Transcript Laboratory tests of renal function and hydration status

Renal function- basic data
for students and residents
Department of Paediatrics
Section for Pediatric Nephrology
University Hospital Motol, Prague
Functions of the kidney
regulation e.g. homeostasis,
water, acid/base
excretion e.g. urea, creatinine
endocrine e.g. renin,
erythropoietin,
1,25 dihydroxycholecalciferolconversion only in kidney!
Renal function tests
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detect renal
damage
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monitor functional
damage
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help determine
etiology
Laboratory tests of renal function
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glomerular filtration
rate (GFR)
plasma creatinine
plasma urea
urine volume
urine urea
minerals in urine
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urine protein
urine glucose
hematuria
osmolality
Kidney Function
 A plumbers
view
Input
Arterial
Filter
Processor
Output
Venous
Output
Urine
Kidney – basic data
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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
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Ultrafiltration (GFR): 180 L/day
Reabsorption: >99% of the amount filtered
How do you know it’s broken?
Input
Arterial
Filter
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Decreased urine
production
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Clinical
symptoms
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Tests
Processor
Output
Venous
Output
Urine
Where can it break?
 Pre-renal
Input
Arterial
 Renal
Filter
(intrarenal)
Processor
 Post-renal
Output
Venous
Output
Urine
(obstruction)
Causes of kidney functional disorders
Pre-renal e.g.
decreased
intravascular
volum
 Renal e.g. acute
tubular necrosis
 Postrenal e.g.
ureteral
obstruction
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Tests of renal function
glomerular filtration
rate=GFR
 plasma creatinine= Pcr
 plasma urea-Purea
 urine volume= V
 urine urea- Uurea
 cystatin C in plasma?
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urine protein
 urine glucose
 hematuria
 osmolality
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Renal Function TestsUrine volumes
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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!
Principle of of Clearance
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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)
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)
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P x x= clearence of substance used
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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!!
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Glomerular filtration rate
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GFR in children, value always adapted to the
BSA!! Ideal BSA in adults is 1.73m2
 Schwartz equation : GFR=
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v
0.808
P (umol/L)
x
cr
 How to assess easy if plasma creatinine is OK?
 Pcr max= Vcm x 0.61 (v= body height in cm
Creatinine and Urea Plasma
Concentration- hyperbolic correlation
pCr,
pUrea
Tendency in individual patients is
more important than the one value,
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
GFR 50%
(0%)
140 mL/min
(100%)
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
Urea cycle
aspartate
CO2
NH3
Urea
Cycle
3 ATP
Urea
Enzymatic conductivity rate
method for measuring urea
Urea + 3H2O
urease
HCO3¯ + 2NH4 + OH¯
Urease
solution
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
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Plasma creatinine and renal functions
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Creatine: main storage compound of high
energy phosphate needed for muscle
metabolism.
Creatine
Creatinine
H2O
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(Waste product)
Creatinine: anhydride of creatine!
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
(0%)
GFR
140 mL/min
(100%)
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)
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!!
BUN: creatinine ratio
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Input
Arterial
Pre-renal disorders
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Filter
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BUN:Cr ratio >20
Renal disorders
 BUN:
Cr nl but both
elevated
Processor
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Output
Venous
Output
Urine
Post-renal
Osmolality of urine
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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
Standardized renal
concentration capacity test
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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
Urine dipsticks
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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:
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Protein, hemoglobin, glucose, urobilinogen, nitrite,
leukocytes, specific gravity, and pH
Should be a tool everywhere on the level of primary
care!!!