kidney function testing

DR. SAID AL GHORA

Kidney Function TestingKidney Function Testing

An Introduction to the Urinary System

Produces urine

Transports urine towards bladder

Temporarily store urine

Conducts urine to exterior

The Function of Urinary System Excretion & Elimination:

removal of organic wastes products from body fluids (urea, creatinine, uric acid)

Homeostatic regulation: Water -Salt Balance Acid - base Balance Enocrine function:

Hormones

A(

B(

C(

Kidney – basic data

Urine excreted daily in adults: cca 1.5LKidney only ca 1% of total body weight, despite itThe renal blood flow= 20% of cardiac outputPlasma renal flow= PRF ca 600 mL/Min./1.73 M2

Reflects two processes Ultrafiltration (GFR): 180 L/day Reabsorption: >99% of the amount filtered

Renal threshold

Renal threshold of a substance is the concentration in blood beyond which it is excreted in urine

Renal threshold for glucose is 180mg/dLTubular maximum (Tm): maximum capacity

of the kidneys to absorb a particular substance

Tm for glucose is 350 mg/min

Kidney Function

A plumbers view

Filter

Processor

InputArterial

OutputVenous

OutputUrine

How do you know it’s broken?

Decreased urine production

Clinical symptoms

Tests

Filter

Processor

InputArterial

OutputVenous

OutputUrine

Where can it break?

Pre-renal

Renal (intrarenal)

Post-renal (obstruction)

Filter

Processor

InputArterial

OutputVenous

OutputUrine

Causes of kidney functional disorders

Pre-renal e.g. decreased intravascular volum

Renal e.g. acute tubular necrosis

Postrenal e.g. ureteral obstruction

Signs and Symptoms of Renal FailureSymptoms of Uraemia (nausea, vomiting,

lethargy)Disorders of Micturation (frequency,

nocturia, dysuria)Disorders of Urine volume (polyuria,

oliguria, anuria)Alterations in urine composition

(haematuria, proteinuria, bacteriuria, leukocytouria, calculi)

Pain Oedema (hypoalbuminaemia, salt and

water retention)

Lab findings

Rising creatinine and ureaRising potassiumDecreasing HbAcidosisHyponatraemiaHypocalcaemia

Why Test Renal Function?

To identify renal dysfunction.To diagnose renal disease.To monitor disease progress.To monitor response to treatment.To assess changes in function that may

impact on therapy (e.g. Digoxin, chemotherapy).

When should you assess renal function?

Older age Family history of Chronic Kidney disease (CKD) Decreased renal mass Low birth weight Diabetes Mellitus (DM) Hypertension (HTN) Autoimmune disease Systemic infections Urinary tract infections (UTI) Nephrolithiasis Obstruction to the lower urinary tract Drug toxicity

Biochemical Tests of Renal Function

Measurement of GFR Clearance tests Plasma creatinine Urea, uric acid and β2-microglobulin

Renal tubular function tests Osmolality measurements Specific proteinurea Glycouria Aminoaciduria

Urinalysis Appearance Specific gravity and osmolality pH osmolality Glucose Protein Urinary sediments


Measurement of GFR Clearance tests Plasma creatinine Urea, uric acid and β2-

microglobulin

In acute and chronic renal failure, there is effectively a loss of function of whole nephrons

Filtration is essential to the formation of urine tests of glomerular function are almost always required in the investigation and management of any patient with renal disease.

The most frequently used tests are those that assess either the GFR or the integrity of the glomerular filtration barrier.

Biochemical Tests of renal function

GFR can be estimated by measuring the urinary excretion of a substance that is completely filtered from the blood by the glomeruli and it is not secreted, reabsorbed or metabolized by the renal tubules.

Clearance is defined as the (hypothetical) quantity of blood or plasma completely cleared of a substance per unit of time.

Clearance of substances that are filtered exclusively or predominantly by the glomeruli but neither reabsorbed nor secreted by other regions of the nephron can be used to measure GFR.

Inulin

The Volume of blood from which inulin is cleared or completely removed in one minute is known as the inulin clearance and is equal to the GFR.

Measurement of inulin clearance requires the infusion of inulin into the blood and is not suitable for routine clinical use

GFR = (U V)P

inulin

inulin

V is not urine volume, it is urine flow rate

Measurement of glomerular filtration rate



microglobulin

1 to 2% of muscle creatine spontaneously converts to creatinine daily and released into body fluids at a constant rate. Endogenous creatinine produced is proportional to muscle mass, it is a function of total muscle mass the production varies with age and sex Dietary fluctuations of creatinine intake cause only minor variation in daily creatinine excretion of the same person. Creatinine released into body fluids at a constant rate and its plasma levels maintained within narrow limits Creatinine clearance may be measured as an indicator of GFR.

Creatinine

Clinical Significance

Elevated Creatinine is found in

Impaired renal functionVery high protein dietVary large muscle mass: body builders, giants, acromegaly

patientsRhabdomyolysis/crush injuryDrugs:

TrimethoprimAmiloride

Clinical SignificanceFor renal transplant patients, an increase in serum

creatinine of 2 mg/L has been used as a criterion of establishing rejection.

In other persons a change in creatinine of 2 mg/L would represent a 20% loss in renal function.

Specimen

One can analyze serum, plasma, or diluted urine. The common anticoagulants (fluoride and heparin) do

not cause interference, though heparin, which can be formulated as the ammonium salt, must be avoided in enzymatic methods that measure ammonia production.

Storage 7 days at 4-25oC At least 3 months at -20oC

Specimen

Urine should be diluted 1:100 Bacterial contamination has been found to falsely lower

creatinine values measured using the Jaffé reaction.The mechanism of this interference appears to be

bacterial production of a substance that retards the rate of the Jaffé reaction.

Enzymatic Method

Creatinine + H2O Creatine

Creatine +ATP Creatine-P + ADP

ADP + Phosphoenolpyruvate ATP + Pyruvate

Pyruvate + NADH Lactate + NAD+

The difference in absorbance at fixed times during conversion is proportional to the concentration of creatinine in the sample

Lactate dehydrogenase

Creatine Kinase

Pyruvate Kinase

Creatinine aminohydrolase

Creatinine Clearance

Creatinine clearance is used to estimate the glomerular filtration rate (GFR).

One method of determining GFR from creatinine is to collect urine (usually for 24-hours) to determine the amount of creatinine that was removed from the blood over a given time interval.

Clearance is defined as the (hypothetical) quantity of blood or plasma completely cleared of a substance per unit of time.

The most frequently used clearance test is based on the measurement of creatinine.

Creatinine is chosen because it is freely filtered at the glomerulus and is not reabsorbed by the tubules.

However, a small amount of the creatinine (about 5%) in the final urine of healthy persons is derived from tubular secretion.

To do the test, one needs a precisely timed urine collection and a blood sample taken during the collection period.

Best results are obtained from a 24-h urine collection. The test is initiated by having patients empty their

bladder at the beginning of the timed period. Urine is collected throughout the period, the bladder is

again emptied at the end of the time period.

The 'clearance' of creatinine from plasma is directly related to the GFR if:

The urine volume is collected accurately There are no ketones or heavy proteinuria present to interfere with the

creatinine determination.It should be noted that the GFR decline with age (to a greater

extent in males than in females) and this must be taken into account when interpreting results.

Creatinine ClearanceCreatinine determinations are performed on both

samples. The creatinine clearance is calculated from the following formula:

A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour produces 60ml of urine with a creatinine concentration of 1.25 mg/mL.

Creatinine clearance (mL/min)= (UV)/P X 1.73/Swhere U is urinary creatinine (mg/L), V is volume of

urine (mL/min), P is plasma creatinine (mg/L), S is the calculated surface area of the patient, and 1.73 is the surface area (m2) of a standard 70 kg person.

The range of creatinine clearance in healthy persons corrected to a surface area of 1.73 m2 is 90 to 120 mL/min.

At low filtration rates, the creatinine clearance does not parallel true glomerular filtration rate because a relatively large portion of the urine creatinine is secreted rather than filtered.

KidneyOutput

PlasmaPool

Content

CreatinineInput

NormalMuscle

Mass

NormalKidneys

DiseasedKidneys

NormalMuscle

Mass

DiseasedKidneys

NormalKidneys

IncreasedMuscle

MassReduced

MuscleMass

Effect of Muscle Mass on Serum Creatinine



microglobulin

Catabolism of proteins and nucleic acids results in formation of so called nonprotein nitrogenous compounds.

Protein

Proteolysis, principally enzymatic

Amino acids

Transamination and oxidative deamination

Ammonia

Enzymatic synthesis in the “urea cycle”

Urea

Measurement of nonprotein nitrogen-containing compounds

Urea is the major nitrogen-containing metabolic product of protein catabolism in humans,

Its elimination in the urine represents the major route for nitrogen excretion.

More than 90% of urea is excreted through the kidneys, with losses through the GIT and skin

Urea is filtered freely by the glomeruli

Plasma urea concentration is often used as an index of renal glomerular function

Urea production is increased by a high protein intake and it is decreased in patients with a low protein intake or in patients with liver disease.

Plasma Urea

Many renal diseases with various glomerular, tubular, interstitial or vascular damage can cause an increase in plasma urea concentration.

The reference interval for serum urea of healthy adults is 5-39 mg/dl. Plasma concentrations also tend to be slightly higher in males than females. High protein diet causes

significant increases in plasma urea concentrations and urinary excretion.

Measurement of plasma creatinine provides a more accurate assessment than urea because there are many factors that affect urea level.

Nonrenal factors can affect the urea level (normal adults is level 5-39 mg/dl) like: Mild dehydration,

high protein diet,

increased protein catabolism, muscle wasting as in starvation,

reabsorption of blood proteins after a GIT haemorrhage,

treatment with cortisol or its synthetic analogous

Plasma Urea

Clinical Significance States associated with elevated levels of urea in blood

are referred to as uremia or azotemia.Causes of urea plasma elevations:

Prerenal: renal hypoperfusion Renal: acute tubular necrosis Postrenal: obstruction of urinary flow

Increased protein catabolism: Increased dietary protein Severe tress: fever, etc Rhabdomyolysis Upper GI bleeding

Causes of urea plasma decrease

Decreased dietary protein Increased protein synthesis ( Pregnant women ,

children ) severe liver disease Overhydration (IV fluids)

SpecimenSerum and heparinized plasma can be used for the

urease/GLDH methods. Fluoride will inhibit the urease reaction; therefore

methods employing urease cannot use serum preserved with fluoride.

Ammonium heparin also cannot be used as an anticoagulant for urease methods.

Stability in serum or plasma: 7 days at 4–8°C 1 year at -20°C

Because of urea’s susceptibility to bacterial degradation, serum and urine samples should be kept at 4° to 8° C until analysis.

Urease/GLDH Method

The method is optimized for 2-point kinetic measurement.

Decrease in absorbance at 340 nm is proportional to concentration of urea

39

BUN / Creatinine RatioBUN / Creatinine Ratio Normal BUN / Creatinine ratio is 10 – 20 to Normal BUN / Creatinine ratio is 10 – 20 to

1 1 Creatinine is another NPNCreatinine is another NPN

Pre-renal increased BUN / Creat ratio BUN is more susceptible to non-renal

factors

Post-renalPost-renal increased ratio BUN / Creat ratio

Both BUN and Creat are elevated

RenalRenal decreased BUN / Creat ratio Low dietary protein or severe liver disease

Increased BUNNormal Creat

Increased BUNIncreased Creat

Decreased BUNNormal Creat

In human, uric acid is the major product of the catabolism of the purine nucleosides, adenosine and guanosine.

Purines are derived from catabolism of dietary nucleic acid (nucleated cells, like meat) and from degradation of endogenous nucleic acids.

Overproduction of uric acid may result from increased synthesis of purine precursors.

In humans, approximately 75% of uric acid excreted is lost in the urine; most of the reminder is secreted into the GIT

Uric acid

Renal handling of uric acid is complex and involves four sequential steps:

Glomerular filtration of virtually all the uric acid in capillary plasma entering the glomerulus.

Reabsorption in the proximal convoluted tubule of about 98 to 100% of filtered uric acid.

Subsequent secretion of uric acid into the lumen of the distal portion of the proximal tubule.

Further reabsorption in the distal tubule.

Hyperuricemia is defined by serum or plasma uric acid concentrations higher than 7.0 mg/dl (0.42mmol/L) in men or greater than 6.0 mg/dl (0.36mmol/L) in women

Uric acid

Greater-than-normal levels of uric acid (hyperuricemia) may be due to:

Alcoholism Diabetes Gout Hypoparathyroidism Lead poisoning Leukemia Nephrolithiasis Renal failure Toxemia of pregnancy Purine-rich diet Excessive exercise Chemotherapy-related side effects

Lower-than-normal levels of uric acid may be due to:

Fanconi syndrome Wilson's disease Syndrome of inappropriate antidiuretic hormone

(SIADH) secretion Multiple SclerosisLow purine diet

Gout

Gout is a kind of arthritis that occurs when uric acid builds up in the joints.

In Gout increased serum levels of uric acid lead to formation of monosodium urate crystals around the joints.

Acute gout is a painful condition that typically affects one joint.

Chronic gout is repeated episodes of pain and inflammation, which may involve more than one joint.

Specimen

Serum or plasma may be used; slight but insignificant positive bias (0.2 mg/dL) has been noted in plasma specimens as compared with serum.

Stability in serum / plasma: 6 months at -20°C 7 days at 4-8°C 3 days at 20-25°C

Enzymatic Colorimetric

Uric acid + H2O + O2 Allantion + CO2 + H2O2

TBHBA + 4- Aminoantipyrine + 2H2O2 Quinoneimine + 3 H2O

Uric acid is oxidized to allantoin by uricase. The generated hydrogen peroxide reacts with 4-

aminophenazone/ESPT to quinoneimine.

POD

Uricase

Acute Renal Failure

Metabolic features:Retention of:

Urea & creatinine Na & water potassium with hyper-

kalaemia Acid with metabolic

acidosis

Classification of Causes:Pre-renal

reduced perfusionRenal

inflammation infiltration toxicity

Post-renal obstruction

Pre-renal versus intrinsic ARF

Test Result

Pre-renal Renal

Urea & Creatinine Disproportionaterise in Urea

Tend to risetogether

Protein in urine Uncommon Present ondipsticktesting

49Reference Ranges

BUN 10 - 20 mg / dl

Creatinine 0.5 - 1.5 mg /dl

Uric Acid 3.0 - 7.0 mg / dl

Creatinine Clearance 90 - 130 ml / min

Ammonia 20 - 60 ug / dl

BUN / Creat Ratio 10 - 20 to 1

Case Study—Renal Failure

Case Study #1

HSL is a 63 year-old male with HTN, CAD, and hyperlipidemia; routine physical examination reveals asymptomatic hematuria.

What do you do?

Case Study 2 Ms. Garcia, a 54 yr old Hispanic female, dx with IDDM

for 10 years. Admitted to the hospital with CHF, ESRD, altered lab values (K+=6.2; BUN 45, Creatinine 3.5; Hgb 6.2; Hct 18.6).

States that her “breathing keeps getting worse and worse, …can’t get around, bones break…decreased appetite but keep gaining weight…funny taste in mouth…blood sugar real high…legs jump at night”. States that a doctor told her she had “bad kidneys”.

1. What lab work is typically done?

2 If ESRD, what lab results would be anticipated?

3. What S&S of ESRD does Ms. Garcia display and WHY?

4. What conservative measures might have delayed ESRD? Discuss dietary, fluid, medications, etc…

Case Study

1. What lab work is typically done? Chem 12; H&H ; 24 hr creatinine clearance most helpful; know normals!

2 If ESRD, what lab results would be anticipated? ^ BUN, serum creatinine; low H&H; ^ K+, metabolic acidosis from kidneys inability to excrete acid load (especially NH3) and from defective reabsorption of bicarbonate.

Case Study

3. What S&S of ESRD does Ms. Garcia display and WHY? Metabolic Acidosis due to decreased ability to excrete acid metabolites therefore Kussmauls breathing in effort to blow off excess CO2. musculoskeletal system affected with renal osteodystrophy as GFR dec., kidney cannot eliminate phosphate; high phosphate binds with Ca which is drawn from bone; in CRF, kidneys do not metabolize vitamin D to its active form which is required for reabsorption of Ca from intestinal tract; weight gain from Na and water retention; uremic damage causing peripheral neuropathy..plus other symptoms including anemia from decreased production of erythropoietin; and HTN..

4. What conservative measures might have delayed ESRD? Discuss dietary, fluid, medications, etc…Control HTN usually by Na and fluid restriction and antihypertensives, esp by ace inhibitors; restrict phosphate intake and use phosphate binders and give with meals; inc. Ca levels by adm. of active Vit D; monitor K levels; adm erythropoietin; avoid use of nephrotoxic drugs…such as aminoglycosides; protein restriction in diet.

Labs

Urinalysis 2+ protein 3+ occult blood >60 RBC per HPF

Biochemical test K+ 4.0 BUN 19 sCr 1.5 Glucose 116

Repeat LabssCr 1.7 (MDRD 45)Glucose 88; HBA1c 5.9Hct 40.4LDL 92

CrCl 46 ml/min24 hr Uprot 741.6 mg

Renal U/S: normal

Modification of Diet in Renal Disease

One month ago… sCr 1.6 (49)

6 months ago… sCr 1.3 (59)

One year ago… sCr 1.5 (54)

To summarize:

1. Use the Creatinine Clearance as the best estimate of GFR2. Use the Serum Creatinine to follow renal function over time3. Use the Creatinine Index to check the adequacy of a urine collection4. Use the BUN to help assess GFR, volume status, and protein intake

kidney function testing

Documents