Renal Functions
and
Laboratory Assessment
Serkan SAYINER, DVM PhD. Assist. Prof.
Near East University, Faculty of Veterinary Medicine, Department of Biochemistry serkan.sayiner@neu.edu.tr
Renal
Morphology
Renal Morphology
▪ The mammalian kidney consists of tens of thousands to millions of nephrons units. The larger the species, the greater the number of nephrons per kidney.
• This ranges from about 10,000 in mice, 175,000 in cats, 300 to 700,000 in dogs and 7 million in elephants, as compared to about 1 million in humans.
▪ The number of nephrons progressively increases during fetal development and is complete at birth.
▪ The number of nephrons in the dog decreases slightly (5%) during the 2 first months of life, whereas the glomerular
volume increases by 33%.
Source: Wiki
1. Renal pyramid 2. Interlobular artery 3. Renal artery
4. Renal vein 5. Renal hilum 6. Renal pelvis 7. Ureter
8. Minor calyx 9. Renal capsule
10.Inferior renal capsule 11.Superior renal capsule 12.Interlobar vein
13.Nephron 14.Renal sinus 15.Major calyx 16.Renal papilla 17.Renal column
Source: Wiki
1. Glomeruli
2. Efferent arteriol 3. Bowman Capsule 4. Proximal Tubule
5. Cortical Collectiong Tubule 6. Distal Tubule
7. Loop of Henle 8. Papillar Canal
9. Peritubular capillaries 10.Vena arcuata
11.Arteria arcuata 12.Afferent arteriol
13.Juxtaglomerular apparatus
Source: Wiki
Source: Wiki
Renal Morphology
▪ Blood is supplied to the kidneys by the renal arteries. These divide into interlobar and arcuate arteries located at the
corticomedullary junction.
▪ The total blood supply to the kidneys (renal blood flow,
RBF) is very high, about 20% of the cardiac output, and most of it goes to the cortex.
▪ Only a fraction of the plasma flow (renal plasma flow, RPF) is filtered resulting in the glomerular filtration rate (GFR).
• This is the filtration fraction (FF), which generally amounts to 20%
to 30% of RPF: GFR = RPF X FF.
• The RBF remains quite stable, because of autoregulation, even with variations in systemic blood pressure.
Renal
Functions
Glomerulus and Filtration
▪ Glomeruluses collect primitive urine by plasma filtration and transmit it to the tubule system.
▪ The driving force of filtration is hydrostatic pressure of cardiac origin (Fig. 16- 3). This is opposed by plasma colloi- dal (oncotic) pressure produced by
plasma proteins and urine hydrostatic pressure within the Bowman’s capsule.
▪ The limits of filtration are as follows:
• Size and shape: Neutral molecules with a diameter <2.5nm diffuse freely. Then as diameter increases, filtration decreases to approximately 0 when the diameter is
>3.5nm (i.e., albumin).
• Charge: The filtration slit tends to repel negatively charged molecules (i.e., most plasma proteins at blood pH).
▪ As a result of glomerular filtration, all small hydrosoluble plasma molecules, including water and ions, are freely filtered but high molecular weight proteins are not.
• Albumin is very close to the limit of filtration so that only a minimal amount is filtered by “normal” kidneys. The albumin concentration in primitive urine is approx. 20 to 30 mg/L and many smaller proteins are also present, most of which are reabsorbed in the tubule.
Tubule: Reabsorption and Secretion
▪ Parts of Tubule
• Proximal Tubule: Begins with a convoluted portion followed by a straight section. This is the portion of the nephron where most
solutes and water are reabsorbed.
• Loop of Henle: Produces a “hairpin” bend within the medulla, ending close to the glomerulus at the juxtaglomerular apparatus.
The loop of Henle is essential to urine concentration mechanism.
• It is often stated that long loops are mostly observed in species living in desert areas. Besides, dogs and cats are long looped henle than humans or pigs. The water and urea permeability of the descending thin limb is high. The ascending thin limb shows very low water and high NaCl permeability.
Tubul: Re-absorption and secretion
• Juxtaglomerular Apparatus: It is a morphological entity at the confluence of the afferent and efferent arterioles of the glomerulus and a differentiated part of the loop called the macula densa. The cells in the macula densa respond to decreases in blood
pressure or hyponatremia by secreting renin stored in the
granules, thus activating the angiotensin-aldosterone response.
• Distal Tubule: It stretches from the macula densa to the confluence into a collecting tubule within the cortex. The
reabsorption capacity is lower than in the proximal nephron (e.g., approx. 5% to 10% of Na and Cl), and secretion of potassium may occur.
• Collecting Tubule: Leading to the renal pelvis. The final
regulation of urine volume and solute excretion occurs in the final segment of the distal tubule and the collecting tubule, and it is
partly regulated by hormones.
Source: Wiki
• The main tubule functions are the reabsorption of water, electrolytes, and small molecules and, to a lesser extent, the secretion of ions and small molecules.
• Reabsorption is dominant in healthy animals and mainly occurs in the proximal tubule by active and passive transport. Further adjustment of urine excretion occurs in the distal tubule and is controlled by hormones, so that the final urine is usually more concentrated than the ultrafiltrate.
• The intake and utilization of ions and small molecules vary with food and water supply, proximity of meals, environment, physical effort, and so on. Urine composition can show large variations in the same healthy or diseased subject (e.g. analyte levels in spot urine).
• Glucose, amino acids, and low-molecular-weight proteins are mostly reabsorbed in the proximal tubule. Almost 100% of the glucose, amino acids, and proteins are reabsorbed. Glucose reabsorption capacity (kidney threshold) is within
certain limits (renal threshold limits); 180-220 mg/ dLfor dogs, 200-300 mg/dL for cat, 180-200 mg/dL for horse and ~100 mg/dL for cattle.
• No urea reabsorption occurs before the medullary part of the collecting duct because of the presence of urea transporters activated by antidiuretic hormone (ADH). This is part of the mechanism creating a high inner medullary osmolality. Urea reabsorption is increased when urine flow is low (e.g., during dehydration or volume depletion).
Source: Wiki
• Electrolytes are mostly reabsorbed in the proximal tubule. The rate of reabsorption differs considerably according to the internal balance of each ion. Under “normal” conditions it is almost 100% for Na, Cl, Ca, and phosphates, but much lower for potassium, especially in ruminants owing to their high dietary intake. The extent of reabsorption can be estimated from the fractional excretion (FE) of solutes.
• Na concentration is kept low in tubule cells, as in other cells, by an Na/K-ATPase in the
basolateral membrane. It also creates a sodium concentration gradient that allows cotransport of amino acids, glucose and other ions, and so on. Further reabsorption of sodium and chloride occurs in the ascending branch of the loop of Henle via an Na-K-2Cl cotransporter in the
luminal membrane. Final adjustment in the distal part of the nephron is hormonally controlled by aldosterone and natriuretic peptides. Cl is the most abundant anion in the extracellular compartment. In metabolic acidosis, bicarbonate ions secreted by the kidney cells are exchanged with chloride.
• K is reabsorbed in the proximal tubule (apporx. 70%) and in the ascending part of the loop of Henle, the distal tubule, and the medullary collecting duct. It is also secreted by the distal tubule and cortical collecting duct, mainly during hyperkalemia.
• Pi is reabsorbed in the proximal tubule by a sodium cotransporter, which is inhibited by PTH.
• Free and complexed Ca ions are freely filtered by the glomerulus. It is mostly reabsorbed in the proximal tubule and in the ascending branch of the loop of Henle (expect in horse and rabbit).
• Non-protein-bound Mg is filtered by the glomerulus. Only about 25% are reabsorbed in the proximal tubule. Most reabsorption occurs in the ascending branch of the loop of Henle (50% to 60%)
Source: Wiki
• Most water ( approx 75%) is reabsorbed passively in the proximal tubule along with ions. It is reabsorbed in the descending branch of Henle’s loop, whereas the ascending branch is impermeable to water. Final reabsorption occurs in the
collecting tubule, mainly under the of ADH.
• Acid-Base Regulation: The main organs involved in acid-base regulation are the kidneys and lungs. High concentrations of carbonic anhydrase occur in many tissues including the kidney tubule. Bicarbonate ions filtered by the glomerulus are mainly reabsorbed in the proximal tubule (approx. 80%) as CO2, which is lipophilic and able to diffuse across the membrane. Within the cell, CO2 is hydrated by carbonic
anhydrase into carbonic acid, which dissociates into bicarbonate ions and
protons. The secretion of protons into the tubule lumen ensures the conservation of bicarbonate and its transfer to the plasma. This occurs principally in the distal tubule.
• Endocrine Functions: Two major hormones, erythropoietin (EPO) and 1,25- dihydroxycholecalciferol (calcitriol), are synthesized by the kidneys and released into the blood. EPO regulates erythrocyte production synthesized in the peritubular cells. Minor amounts are also produced in the liver, mainly in the newborn.
In advanced chronic renal disease, the synthesis of EPO decreases and is insufficient to meet the demands for new red cell production, resulting in anemia.
Calcitriol is produced in the proximal tubule cells by the action of 1α-hydroxylase on 25-hydroxyvitamin D3 produced by liver hydroxylation of vitamin D3. Calcitriol synthesis is decreased in chronic renal failure (CRF). Other than these, renin, prostaglandin E2, bradykinin, natriuretic hormones/peptides are synthesized.
Kidney Functions in Brief
▪ Regulation of Fluid and Electrolyte Balance: Water, Na, K, H, HCO3, Ca, P, Mg
▪ Excretion of metabolic waste products: Urea, uric acid, creatinine...
▪ Detoxification and excretion of drugs, toxins and metabolites
▪ Regulation of extracellular fluid volume and blood pressure:
RAAS, Prostaglandins...
▪ Hormone Synthesis: Active Vitamin D, EPO...
▪ Degradation of peptide hormones: Insulin, glucagon, calcitonin...
▪ Degradation of small molecular weight proteins: Microglobulin...
▪ Metabolic Effect: Gluconeogenesis ...
Tests of Renal
Function
Renal Function Tests
▪ Kidney function can be evaluated from the concentrations of plasma or urine analytes, which are mainly dependent on their elimination.
▪ These indirect markers can be easily and rapidly
measured, but their sensitivity is poor and generally remains unaltered until 75% of renal function has been lost and their concentrations may be modified by extrarenal factors.
▪ Direct tests of kidney function are based on the elimination kinetics of markers of glomerular filtration, blood flow, or
tubule reabsorption/secretion and are based on the
clearance concept. These tests are more difficult and
take longer to perform but allow earlier detection of
reduced function.
Indirect Tests of Glomerular Function
▪ Serum/plasma creatinine is the test most often used to diagnose and monitor kidney disease in human and animal clinical pathology.
▪ Serum/plasma urea is also used frequently but is subject to more numerous extrarenal factors of variation.
▪ These molecules are almost totally eliminated by glomerular filtration, so that in the case of kidney failure their plasma
concentration increases. However, neither test is sensitive in
the early diagnosis of kidney disease because of the large
functional reserve of the kidneys. Moreover, variations are
not proportional to the number of functional nephrons.
Indirect Tests of Glomerular Function
▪ Creatinine
• Creatinine is a small molecule produced by degradation of creatine and creatine-phosphate; energy storing in skeletal muscles.
• Creatine is synthesized from the amino acids glycine, arginine,
and methionine, the final step occurring in the liver. It is then taken up by the muscles where it is reversibly phosphorylated by
creatine-kinase into creatine-phosphate. Skeletal muscles contain about 95% of the total body creatine and creatine-phosphate pool.
The estimated turnover is about 2%.
• In carnivores and omnivores, creatinine can also originate from the creatine and creatinine in food.
Indirect Tests of Glomerular Function
• Creatinine mainly circulates in a free form in the plasma and is distributed into the whole body water compartment.
• Creatinine is freely filtered by the glomerulus.
• It is not reabsorbed or secreted in cats and ponies.
• It may be strongly secreted in horses.
• In dogs, either no secretion has been observed or very weak proximal tubule secretion has been reported in males but not in females.
• Secretion of creatinine by active transport in the proximal tubule has been reported in humans, sheep, rabbit, pig, and goat.
Indirect Tests of Glomerular Function
• Serum and plasma specimens can store up to 8 months at -20 °C.
Urine samples can store up to 30 days at +4 °C.
• Especially the increase in cats and dogs consuming cooked meat or applying oral creatine is observed. It was higher in dogs fed
chicken-based diets.
• It was only moderately increased in dogs deprived of water for 4 days.
• It was not significantly changed after strenuous physical exercise in untrained dogs.
• It was slightly higher in dogs kept indoors than outdoors.
• Plasma levels decrease, whereas urine levels increase in dogs
receiving glucocorticoids. NSAIDs, halothane, ACE inhibitors have little or no effect on plasma levels. Furosemide increases plasma levels moderately.
Indirect Tests of Glomerular Function
• HPLC is considered to be the reference method.
• Routine analyses are based on the nonspecific Jaffé reaction (alkaline picrate) and enzymatic procedures.
• The enzymatic methods give slightly lower results than HPLC and Jaffé reaction.
• The main interferents in the Jaffé reaction are high
concentrations of glucose, ketones, hemoglobin, vitamin C, cephalosporins, amino acids.
• Irk, yaş, cinsiyet ve biyolojik ritimler sonuçlarda varyasyona neden olabilir.
• Breed, age, sex, and biological rhythms can cause variations in results.
Indirect Tests of Glomerular Function
• It is more specific in assessing GFR than urea. Because synthesis and excretion are fixed almost constantly and are not metabolized
extrarenal/renal.
• Urea may increase with a number of non-renal causes. In addition, a certain portion of urea can be reabsorbed depending on the hydration and glomerular blood flow of the animal.
• It was suggested that the evolution of renal disease in dogs could be monitored by repeated creatinine measurement.
• It is the most efficient indirect marker of GFR in mammals. It is increased in chronic and acute renal failure.
Indirect Tests of Glomerular Function
• Normal plasma levels may not indicate that the kidney is not normal. If 25% of the kidney mass is functional, it will keep the plasma level at normal levels. Therefore, endogenous creatinine clearance (GFR direct test) gives better information.
• There is also a exogenous application and is more sensitive (inulin or iohexol).
• Especially in cases of suspected renal disease - if the serum levels of urea and creatinine are normal - the clearance must be evaluated.
Creatinine Clearance = (Urine Creatinine x Urine Volume/Time/kg) : Serum Creatinine
• The animal must be discharged before the urine collection starts. The next urine should be collected and the time recorded.
Indirect Tests of Glomerular Function
High Plasma Creatinin Concentration
Primary Renal Diseases Amyloidosis, glomerulosclerosis, polycystic crisis, kidney graft rejection, congenital renal disease,
intoksikasyonlar (fluoride, citrinin, ochratoxin, vitamin D)
Secondary Renal Diseases Babesiosis, Leptospirosis, Leishmaniasis, Borreliosis, Heartworm disease
Extra-Renal Diseases Ureteral obstruction, uroperitoneum
Low Plasma Creatinin Concentration
Portosystemic shunts, early babesiosis, hyperthyroidism, cachexia, kidney graft
Indirect Tests of Glomerular Function
▪ Urea
• Urea is a small hydrosoluble molecule synthesized in the liver from bicarbonate and ammonium in the Krebs-Henseleit cycle.
• Urea is the main form in which nitrogen is eliminated in mammals.
• After synthesis, it is distributed into the total body water compartment.
• It is freely filtered by the kidney glomeruli and reabsorbed from the collecting tubule. Its passive reabsorption is increased when urine flow in the tubule is reduced which can lead to increased plasma urea in dehydrated patients or in patients with hemorrhage or to decreased plasma urea in overhydrated patients.
• Some urea also filters into the intestine, where it is degraded by bacteria into ammonium, which is absorbed and provides a
notable proportion of the ammonium supply to the liver. Another important source of ammonium is the catabolism of amino acids.
Indirect Tests of Glomerular Function
• Serum or plasma samples is used for analysis.
• It is little affected by hemolysis .
• It is resistant to 8 months at -20 °C.
• Plasma levels are higher in animals fed on a high protein diet.
• Fasting levels may be lower in dogs, cats, horses, sheep and
goats who are fed a low protein diet and have normal or reduced renal function.
• Plasma urea was also increased by prolonged fasting, because of catabolism of body proteins.
• Most techniques are based on the specific action of a bacterial urease.
Indirect Tests of Glomerular Function
• BUN measurement methods are no longer preferred. The amount of BUN can be calculated from urea.
• BUN (mg/dL) x 2,14 = Urea (mg/dL)
• BUN (mg/dL) x 0,356 = Urea (mmol/L)
• There can often be minor differences depending on gender, age, individual differences and biological rhythms.
• Individual differences can be observed in horses.
• It has been reported that sheep may be up to 30% higher in
summer than in winter, but otherwise it is reported that there is no change.
Indirect Tests of Glomerular Function
• The variations in plasma urea with disease are similar to those of plasma creatinine, but numerous extrarenal factors may contribute to increased or decrease in plasma urea.
• Increase: Gastrointestinal hemorrhage, fasting, or sepsis.
• Decrease: Thyrotoxicosis, decreased renal perfusion,
portosystemic shunts, liver insufficiency, urea cycle enzyme defects.
• These extrarenal factors of variation explain why urea is less specific than creatinine.
• Plasma urea greatly depends on protein supply, it is a useful tool for monitoring the effects of dietary protein restriction.
• Only 30% of cattle with plasma urea above the upper limit of the reference interval had renal disease.
Indirect Tests of Glomerular Function
▪ Incompatible Urea and Creatinine Results
Urea Creatinine
Normal / Early prerenal azoemia; normal GFR with high Urea
• High protein diet, bleeding in the upper gastrointestinal tract Decreased GFR and decreased creatinine
• Loss of muscle mass; cachexia
Normal / Decreased GFR and decreased urea
• Hepatic insufficiency, polyuria-polydipsia (in the absence of chronic renal insufficiency), low protein diet, metabolism of urea by the
intestinal flora in horses and cattle Normal GFR and increased creatinine
• A normal finding in the Greyhounds (due to increased muscle mass)
Indirect Tests of Glomerular Function
▪ Cystatin C
• Cystatin C is a small constitutive protein synthesized by all nucleated cells and only cleared by glomerular filtration.
• After filtration, it completely reabsorbed by the proximal tubule and degraded. So the amount in urine is very low. Increase in urine
can be evaluated as a sign of proximal tubule injury.
• In the case of GFR decline, the amount of serum will increase as the excretion decreases. Correlation with plasma creatinine and GFR is very good in both healthy and GFR-depleted dogs and is a superior GFR marker than creatinine.
• However, depending on the extra-renal causes, it may increase in serum.
Indirect Tests of Glomerular Function
• It is considered the most sensitive marker of renal failure in humans.
• It can be measured with reagents used in humans in plasma (especially for dogs)
• It can be measured in serum and urine.
• Absolutely must be taken after 12 hours fasting. Because plasma levels after feeding are reduced by 50%.
• It is stable 2 days at +4 °C and 1 month at -20 °C.
Indirect Tests of Glomerular Function
• Studies in dogs have been revealed that it is more closely related to GFR and more sensitivethancreatinine for renal diseases.
• Nevertheless, it is not certain whether there will be an increase due to prerenal azotemia.
• Cystatin C concentration in cats with chronic kidney disease and healthy cats may overlap.
• On the other hand, clinically healthy cats are likely to have subclinical renal disease.
• Increased levels in the urine may be a sign of proximal tubular injury.
• Further research is needed.
• It is not affected by muscle mass like creatinine.
Indirect Tests of Glomerular Function
• Reference values reported in cats and dogs are 0.4-1.6 mg/L and 0.4-1.0 mg/L, respectively.
• Increased Serum levels: Is used as a sensitive indicator for decreased GFR.
• Prerenal Azotemia: The effect is not certain.
• Non-renal diseases: Increased in cats with hyperthyroidism.
• Acute kidney damage: Needs work.
• Chronic Kidney Disease: Increased. However, the use of creatinine is still preferred when the disease is graded.
• Increased urine levels: Potential indication of proximal renal
damage. Proteinuria may mask the result. It is therefore advisable to look at urine with protein and creatinine.
Indirect Tests of Glomerular Function
▪ SDMA (Symmetrical dimethylarginine)
• SDMA (symmetric dimethylarginine) is the amino acid, arginine, that contains two methyl groups (dimethyl) in a symmetrical orientation.
• It is the structural isomer of the endogenous nitric oxide synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA).
• SDMA is a relatively newly discovered renal biomarker. SDMA is primarily eliminated by renal excretion. Therefore, it is an
endogenous marker of GFR.
• It is not influenced by muscle mass, which is an advantage in comparison with creatinine.
• It has been used successfully to diagnose CKD in dogs and cats;
particularly in the early stage.
Direct Tests of Glomerular Function
▪ The determination of glomerular filtration rate (GFR) is considered the best way to evaluate kidney function.
▪ GFR is the volume of ultrafiltrate produced per unit of time by glomerular filtration.
• It is 3-6 ml/minute/kg healthy dogs and 2-4 ml/minute/kg in cats.
▪ The surviving nephrons of the diseased kidney largely retain their essential functional integrity and retain a remarkably
uniform relationship between glomerular and tubular function.
▪ GFR may vary depending on the size of the animal.
Therefore, weight and surface area of the body is important
in the calculation.
Direct Tests of Glomerular Function
▪ There is no easy method for determining GFR from a sin- gle blood or urine specimen.
▪ In humans, there are equations to estimate GFR from plasma
creatinine, gender, weight, and age. The most frequently used are the Cockroft-Gault’s equations, but these are imprecise and the results depend on the techniques used for P-Creatinine.
• This equation has been tested in dogs, but its use has not been determined appropriately.
▪ The accepted reference for GFR determination is the urinary clearance of inulin.
• GFR= (Urine volume) x (Urine inulin) : (Plasma inulin) x t
▪ Decreased GFR is the gold standard for the diagnosis of
renal failure.
Tests of Tubule Function
▪ Urine Osmolality versus Urine-Specific Gravity (Density)
• Urine concentration is best evaluated by osmolality.
• The urine concentration is determined in routine tests from the specific gravity (SG). Urine osmolality and specific gravity were highly correlated in dogs, sheep and cats; weaker in calves.
• It may vary depending on diet, physical exercise, environmental conditions, medications, anesthetics, breed, sex, age, individual differences, and biological rhythms.
• Measuring osmolality requires expensive equipment. Thus, SG can be measured with the refractometer. In routine, test strips are used.
• Repeated measurements are recommended due to variations in healthy animals.
Species Amount of Daily Urine
Specific Gravity
pH
Mean Min-Max
Horse 3 – 10 L 1040 1025-1060 6,8 – 8,4
Cattle 6 – 25 L 1032 1030-1045 6,0 – 8,7
Sheep/Goat 1 – 1,5 L 1030 1015-1045 6,0 – 7,0
Dog 0,5 – 2,0 L 1025 1016-1060 6,1
Cat 75 – 200 mL 1030 1020-1040 6,0
Tests of Tubule Function
▪ Urine Excretion of Ions; Fractional Excretion (FE)
• Many electrolytes are intensely reabsorbed after filtration, mainly in the proximal tubule; their excretion is thus increased when tubule dysfunction occurs.
• Urine electrolyte concentrations also depend on the alimentary supply as the homeostatic mechanisms aimed to stabilize plasma
concentration modulate tubule reabsorption. The most meaningful information would be obtained from daily urine excretion, which is often impossible to obtain because of the difficulties associated with urine collection.
• Expressing the urinary elimination of a solute as the ratio of the filtered load that is found in urine has been proposed, whence the name fractional excretion (FE).
• FEX = (Urine-X x Urine Volume) : (Plasma-X x GFR)
Tests of Tubule Function
• If creatinine clearance is used as a measurement of GFR, it can be demonstrated that FE is equal to the ratio of the solute clearance to creatinine clearance.
• FEX = (Urine-X : Plasma-X) x (Plasma-Creatinine : Urine Creatinine)
• Such spot measurements are often well correlated with daily elimination.
• It shows high variation in cats.
• Na, Cl, K, Pi can be used for this purpose. There are differences between animals.
Tests of Tubule Function
• It may vary depending on diet, physical exercise, environmental
conditions, medications (especially fluid therapy), anesthetics, breed, sex, age, individual differences, and biological rhythms.
• Analytically, plasma techniques can be used in the urine (dilutions may be needed). Attention should be paid to the substances that create interferences.
• Especially in sheep, cattle, horses and cattle, an erroneous low K may be determined.
• The greatest difficulty in evaluating FE is that it varies with a number of non-renal problems. For this reason, more appropriate results can be obtained with controlled and repeated measurements.
Tests of Tubule Function
▪ Urine Excretion of Ions; Fractional Excretion (FE)
FE
XDog Cat
Na < 1 < 1
K < 6-20 < 6-20
Cl < 1 < 1.5
P < 20 < 73
Tests of Renal
Damage
Glomerular Damage
▪ Proteinuria
• It is the most common abnormal condition in routine urine analysis.
• Although glomerular damage is the cause of the most intense
proteinurias, it can also originate from the tubules and their cause may be pre- or postrenal.
• The following systematic approach is required in cases of confirmed proteinuria.
1. Is it persistent?
2. Evaluate the magnitude.
3. Localize the origin.
Glomerular Damage
• In glomerular filtrate, plasma proteins are either not found or in trace amounts.
• The molecular weight of albumin is closest to the filtration
threshold, so this is the first plasma protein to escape into urine in the case of glomerular disturbance.
• Almost all of the filtered proteins are reabsorbed in the tubule, and the remaining molecules which are degraded, are then discarded in the urine. This may lead to underestimation of urinary proteins with certain techniques (e.g. Biuret test) (<LOD).
• Protein concentration in spot urine may vary considerably
depending on urine concentration. Therefore, better estimates of proteinuria would be obtained by measuring total daily excretion;
need to collect urine for 24-hour, which is difficult.
Glomerular Damage
• The concentration of creatinine, which is inversely related to urine dilution, is used as a correction in spot samples as its excretion in a given animal is supposed to be fairly constant.
• It is an accepted equation today (mainly by IRIS).
• The U-P/C ratio in spot urines is well correlated with 24-h urine excretion in healthy and chronic renal failure dogs and cats.
Urinary Protein/Creatinine Ratio (U-P/C) =
Urine Protein (mg/L) Urine Creatinine (mg/L)
Glomerular Damage
• Variations in U-P/C values can be seen depending on the sample, diet, physical exercise, medication, sex, age, individual differences and biological rhythms.
• Proteinuria detection is most often based on the use of test strips, the detection limit being 0.25 to 0.30g/L for albumin.
Quantifications cannot be done.
• Alkaline urine can give erroneous results. The result must be evaluated together with the density.
• For quantitative results, methods such as Ponceau S, Coomassie Blue and Pyrogallol Red should be used.
• The biuret reaction cannot be used as its quantification limit (ŞOQ) is too high (~0,2 g/dL = 20 g/L).
Glomerular Damage
• Borderline result should be repeated and re-evaluated within two months.
• Non-proteinuric or borderline values can also be considered as
«microalbuminuric».
• Proteinuria may decrease as renal dysfunction increases.
• The treatment can be followed by U-P/C.
U-P/C Value
Assessment
Dog Cat
<0,2 <0,2 Non-Proteinuric
0,2 - 0,5 0,2 - 0,4 Borderline proteinuric
>0,5 >0,4 Proteinuric
Glomerular Damage
▪ Albuminuria-Microalbuminuria
• More than 99% of filtered albumin is reabsorbed in the proximal tubule.
• The word microalbuminuria is used to qualify the urinary elimination of traces of albumin.
• It can be used for early diagnosis in humans.
• Methods used for human can not be used for cats and dogs.
Semi-quantitative commercial test kits for dogs are available.
• Microalbuminuria was observed in a large proportion of dogs
without any clinical sign of renal disease. However, more studies are required.
▪ Urine Protein Electrophoresis
• Routine use in animals is rare.
Tubule Damage
▪ Urine Enzyme Activities
• Enzymes found in urine have two sources. Those with a small molecular weight may leak from the glomerulus, many of which are absorbed. The second source is urine leakage due to tubular cell damage. Most originate from the proximal tubule.
• Renal damage causes their excretion into urine to increase, but there are no increases in plasma enzyme activity (except in
severe cases).
• ALP, GGT, LDH, GLDH, NAG
• Increased urine enzyme activities indicate acute kidney damage regardless of the cause (not dysfunction). In many cases, enzyme activities increase before function parameters (sometimes function parameters may not increase).
• Enzyme activities may also increase depending on the secondary causes. Ex. Canine Leishmaniasis and pyometra.
Tubule Damage
▪ Blood: Hematuria
• It can originate from any part of the kidney or urinary tract.
• It is detected by the color of the urine, ranging from light pink to red in macrohematuria, or more frequently by rou- tine urinalysis for invisible microhematuria.
• The limit of detection of hemoproteins is low, so that occult blood can be detected in the absence of a positive reaction for proteins (peroxidase activity).
• Depending on the sampling technique, blood can be seen in urine (especially in catheterization).
• It can also be detected with a microscope examination.
Your Questions?
Send to serkan.sayiner@neu.edu.tr
References
▪ ECLINPATH. http://www.eclinpath.com/chemistry/kidney/cystatin-c/Access Date: 8.5.2018
▪ Dahlem DP, Neiger R, Schweighauser A, et al. (2017). Plasma Symmetric Dimethylarginine Concentration in Dogs with Acute Kidney Injury and Chronic Kidney Disease. Journal of Veterinary Internal Medicine, 31(3):799-804. Doi:10.1111/jvim.14694.
▪ Karagül H, Altıntaş A, Fidancı UR, Sel T, 2000. Klinik Biyokimya. Medisan, Ankara
▪ Kaneko JJ, Harvey JW, Bruss ML, 2008. Clinical Biochemistry of Domestic Animals, 6th edi. Academic Press-Elsevier
▪ Thrall MA, Weiser G, Allison RW, Campbell TW, 2012. Veterinary Hematology and Clinical Biochemistry, 2nd edi. Wiley-Blackwell
▪ Turkish Society of Nephrology. http://www.nefroloji.org.tr/folders/file/bobrek_yetmezligi.pdf Erişim Tarihi: 8.5.2018
