3. Renal Function Today.ppt

Renal function
BCM 242
Clinical Biochemistry
9/20/2023 1

Nephron
• Structural and functional portion of the
kidney.
• About 1million per kidney.
• Consists of two parts the glomerulas
(filtering part) and the tubules.
• Glomerulas (tuft of capillaries surrounding
the bowman’s capsule.
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Functions of the kidneys
• Urine formation.
• Regulation of fluid and electrolyte
imbalance.
• Regulation of acid and base balance.
• Excretion of waste products of metabolism
• Hormonal function.
• Protein conservation.
9/20/2023 3

Summary of filtration events.
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Summary of structure
• Afferent and Efferent Arterioles
– Juxtaglomerular apparatus
• Capillary bed:
– Endothelial Cells: Fenestrated/negatively
charged
– Epithelial Cells (“Podocytes”)
– Basement Membrane
– Mesangium: Intrinsic glomerular cells and
macrophages
9/20/2023 6

Renal functions
• Rate (GFR): 120 mL/min (normal)
• Substances “Filtered”:
– water, electrolytes (Na, K, etc.), sugars
(glucose), nitrogenous waste (urea,
creatinine)
• Substances “Excluded”:
– Substances of size > 70 kDa
– Plasma protein bound substances
9/20/2023 7

• Re-absorbtion may be passive or active
depending on the substances involved.
• Active against a concentration gradient e.g.
glucose, amino acids,low molecular weight
proteins etc.
• Passive- no energy involved- water and urea.
• Tubular secretion may also be active or
passive.
9/20/2023 8

Proximal convulated tubule
• 80% of water absorbed from the glomerular
filtrate.
• All glucose and amino acids.
• Low molecular weight protein, urea, uric
acid, bicarb etc absorbed to various
degrees.
• Organic acids and bases, hydrogen ions and
ammonia may be secreted into the tubular
fluid.
9/20/2023 9

Loop of Henle
• Decending limb is highly permeable to
water.
• Passive re-absorbtion of water in the
descending loop.
• Ascending loop is impermeable to water,
but actively reabsorbs sodium and chloride
hence called the diluting segment.
• Diluting segment as it lowers thesalt
concentration.
9/20/2023 10

Distal convulated tubule
• Little amount of sodium, chlorides, water
reabsorbed.
• This is done under the influence of
antidiuretic hormone.
• ADH increases the porosity of the tubule
for water reabsorbtion.
• Potassium reabsorbed or secreted.
9/20/2023 11

Collecting duct
• Also under control of ADH.
• Re-absorbtion of sodium and chloride.
9/20/2023 12

Regulation of fluid and electrolyte
balance
• Water: body water weight remains constant day
by day-ability of the kidney to excrete or
conserve water.
• Water level is control by voluntary intake (thirst)
and excretion (urine loss).
• Sodium freely filtered through the glomerulas
and reabsorbed.
• Active reabsorbtion of Na+ results in passive
transport of Cl- and bicarbonate ions
9/20/2023 13

• Chloride parallels sodium, but reabsorption is
passive in proximal convulated tubules and
active in distal convulated tubules.
• Potassium- aldosterone enhances potassium
secretion hence maintaining its body levels.
• Calcium reabsorption in proximal convulated
tubules parallels that of sodium.
• Calcium balance is maintained through
absorption of calcium from the intestines.
• High calcium diet results in increased renal
secretion.
9/20/2023 14

• Phosphorous level through renal excretion.
-as phosphate ion in proximal convulated
tubules.
-Parathyroid hormone increases reabsorption.
• Magnesium reabsorption in proximal tubules,
parallels that of Calcium.
-also under influence of parathyroid hormone.
9/20/2023 15

Acid and base balance.
• Secretion of hydrogen ions from tubular cells.
• Hydrogen ions generated in the proximal and
distal convulated tubules as a result of carbonic
anhydrase activity is involved in;
- can react with bicarbonate ions.
-react with buffers-phosphate ions.
-react with ammonia (deamination of glutamine)
to form ammonium ion.
- secreted as hydrogen ions.
9/20/2023 16

Nitrogenous waste excretion
• When the GFR rate is low nitrogenous
waste accumulates in blood i.e. NPN.
• NPN e.g. urea is toxic.
• Ammonia from amino acids deamination is
converted to Urea via the urea cycle and
excreted via the kidneys.
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Generation of Urea
9/20/2023 18

• Urea is a major non protein Nitrogen.
• About 40% of urea is reabsorbed by the renal
tubules when glomerular fitration rate slows.
• This leads to a rise in blood urea
concentration.
• Often called Blood Urea Nitrogen (BUN) as
traditional methods used the kjeldahl method
of analysis.
• BUN is affected by a number of factors.
- High protein diet can double BUN.
- Steroid use increases the mobilisation of
protein for energy use.
- Increase tissue breakdown.
9/20/2023 19

• Urea is readily filtered by the glomerulas 40 to
50% re-absorbed.
• Not a specific indicator of renal function.
• Normals plasma/serum 0.15 to 0.45g/L
- Urine 20 to 35g/24 hour (338-538mmol/24hr).
• Decreased in liver failure, late in pregnancy,
starvation and low protein diet.
• To convert Urea N to urea in mg/dl you
multiply by 2.14.
9/20/2023 20

Creatinine
• Non enzymatic dehydration of creatine in
skeletal muscle.
• Plasma levels higher in males.
• Blood concentration dependent on only
muscle mass.
• Produced at a constant rate.
• Elevated only with impaired renal function.
• More specific and sensitive indicator of
renal disease than BUN.
9/20/2023 21

Creatinine determination using (Jaffe’
method)
• Principle invoves creatinine reacting with
sodium picrate at alkaline pH to form a yellow
orange coloured complex.
• Reagents include NaOH (2mM) and picric acid
(20mM) kept separately and used to form
Sodium picrate.
• Absorbance is measured at 500 to 520nm
against a distilled water blank.
9/20/2023 22

Measurement of Urea
• Urea can be determined using the urease modified
Berthelot reaction
Urea+H20 → 2NH3 +C02 (In the presence of Urease).
• In an alkaline medium, the amonium ions react with
salicyclate and hypochloride to form a green coloured
indophenol (2,2 dicarboxyl-indophenol).
• Samples include EDTA, citrated or heparinised
plasma, urine usually diluted first e.g. 1:100 and the
result multiplied by 99.
• Wavelength is 580nm (Hg 578).
9/20/2023 23

Uric acid
• From the oxidation of purine bases
• Circulates in serum as sodium urate.
• Removed by renal system and partially
reabsorbed in tubules.
• Not primarily a renal function test, used to
diagnise gout.
• Normal (M: 3.5 to 7.5 and F: 2.5 to 6.5mg/dl).
9/20/2023 24

Level rises in
• renal failure
• gout (remember all patients with gout have
hyperuricemia, but most patients with hyperuricemia
do not have symptomatic gout)
• liver-and-sweetbread gourmets ("organ meats" are
rich in purines)
• increased breakdown of nucleoprotein (burns, crush
injuries, very severe hemolytic anemia, plasma cell
myeloma, myeloproliferative disorders, and especially
leukemias under treatment)
• lead poisoning (why?)
• patients receiving thiazide diuretics or high doses of
aspirin
• "idiopathic" (relatives of gout victims, etc.)
9/20/2023 25

Glomerular filtration rate
• Creatinine clearance is widely used to approximate
glomerular filtration rate.
• You need a timed urine sample and a blood sample.
• Clearance=(conc. in urine)x(urine volume)/(conc. in
plasma).
• In deciding how to "time" your collection, remember
that you don't really need to collect urine for a full 24
hours.
• You can get more reliable results by a controlled
collection over 4 hours, monitoring body position (keep
them lying down) and good hydration,
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Example
• Glomerular filtration rate is proportional to body
surface area and the above range is for the
average adult with a surface area of 1.73
square meters.
• It is particularly important when evaluating renal
function of children to correct the measured
creatinine clearance to what it would be if the
child had a body surface area of 1.73 sq.
meter..
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• The "corrected" creatinine clearance is obtained
by multiplying the measured creatinine
clearance X 1.73 and dividing by actual body
surface area.
• Nomograms are available from which body
surface area is related to height and weight.
• Sample calculation of creatinine clearance:
Crs = 3.3 mg/dl and Cru = 60.0 mg/dl
V = 2400 ml t = 24 hours = 24 hr. x 60 min/hr = 1440
min.
Creatinine clearance = [(60 mg/dl)x2400 ml/1440 min]
/(3.3 mg/dl) = 30 ml/min
9/20/2023 28

Note that (Cru x V/t) = creatinine excretion rate
(amount of creatinine excreted per day) and for
the example above
= (60 mg/dl)x(2400 ml)/(1440 min) = 1 g/day
NB: Don't confuse the terms excretion rate and
clearance.
9/20/2023 29

The Mosteller¹ formula
BSA (m²) = ( [Height(cm) x Weight(kg) ]/ 3600 )½ e.g. BSA = SQRT( (cm*kg)/3600 )
or in inches and pounds: BSA (m²) = ( [Height(in) x Weight(lbs) ]/ 3131 )½
The DuBois and DuBois² formula
BSA (m²) = 0.20247 x Height(m)0.725 x Weight(kg)0.425
A variation of DuBois and DuBois15 thats gives virtually identical results is:
BSA (m²) = 0.007184 x Height(cm)0.725 x Weight(kg)0.425
The Haycock³ formula
The Gehan and George4 formula
The Boyd formula5
BSA (m2) = 0.0003207 x Height(cm)0.3 x Weight(grams)(0.7285 - ( 0.0188 x LOG(grams) )
9/20/2023 30

• Normal GFR is 100 +20ml/min.
• Increase no medical significance.
Inhibitors of creatinine secretion
• Cimetidine
• Trimethoprim
• Pyrimethamine
• Dapsone
• NB. Look at Inulin clearance and non endo-genous
markers for GFR e.g. Para-aminohippurate (PAH)
clearance, Ortho-iodo-hippuran, Radioiodine labelled
PAH analogue, Mercapto-acetyl-triglyerine (99mTc-
MAG3), 5-hydroxyindoleacetic acid (5-HIAA)
9/20/2023 31

Estimation of creatininine clearance
• A number of formulae exists for predicting
creatinine clearance (or GFR) from plasma
[creatinine] & other readily available
information, such as age, sex and weight
• The best known of these is that of Cockcroft
and Gault (1976)
Creatinine clearance =
9/20/2023 32

• Creatinine clearance is in ml/min, age in years, weight
in kg, [creatinine] in μmol/L
• The above equation has been shown to be as reliable
an estimate of creatinine clearance as the actual GFR
measurement
• But since it estimates creatinine clearance (not GFR) it
suffers from the same overestimation of GFR as renal
function declines just like GFR actual measurement
• These formulae should not be used when diet is
unusual (strict vegetarian or creatinine suppliments),
in extremes of muscle mass (mulnutrition, muscle
wasting, amputations) or obesity
• Note endogenous production of creatinine remains
constant, and the plasma reference range is 55 to
120μmol/L
9/20/2023 33

Urine Protein (Proteinuria)
• Protein in urine is noticeably increased in renal
disease of any etiology, except obstruction, and
is therefore a very sensitive, general screening
test for renal disease, though not specific.
• The extent of proteinuria also provides useful
information.
• The greatest degree of proteinuria is found in
the nephrotic syndrome ( > 3 - 4 g/day).
• What is the difference between nephrotic and
nephritic syndrome?
9/20/2023 34

• Nephrotic syndrome is a nonspecific kidney disorder
characterised by a number of signs of disease
proteinuria, hypoalbuminemia and edema
• It is X-terized by an increase in permeability of the
capillary walls of the glomerulus leading to the
presence of high levels of protein passing from the
blood into the urine (proteinuria at least 3.5 grams per
day per 1.73m2 body surface area)
• Low levels of protein in the blood (hypo-proteinemia or
hypoalbuminemia), ascites and in some cases, edema
high cholesterol (hyperlipidaemia or hyperlipemia) and
a predisposition for coagulation
9/20/2023 35

• Nephritic syndrome (or acute nephritic syndrome)
is a collection of signs (known as a syndrome)
associated with disorders affecting the kidneys, more
specifically glomerular disorders.
• It is characterized by having a thin glomerular
basement membrane and small pores in the
podocytes of the glomerulus, large enough to permit
proteins (proteinuria) and red blood cells (hematuria)
to pass into the urine.
• By contrast, nephrotic syndrome is x-terized by only
proteins (proteinuria) moving into the urine. Both
nephritic syndrome and nephrotic syndrome involves
hypoalbuminemia due to protein albumin moving from
the blood to the urine.
9/20/2023 36

• In renal disease with the nephritic syndrome,
the urinary protein excretion rate is usually
about 1 - 2 g/day.
• In tubulo-interstitial disease, urine protein is
generally less than 1 g/day.
• Only in the nephrotic syndrome is the urine
protein loss sufficiently great to result in
hypoproteinemia.
• Protein in serum can generally be maintained
at concentrations above the lower limit of
normal by increased hepatic protein synthesis
so long as protein loss is less than about 3
g/day.
9/20/2023 37

Hyperphosphatemia, hyperuricemia and
acidosis
• Metabolic with an anion gap ([Na+]+[K+] – [Cl-]
+[CO2] do not develop until there is 70-80%
loss of functional capacity (i.e., in the renal
failure stage)
• Therefore measurements of serum phosphate,
uric acid or bicarbonate, do not provide a very
sensitive indication of the possibility of renal
disease.
• However, measurement of serum phosphate
and uric acid is useful for monitoring the effect
of therapy during chronic renal disease
9/20/2023 38

• and/or the effects of attempts to minimize
increased serum concentrations by restricting
dietary intake of phosphate and nucleotides
• Acidosis is generally mild and without
pathophysiologic consequences until the uremic
stage (Uremia describes the final stage of
progressive renal insufficiency and the resultant
multiorgan failure)
• NB: AG is the diff between cations and
anions,it represents the umeasured anions
and includes proteins, phosphates,sulphate
and lactate ions
9/20/2023 39

Calcium defiecency
• This occurs during the renal failure stage of
chronic renal disease because of deficient renal
activation of vitamin D.
• But does not usually result in frank
hypocalcemia because 2˚ hyperparathyroidism
develops as a compensatory response to
maintain serum calcium concentrations within
the normal range.
9/20/2023 40

• Bone calcium, however, is lost in the process
causing renal osteodystrophy
• Renal osteodystrophy is typically a mild form of
osteomalacia and is only rarely the more severe
osteitis deformans (Van Ricklenhausen's
disease) classically associated with primary
hyperparathyroidism
• Calcium deficiency and consequent
osteomalacia is treated with activated vitamin
D.
9/20/2023 42

• Although calcium metabolism is abnormal,
serum calcium determinations are not useful for
detecting or assessing the severity of renal
disease, but its useful for monitoring the
effectiveness of vitamin D therapy and assuring
that hypocalcemia is prevented
9/20/2023 43

Sodium, potassium and renal
function
• Sodium and potassium homeostasis is
maintained, so long as intake is not extremely
variable, unless renal dysfunction is sufficiently
severe to cause oliguria (Secretion of a
diminished amount of urine in relation to the
fluid intake)
• Electrolyte determinations are, therefore, not
useful for detecting or assessing the
progression of renal disease.
9/20/2023 44

• Evaluation of renal sodium reabsorption is,
however, valuable for assessing tubule function
and is useful for distinguishing whether rapidly
developing azotemia is due to acute renal
failure or to prerenal azotemia from a
compensatory decrease in renal blood flow due
to hypovolemia.
• In cases of prerenal azotemia, without hypoxic
damage to renal tubule cells despite decreased
renal blood flow, aldosterone effectively
stimulates sodium reabsorption and urine
sodium concentration is generally less than 30
mEq/L.
9/20/2023 45

• In acute renal failure, aldosterone elevation is
ineffective in stimulating sodium reabsorption
because of tubular damage so that urine
sodium concentration is generally at least 25
mEq/L and often greater than 50 mEq/L.
• However, there is considerable overlap in urine
sodium concentrations between cases of
prerenal azotemia and acute renal failure.
9/20/2023 46

• The fraction of filtered sodium excreted (FENa) is a
more discriminating parameter for evaluating tubular
function and is determined by measurement of both
serum and urine concentrations of sodium and
creatinine:
FENa = ( Nau/Nas) / ( Cru/Crs)
• Normal values - range from 1-3%.
• If the value is below 1% dehydration can be the cause
of impaired kidney function.
• To confirm dehydration, the below factors, which
also give values below 1% must be ruled out :-
9/20/2023 47

1. Acute Glomerulonephritis - a kidney disease in
which the kidneys' glomeruli (filters) become inflamed
and scarred, and slowly lose their ability to remove
wastes and excess water from the blood to make urine
2. Hepatorenal syndrome - acute kidney failure
occurring without other cause in a person with severe
liver disease.
3. Prerenal azotemia - abnormally high level of
nitrogen-type wastes in the
bloodstream caused by conditions that reduce blood
flow to the kidneys such as congestive heart failure.
4. Their urinary tract may be partially obstructed.
9/20/2023 48

Urinalysis
• Urine specimens are examined in a routine
manner by the use of "dip sticks" for
semiquantitatively estimating the concentrations
of a number of components.
• Commonly glucose, ketones, bilirubin,
urobilinogen, protein, hemoglobin, and pH.
• The routine urinalysis also includes
measurement of specimen volume,
determination of specific gravity or osmolality
and microscopic examination of the urine
sediment for cells, casts and crystals.
9/20/2023 50

Urine Dipstick
• The entire group of tests is
clinically useful for evaluating a
wide variety of pathologic
conditions
• The following are particularly
useful for evaluating renal
disease.
9/20/2023 51

Appearance of Urine
• The appearance of urine is altered by the presence of
blood or leukocytes.
• Blood imparts a red to brown color and leukocytes
cause urine to appear turbid.
• Blood in the urine is characteristic of
glomerulonephritis, but may be due to trauma or tumor
anywhere along the urinary tract or bladder.
• Urinary white cells are characteristic of
tubulointerstitial disease, but may also result from
bladder infection. The presence of cells in casts is far
more specific for renal disease.
9/20/2023 53

Urine Volume
• Urine volume is influenced by water intake and by
concentrating ability
• Minimum urine volume during fluid fasting is normally
about 300 - 500 ml/day
• During the progression of renal disease, minimum
urine volume increases to 2 L/day when concentrating
ability is completely lost at the renal failure stage.
• Polyuria is defined as a daily urine volume greater
than 2 L/day.
• In end stage, GFR is so reduced that maximum urine
volume is less than 400 ml/day (oliguria). Daily urine
volume rarely drops so low as to be considered anuric
( <50 ml/day).
9/20/2023 54

Proteinuria
• Proteinuria is present in renal disease of almost any
etiology to varying degree
• The only other condition associated with proteinuria is
multiple myeloma
• The reagents impregnated in urine dipsticks react
only to albumin
• This is sufficient for detecting increased protein from
renal disease but increased urine protein from light
chain excretion in cases of multiple myeloma will not
be detected..
9/20/2023 55

• The sensitivity of dipsticks for detecting protein is
about 25 mg/dl which is equivalent to about 200 - 500
mg in a 24 hour urine collection
• Quantitative methods are used to determine total
protein in 24 hour urine collections
9/20/2023 56

Hematuria
• Hematuria is most characteristic of glomerulonephritis,
but may be due to trauma or tumor anywhere along
the urinary tract or bladder
• The presence of red cells in casts is more specific for
renal disease.
• The finding of casts is the most important observation
from the microscopic examination of the urinary
sediment
• Casts are normally formed within tubules from
gellation of the Tamm-Horsfall mucoprotein secreted
by the tubule epithelia of the ascending loop of Henle.
9/20/2023 57

• The mucoprotein accounts for about 30 - 50 mg of
total urine protein excreted per day
• Cast formation is favored by sluggish flow, acid, and
high protein content
• Sluggish flow permits the formation of larger casts
within larger collecting ducts
• Cells trapped in casts disintegrate to a greater degree
the slower the flow and the longer the cast remains in
the collecting ducts
• Hyaline casts of Tamm-Horsfall protein are present in
normal urine.
9/20/2023 58

• Red cell casts predominate in acute
glomerulonephritis
• White cell casts predominate in tubulointerstitial
disease.
• Granular casts result from disintegration of cellular
casts and may be coarsely or finely granular
depending on the degree of disintegration of the
cellular elements.
• Broad, waxy casts represent complete disintegration
of embedded components, suggesting sluggish flow;
the broad nature of these casts is due to their
formation in larger collecting ducts
• These casts appear in more severe stages of renal
disease and are sometimes referred to as "renal
failure casts".
9/20/2023 59

Microscopic appearance of granular casts
9/20/2023 60

Specific gravity
• The number of times a substance is more
denser than water
• Specific gravity is measured for the purpose of
determining concentrating ability
• The result has meaning for this purpose only
when the specimen has been collected
following a 12 hour fluid fast
• Normal kidneys can concentrate urine so that
the concentration of osmotically active
substances is 3 - 4 times greater than that of
plasma.
9/20/2023 61

• Normal specific gravity of a concentrated urine
specimen is greater than 1.025.
• The specific gravity of a urine specimen from a fluid
fasted patient with renal disease, sufficiently severe so
that concentrating ability is completely lost, is 1.010
• At end stage, diluting ability is minimal, and specific
gravity becomes fixed at 1.010 regardless of fluid
intake
• Specific gravity results are not reliable when urine
contains an appreciable amount of protein.
• Instead osmolarity measurements are done (Check
details of osmolarity measurements)
9/20/2023 62

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