equation : GFR= v x 0.808
® P (umol/L)
cr
®How to assess easy if plasma creatinine is OK?
16. Creatinine and Urea Plasma
Concentration- hyperbolic correlation
Tendency in individual patients is
pCr, more important than the one value,
pUrea ever test if the hydration is OK. In
patients with CRI always note also
the BSA!
Lower limit today not 80 ml/Min. /
1.73 m2 but 90 ml/Min./1.73 m2
Normal
range->
0 mL/min 140 mL/min
(0%) GFR 50% (100%)
17. Plasma urea (BUN)
® = BUN (blood urea nitrogen)
® Urea: product of protein catabolism
® Synthesized by liver, majority
excreted by kidney, partially
reabsorbed in tubuli
® Plasma concentration increases with
decreased GFR
20. Urea in patients
with kidney diseases
® Useful test but must be interpreted with
great care, urea plasma level is more than
creatinine dependent on protein intake
Most useful when considered along with
creatinine
® High in high protein intake, low in severe
liver dysfunction
® Urea EF may be useful in pts. on diuretics
21. Plasma creatinine and renal functions
® Creatine: main storage compound of
high energy phosphate needed for
muscle metabolism.
Creatine Creatinine
H2O (Waste product)
® Creatinine: anhydride of creatine!
22. Plasma creatinine vs. GFR
not linear, hyperbolic correlation!
[pCreat]
Change within an
individual patient is usually
more important than the
absolute value
0 mL/min 140 mL/min
(0%) GFR (100%)
23. Jaffe´ reaction for measuring
creatinine, simple, but better is
enzymatic method
Creatinine + alkaline picrate solution
Bright orange/red colored complex
absorbs light at 485nm
(many interfering substances in blood
Can be minimized using rate method)
24. Analytical methods (Cr)
® Normal range Pcr
Male 0.6-1.2 mg/dL,
Female 0.5-1.0 mg/dL
Be careful in children!!
Remember the max.
plasma creatinine
value!!
25. BUN: creatinine ratio
Input
Arterial
® Pre-renal disorders
® BUN:Cr ratio >20
Filter ® Renal disorders
® BUN: Cr nl but both
Processor
elevated
® Post-renal
Output Output
Venous Urine
26. Osmolality of urine
® Measures urine concentrating ability
® Depends on # of particles, not size or charge
® Largely due to ADH (anti-diuretic hormone)
® Can reach maximum of 1200 mOsm/L
® Normal range: 300-900mOsm/L, plasma 285+10
® prior to collection, fluid intake restricted, first
void submitted for evaluation
® Measuring using the fact of freezing point
depression
27. Standardized renal
concentration capacity test
® 1. Voiding completely at 9 p.m. (WC)
® 2. Desmopressin administration (since 2006 as
nasal spray). DDAVP is a Czech invention !!
® 3. Collection of urine (9 p.m. – 7 a.m.)
® 4. Testing of urine osmolality in this sample (not
the morning urine only!)
® 5. The lower limit of normal value= 950
mOsm/kg of urine
® 6. Short testing- Desmopressin, collection for 4
hours only= at least 900 mOsm/kg of urine
28. Urine dipsticks
® Strip impregnated with reagents for the substances in
question within a urine sample
® Substance level can be altered in the setting of pathology
within the urinary tract
Measured substances:
®
® Modern dipsticks with multiplied zones:
® Protein, hemoglobin, glucose, urobilinogen, nitrite,
leukocytes, specific gravity, and pH
® Should be a tool everywhere on the level of primary
care!!!
Notas do Editor
The kidneys excrete the end products of metabolism, urea from amino acid breakdown, uric acid from purine (nucleic acids) metabolism and creatinine from the catabolism of creatine an amino acid found in muscle. Homeostatic functions include the maintenance of water balance by regulating urine volume, acid base balance by altering hydrogen ion excretion, sodium balance by altering the rate of sodium reabsorption. Endocrine functions include the secretion of renin from the JGA which influences aldosterone. Erythropoietin effects the rate of red cell production and 1,25-dihydroxycholecalciferol is the active form of vitamin D, effecting calcium homeostasis. Patients with chronic renal disease and impaired renal functions will show defects in endocrine and excretory functions before the loss of homeostatic control. When the homeostatic functions cease then the patient is in renal failure and would die if there were no interventions.
From a clinical perspective it is important to have test which would have these characteristics. No such test exists. An early test to detect renal damage, for instance a simple strip test for haematuria is important in screening for heavy metal poisoning. There is a clinical need to monitor a patient with renal disease and this is achieved by serial plasma measurements. We need to know when to start dialysis in renal failure and laboratory tests assist the clinical decision making. There are about a million nephrons in each kidney and this represents a considerable functional reserve. In renal disease about half the nephrons have to lose their functioning before the abnormality can be detected by conventional laboratory tests.
I shall review the tests in the left column today. The measurement of urine protein is important in certain conditions, e.g.diabetes. The detection of substances such as red cells or glucose could be an early indicator of renal damage.
Oliguria is a significant finding in a patient. An example is provided by Case 3 in the Chem Path tutorials. The traditional classification of causes is into prerenal, renal and postrenal. Usually the cause of the oliguria is obvious and can be appropriately managed.
I shall review the tests in the left column today. The measurement of urine protein is important in certain conditions, e.g.diabetes. The detection of substances such as red cells or glucose could be an early indicator of renal damage.
Urine volume depends on how much you drink and sweat. In health it is closely matched to water balance by the hormone ADH or vasopressin, AVP. We define abnormally low urine volume as a 24 hour volume less than 400 mL. This is known as oliguria. A patient is considered anuric when there is no or little urine, less than 100 mL/24 h. There is no absolute definition for polyuria as some people can drink an awful lot and match it with a high urine output. If a patient has a urine volume greater than 3 litres per day and is not drinking then this is polyuria.
I shall review the tests in the left column today. The measurement of urine protein is important in certain conditions, e.g.diabetes. The detection of substances such as red cells or glucose could be an early indicator of renal damage.
Urea is easily measured. It has a wide reference range and the value increases after a meal. Its concentration is increased in many different conditions which makes it sensitive to the presence of disease but a non-specific test.
Urea is easily measured. It has a wide reference range and the value increases after a meal. Its concentration is increased in many different conditions which makes it sensitive to the presence of disease but a non-specific test.
In health only about 60% of filtered urea is excreted the rest is reabsorbed passively by the renal tubules. The rate of urea reabsorption is variable and depends on the rate of tubular flow. More urea is reabsorbed if the flow rate is slow as there is more time for urea to diffuse into the peritubular capillaries. Flow rate is slow when there is a decrease in RBF, following myocardial infarction for example. More urea is reabsorbed and plasma urea increases. Many conditions result in renal hypoperfusion including fluid loss, circulatory insufficiency, renal artery stenosis
From a clinical perspective it is important to have test which would have these characteristics. No such test exists. An early test to detect renal damage, for instance a simple strip test for haematuria is important in screening for heavy metal poisoning. There is a clinical need to monitor a patient with renal disease and this is achieved by serial plasma measurements. We need to know when to start dialysis in renal failure and laboratory tests assist the clinical decision making. There are about a million nephrons in each kidney and this represents a considerable functional reserve. In renal disease about half the nephrons have to lose their functioning before the abnormality can be detected by conventional laboratory tests.
In most circumstances the measurement of plasma creatinine can provide a specific test of glomerular function. The reference range is wide. A body builder may have a plasma creatinine at the top end and an old lady a value at the low end and this reflects muscle mass. Plasma creatinine should not be measured until 8 hours after a meal as there is some evidence that the concentration increases after meat ingestion. Plasma creatinine concentration increases when GFR falls. The problem is that GFR has to fall quite a bit before plasma creatinine concentration reliably increases. There are some important analytical interferences which you should check with the laboratory. A patient with ketoacidosis, jaundice or infection might have agents in the plasma which could invalidate the measurement of creatinine. Overhead 1 follows
GFR is not often measured in clinical practice. It requires a patient to come to hospital. Currently people who are considering donating a kidney whilst they are alive have their GFR measured. Before administering a drug with potentially toxic effects some patients will require a GFR measurement before the chemotherapy. This enables the oncologist to calculate the exact dose of drug after estimating its elimination rate. GFR used to be measured by calculating the clearance of inulin. Nowadays radioactive substances are used, either technetium labelled diethylenediaminetetra acetic acid DTPA or 51-chromium labelled EDTA ethylenediaminetetra acetic acid.
