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 dihydroxycholecalciferol-

conversion only in kidney!

Renal function tests

detect renal damage

monitor functional damage

help determine etiology

Laboratory tests of renal function

glomerular filtration

rate (GFR)

plasma creatinine

plasma urea

urine volume

urine urea

minerals in urine

urine protein

urine glucose

hematuria

osmolality

Kidney Function

A plumbers view

Filter

Processor

Input

Arterial

Output

Venous

Output

Urine

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

How do you know it’s broken?

Decreased urine

production

Clinical

symptoms

Tests

Filter

Processor

Input

Arterial

Output

Venous

Output

Urine

Where can it break?

Pre-renal

Renal

(intrarenal)

Post-renal

(obstruction)

Filter

Processor

Input

Arterial

Output

Venous

Output

Urine

Causes of kidney functional disorders

Pre-renal e.g.

decreased

intravascular

volum

Renal e.g. acute

tubular necrosis

Postrenal e.g.

ureteral

obstruction

Tests of renal function

glomerular filtration

rate=GFR

plasma creatinine= Pcr

plasma urea-Purea

urine volume= V

urine urea- Uurea

cystatin C in plasma?

urine protein

urine glucose

hematuria

osmolality

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!

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)

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

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!!

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

cr (umol/L)

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

GFR 50%

pCr,

pUrea

140 mL/min

(100%) 0 mL/min

(0%)

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->

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

CO2

NH3

aspartate

Urea

3 ATP

Urea

Cycle

Enzymatic conductivity rate

method for measuring urea

Urease

solution

Urea + 3H2O

HCO3¯ + 2NH4 + OH¯

urease

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

Plasma creatinine and renal functions

Creatine: main storage compound of high

energy phosphate needed for muscle

metabolism.

Creatinine: anhydride of creatine!

Creatine Creatinine

(Waste product) H2O

Plasma creatinine vs. GFR

not linear, hyperbolic correlation!

GFR

[pCreat]

140 mL/min

(100%) 0 mL/min

(0%)

Change within an

individual patient is usually

more important than the

absolute value

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

Pre-renal disorders

BUN:Cr ratio >20

Renal disorders

BUN: Cr nl but both

elevated

Post-renal

Filter

Processor

Input

Arterial

Output

Venous

Output

Urine

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

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

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!!!