Renal Function Test

Outline

• Introduction

• Anatomy

• Kidney Functions

• Components of Renal function Tests

• Conclusion

Introduction

• After the age of 35 years, kidney function progressively deteriorates,

• Then the importance of some knowledge of normal renal functions and diseases of the kidney becomes imperative.

• Renal disease is not only common in its own right but also a frequent complication of other common diseases such as DM and HTN

Anatomy

• The kidneys are paired organ system located retroperitonealy

• Each is about 12cm long and weigh about 150g• Each has a renal artery supply (25% of cardiac

output for both) and renal vein drainage• Each consists of an outer cortex and an inner

medulla• The functional unit is the nephron• About 200L of blood is filtered/day• urinary volume/day is 1.5 – 2.5L (≈ 1% of filtrate)

(A) Glomerulus

B) Glomerular Capsule

C) Renal Tubule

proximal convoluted tubule

• loop of Henle

• distal convoluted tubule

D) Collecting Duct

Each kidney consists of one million functional units:

Nephrone

Cont’d

• Each kidney contains 4x105 and 1x105

nephrons

• Each nephron consists of a:

glomerulus

proximal tubule

loop of Henle

distal tubule

collecting duct

Kidney Functions

• The major function of the kidney water homeostasis electrolyte homeostasis acid-base balance (H+ homeostasis) excretion of metabolic products (Urea,urate &

Creatine) endocrine immunological regulation of blood pressure reabsorption of essential metabolities eg glucose,

amino acids

Cont’d

• Other functions of the kidney involve Vit. D metabolism,

• Synthesis and secretion of rennin and erythrocyte production

• Miscellaneous functions such as the catabolism of peptide hormones and synthesis of glucose (gluconeogenesis) during fashing.

REVIEW OF PHYSIOLOGY

What are the normal functions of the kidney?

1. To maintain the constancy of the extra-cellular fluid by:

I. Excreting dietary surpluses and metabolic end-products e.g. urea, creatinine, urate, H+

II. Retaining necessary substances, either by not letting them be filtered (e.g. proteins) or by reabsorbing them in the tubules (e.g. glucose, amino-acids, HCO3-)

2. To act as an endocrine gland

I. Erythropoietin

II. Renin

III. 1-alpha-hydroxylation of Vitamin D (to make 1:25 di-hydroxycholecalciferol, calcitriol)

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

Renal Function Tests

• Renal functon test can be divided into• Glomerular function test

ERPF Clearance test e-GFR

• Tubular function test Urine pH Urine Osmolarity Urine Specific gravity Urine volume

Glomerular function testGlomerular filtration rate (GFR)

-This is the rate (ml/min) at which a given amount of a substance is filtered through the glomeruli

-It is thought to be the most reliable measure of renal functional capacity as it reflects the number of functioning nephrons

-Factors affecting it include

• renal perfusion

• obstruction to urine outflow

• hormonal regulation (aldosterone, renin-angiotensin, ADH)

Cont’d

Renal clearance test (Glomerular filtration rate)

-This is volume of plasma from which the substance is completely cleared by the kidneys per unit time- Mathematically, it is expressed as

Cx = (Ux x V)

Px

- where

• Cx = clearance of substance x (ml/min)

• Ux = urinary conc of substance x

• Px = plasma conc of substance x

• V = urinary volume

• the result is “normalized” to a standard body surface area of 1.73m2

Cont’d

• The glomerular filtration rate (GFR) in young adult males is about 130 ml /min/ 1.73m2 body surface area.

• After the age of 35 years due to age-related nephron drop out, the rate falls 10% per decade. There is an increase of 30 – 50% before the end of the first trimester of pregnancy.

• A high protein diet will increase the GFR slightly and there is a small decrease during protein restriction

Ideal Marker

• Freely filtered

• Not secreted nor reabsorbed

• Non-toxic

• Not metabolized/changed in kidney

Markers used include1. Endogenous I. urea, II. creatinine, III. transthyretin, IV. cystacin CV. α1-microglobulin,VI. β2-microglobulinVII. BTP

2. Exogenous I. inulin (“gold standard”),II. 51Cr-EDTA, III. 131I-iodoacetateIV. IothalamateV. DTPAVI. Iohexol

Uses of GFR• Use to determine presence/progression of kidney

disease

• Prediction of risk of complications of CKD

• Predictor of the time of onset of kidney failure

• Proper dosing of drugs excreted by glomerularfiltration in order to avoid potential toxicity

GFR Assessment

• GFR ‘direct’ Assessment tools

• GFR ‘indirect’ assessment tools

GFR ‘direct’ Assessment tools

• Usually entails exogenous markers

• Exogenous markers give more accurate assessment of GFR compared to endogenous markers

• Assays complicated, time consuming, expensive and have potential side effects

• Prone to variations; measurement errors within a single clearance procedure or between clearance procedures on different days

• Difficult for routine clinical applications

Inulin• Gold standard

• Plant storage molecule

- Jerusalem artichoke, Dandelion, Garlic, Onion

- Polymer of fructose

• Filtered, not reabsorbed or secreted

• Not readily available

• Complicated and expensive assay

• Requires infusion/urine collection( if ‘gold std’ form)

Cont’d

• Can be used as a plasma or urinary clearance

• Invasive

• Urine collection errors(‘gold std’ form)

• Urinary catheter

• Careful timing of blood sampling

NB: inulin has extrarenal clearance

Radioisotope & contrast studies1. 125I‐iothalamate

- Expensive

- Radioactive

- Tubular secretion

- Iodine allergy

- s.c mode of adminstration instead of i.v

2. 99mTc-DTPA

- radioactive

- protein binding by tracer

Cont’d3. Iohexol

- inexpensive

- non-radioactive

- stable; allowing for later analysis

- iodine allergy

- extrarenal clearance

- good for assessment in diabetic nephropathy

- urography can be done concurrently

Cont’d

4. 51Cr-EDTA

- efficiency approximates inulin closely

- extrarenal clearance

- radioactive

- not easily available

- commonest exogenous marker in use in

Europe

Others1. RBP and α1-microglobulin have not found use

clinically because of significant effects on their production by factors outside the kidneys

2. BTP (Beta trace protein)

- glycoprotein involved in PGD synthesis

- independent of age & muscle mass

- useful in the creatinine-blind range

- lack of international standardization

3. B2 microglobulin

- independent of age & muscle mass

- affected by various malignancies

GFR ‘indirect’ assessment tools• Plasma Urea

• Urea clearance

• Plasma Creatinine

• Creatinine clearance

• Cystatin C

• Estimation equations

- Cockcroft-Gault

-MDRD

-Schwartz

-Counahan-Barratt

- CKD-EPI

Plasma urea/urea clearance• No longer accepted for GFR assessment• Urea is produced in the liver as an end product of

amino acid metabolism.• At high flow rates (>2.0 ml/min) about 40% is

reabsorbed whereas at low flow rates (0.5ml/min) around 60% is reabsorbed

• Underestimates GFR • Affected by;

- Diet- ECF volume changes- Liver disease- Severe malnutrition- GIT haemorrhage- Drugs(tetracycline, cortocosteroids)

Creatinine• Fairly constant rate of production

• In steady state, excretion=production

• Creatinine ‘blind range’; plasma Cr levels only increase when >50% 0f renal function is lost

• Freely filtered

• Secreted

• Overestimates GFR

• As GFR declines, tubular secretion of Cr rises

• Extrarenal elimination via GIT(bacterial degradation)

Cont’d• Cr generation affected by;

- muscle mass(age,sex,race)

- diet(meat, vegetables)

• Secretion affected drugs(cimetidine, trimethoprim)

• Assay standardization issues

• Common assay methods affected by;

- cephalosporins

- bilirubin

Plasma Cr & Cr clearance

• Plasma Cr alone should not be used for GFR assessment

• Plasma Cr as a screening tool in some situations

• Cr clearance is the commonly applied clearance study in the clinical setting

• Cr clearance involves timed urine collections which are fraught with problems

• Cr Cl is prone to the analytical & other challenges of creatinine as a substance; overcomes some challenges

Cystatin C

• 13kd non-glycosylated basic protein

• Produced by all nucleated cells

• Generation is constant and unrelated to muscle mass or diet

• Cysteine protease inhibitor

• Predictor of CVS risk

• Freely filtered; catabolized by tubular cells

Cont’d

• Virtually not excreted in urine

• Urinary clearance of Cystatin C cannot be measured

• Assays not yet widely available

• Standardization of assays still ongoing

• No analytical interferences noted

• May be affected by smoking

Cont’d• Cystatin C vs Cr useful in;

- Children and infants

- Patients at high risk for CKD where monitoring is required, e.g. with diabetes, hypertension, etc.

- Patients with suspicion of mild to moderate reduction of GFR

- Patients at risk for acute renal failure

- Patients with liver disease

- Elderly patients

Estimated GFR

• Concept evolved from measures to obviate the problems of plasma creatinine(alone) and CrCl (ie measured GFR) in assessment of GFR(to improve the accuracy plasma Cr levels)

• Involves derived mathematical calculations that try use adjustment factors for measurable determinants of Cr concentration(age, sex etc)

• Has evolved to involve incorporation of other endogenous markers

Key points about estimation equations

• Provides a good platform for following GFR trends over time

• Most are dependent on creatinine

• Tries to model the variabilities in endogenous markers due to their lack of ideal marker properties

• These non-GFR variablities are modelled using demographic & clinical factors(sex, age, sex, wt, etc)

Cont’d• Models the variabilities in a specific population(e.g

MDRD(modification of diet in renal disease) was modelled in CKD patients)

• Modelling in a specific population has some drawbacks;

- causes differences in range of GFR between populations e.g CKD vs healthy

- there is a limitation to which non-GFR

- variability of a marker can be modelled e.gmuscle mass, diet

Cockcroft-Gault EquCockcroft-Gault equation; GFR =

(140-Age) X lean body wt. kg x 100

72 X serum creatinine

• Good accuracy for CKD

• less accurate at:

- normal or near normal GFR

- obesity

- in asian populations

• underestmates at older age (>70yrs)

• overestimates at younger age

MDRD(modification of diet in renal disease)

• GFR (ml/min/1.73m2) = 186 x (Pcr)1.154 x Age0.203 x (0.742 if female) x (1.210 if African American)

• more accurate than Cockcroft-Gault

• Accurate in non-hospitalized, CKD

• Less accurate in;

- normal or near normal GFR

- obesity

- asian populations

• Has 4-item & 6-item(plasma urea & albumin) equations

Schwartz- Used in children

- Based on height

- Also based on Cr measurements (as C-G & MDRD)

- Utilizes enzymatic methods for Cr assessment not Jaffe’s method

- Caution when applied to premature infants and infants(modified equations available)

- Modified equation has different versions for different age-groups

Counahan-Barratt

• used in children

• utilizes height

• based on Cr measurements

• not as widely applied as Shwartz

eGFR equations

• in order to increase effeciency of the formulae, combinations of Cr and cystatin C measurements have been developed

• these equations have been shown to give better values than the ”Cr alone”

ones

• equations using Cys C(alone) have also

been developed

Clinical practice guidelines for CKD:Part 5;guideline 4

• In adults use MDRD & C-G equations

• In children use Schwartz & Counahan-Barratt

• Plasma Cr conc alone should not be used to assess the level of kidney function

• Clinical laboratories should report an estimate of GFR using a prediction equation in addition to reporting the plasma Cr measurement

• Clinical laboratories should use calibrated Cr assays of international standard( calibration is against IDMS)

Tubular function tests• Urinary volume

• Urinary specific gravity

• Urinary osmolality

• urinary pH

• fractional Na excretion

- FENa (%)=Urine sodium/plasma sodium x 100

Urine creat./plasma creatinine

• LMW protein markers include

- α1-microglobulin

- β2-microglobulin

Urine Protein

• Overflow: Capacity to reabsorb normally filtered protein in proximal tubules over whelmed due to overproduction: e.g: light chains, hemoglobinuriaand myoglobinuria

• Tubular proteinuria: Decreased re-absorption of filtered proteins by tubules due to tubulointerstitialdamage ; usually <2 gm

• Glomerular proteinuria: Microalbuminuria to overt proteinuria usually>3.5 gm

Screening for Urine protein

• Dipstick: Gives green color, does not check for light chains

Negative – 10 mg/dl

Trace – 15-25 mg/dl

1-2+ – 30-100 mg/dl

3+ – 300 mg/dl

• Sulfosalicylic acid: white precipitate

Urine protein :Quantitative measurement

• 24 hour collection of urine for protein normal excretion is <150 mg/24 hour.

• Spot urine protein/urine creatinine ratio : (as 24 h urine creatinine excretion is a function of muscle mass i.e. 15 mg/kg for females and 20mg/kg for males )

• a normal ratio is 150/1500 or <0.1 .

• A ratio >3 indicates nephrotic range proteinuria

Microalbuminuria

• Urine albumin excretion below detection by regular dipstick

• First clinical sign of diabetic nephropathy

• Incidence increases with the duration of diabetes and may be present at the diagnosis of NIDDM

• Transient albuminuria may occur with fever,infection,exercise,decompensated CHF

• Associated with poor glycemic control and elevated BP

Cont’d• Normal urine protein excretion : <150mg (20% of this is

albumin)

• Therefore, normal urinary albumin excretion is < 30 mg/day

• Microalbuminuria :urinary albumin excretion 30-300 mg/day

• BP control with Ace_I and ARB’s have been known to reduce microalbuminuria and delay the progression of kidney disease in diabetics

• IDDM patients should be screened yearly,beginning 5 years after the onset of disease

• Patients with NIDDM should be screened at presentation

Why and When to Screen Patients for Microalbuminuria

• BP control with Ace_I and ARB’s have been known to reduce microalbuminuria and delay the progression of kidney disease in diabetics

• IDDM patients should be screened yearly, beginning 5 years after the onset of disease

• Patients with NIDDM should be screened at presentation

Summary• How to evaluate a patient with renal disease

• How to interpret urine protein to creatinine ratios

• Interpretation of urea nitrogen and creatinineratios

• Estimation and measurement of GFR& to see when a patient would need renal replacement therapy

• Interpret urine indices in evaluation of various causes of ARF