Renal function tests

1. RENAL FUNCTION TESTS
By-
Dr. Amita Yadav
MBBS, MD (Pathology)

2. Functions of kidney---
• Maintenance of extracellular fluid volume
• and composition.
• Excretion of metabolic waste products.
• Regulation of blood pressure.
• Synthesis of erythropoietin.
• Production of vitamin D3

3. Factors affecting renal function---
•Pre renal conditions- decrease in renal blood flow as in
dehydration,congestive cardiac failure and shock.
•Diffuse renal disease.
•Post renal conditions- obstruction to urinary outflow.

4. Indications for renal function tests---
 Early identification of impairment of renal function in
patients with risk of chronic renal disease.
 Diagnosis of renal disease.
 Follow the course of renal disease and assess
response to treatment.
 Plan renal replacement therapy (dialysis or renal
transplantation) in advanced renal disease.
 Adjust drug doses according to renal function.

5. Conditions with increased risk of chronic
renal disease---
Diabetes Mellitus
Hypertension
Autoimmune diseases like SLE
Older age
Family history of renal disease
Systemic infection
Urinary tract infection
Lower urinary tract obstruction

6. Classification of renal function tests---
Routine urine analysis- 1. Physical examination
2.Chemical examination
3. Microscopic examination
Tests to evaluate glomerular function
Tests to evaluate tubular function

7. Tests to evaluate glomerular function---
 Clearance tests to measure GFR-
Inuline clearance
I125-iothalamate clearance
Cr51 EDTA clearance
Cystatin C Clearance
Creatinine clearance
Urea clearance

8. Glomerular Filtration Rate (GFR )-
 Best test for assessment of excretory renal function.
 Rate in ml/min at which a substance is cleared from
circulation by glomeruli.
 Normal GFR in young adults is 120-130 ml/min/1.73m2
 Creatine claerance is most commonly used as a
measure of GFR.
 GFR varies according to age,sex and body weight of
an individual,it also depends upon renal blood flow and
pressure.

9. Significance of GFR---

10. Blood Biochemistry---
 Serum creatinine
 Blood urea nitrogen (BUN)
 BUN/Serum creatinine ratio

11. Tests to evaluate tubular function---
 Tests to assess proximal tubular function-
Glycosuria,phosphaturia,uricosuria
Aminoaciduria
Tubular proteinuria
Fractional sodium excretion

12. Tests to assess distal tubular function—
 Specific gravity and osmolality of urine
 Water deprivation test and water loading test
 Ammonium chloride loading test

13. Clearance tests-
Clearance of a substance refers to the volume of
plasma,which is completely cleared of that
substance per minute.
 CLEARANCE = U V / P
U- concentration of a substance in urine in mg/dl
V- Volume of urine excreted in ml/ min
P- concentration of a substance in plasma in mg/dl

14. Properties of agents used for measurement
of GFR-
 Physiologically inert
 Preferably endogenous
 Freely filtered by glomeruli
 Neither reabsorbed nor secreted by renal tubules
 Should not bind to plasma
 Should not be metabolised by kidneys
 Should be excreted only by kidneys

15. Agents used for measurement of GFR-
 Exogenous- Inulin,
Cr51- EDTA ,
I125-iothalamate
 Endogenous- Creatinine,
Urea,
Cystatin C

16. Inulin clearance---
 Inert fructose polymer
 Ideal agent for measuring GFR-Freely filtered by
glomeruli,neither reabsorbed nor secreted by
tubules.
 Bolus dose- 25 ml of 10% solution iv
 Constant iv infusion 500 ml of 1.5% solution at the
rate of 4 ml/min
 Gold standard but rarely used because it is time
consuming,expensive,constant iv infusion
needed,difficulty in laboratory analysis.

17.  Inulin clearance in a healthy young adult has mean
value of
127 ml/min/1.73 m2 – Men
118 ml/min/1.73 m2 - Women

18. Clearance of radiolabelled agents---
( I125-iothalamate,Cr51-EDTA, Tc99-DTPA)
 Expensive
 Risk of exposure to radioactive substances.

19. Clearance of Cystatin C---
 Cysteine protease inhibitor
 Completely reabsorbed by proximal tubule, so its
appearance signifies proximal tubular damage.
 Measured by immunoassay
 More sensitive and specific marker than creatinine
because its level is not affected by sex,diet or
muscle mass.

20. Creatinine Clearance---
 Most commonly used test for measuring GFR.
 Produced constantly by creatine in muscles.
 Completely filtered by glomeruli , is not reabsorbed
by tubules. However a small amount is secreted by
tubules.
 A 24 hour urine sample is preferred to overcome
problem of diurnal variation of creatinine excretion.
 A blood sample for estimation of plasma creatinine
is obtained at midpoint of urine collection.

21. Creatinine clearance= U V / P
• Formula overestimates GFR by 10% because of
secretion of creatinine by renal tubules.
• Level of creatinine is affected by-
Secretion of small amount in renal tubules.
Collection urine is incomplete.
Intake of meat.
Muscle mass.
Certain drugs- Cimetidine,probenecid, trimethoprim

22. Creatinine clearance-
 Men- 110-150 ml/min/1.73 m2
 Women – 105-132 ml/min/1.73 m2

23. Prediction Equations---
( Estimation of creatinine clearance from
serum creatinine )
Cockcroft and Gault Formula-
Creatinine clearance in ml/min =
(140-age in years) x (Body weight in kg )
( 72 x Serum creatinine in mg / dl )
In females value obtained is multiplied by 0.85.

24. SCHWARTZ FORMULA-
CrCl(ml/min/1.73m2) = k x ht in cm/S.Cr(mg/dl)
k = 0.45 ,infants < 1 year of age
k = 0.55 ,children and adolescent
females.
k = 0.7, adolescent males.

25. MDRD formula-
GFR=170 x S.Creat.-0.999 x age-0.176 x BUN-0.170 x
Albumin0.318
(multiplied by 0.742 if female)
( Includes age,race,sex,serum urea nitrogen,serum
creatinine and serum albumin )

26. Simplified MDRD formula-
GFR =186.3 x S.Creat.-1.154 x age-0.203 x 1.212
(multiplied by 0.742 if female)
( Includes age, race, sex, serum creatinine )

27. Urea clearance---
 Urea is filtered by glomeruli but about 40% is
reabsorbed by the tubules.
 Underestimates GFR.

28. Blood biochemistry---
 Blood Urea Nitrogen (BUN)
 Serum Creatinine
 BUN / Serum creatinine ratio

29. Blood Urea Nitrogen---
 Proteins Amino acids
Synthesis of tissue Ammonia
Proteins and Energy Urea Cycle
Other Urea
Compounds
Excretion in urine
Concentration of blood urea is usually expressed as
BUN.

30. Real concentration of urea is BUN x 60/28
Azotemia- Increase in blood level of urea.
Uremia- Clinical syndrome resulting from increase in
blood level of urea.

31. Causes of increase in BUN-
 Pre renal azotemia- shock,congestive heart
failure,salt and water depletion.
 Renal azotemia- impairment of renal function.
 Post renal azotemia- obstruction of urinary tract.
 Increase in rate of production-
High protein diet,
Increase in protein catabolism(trauma,burn,fever)
Gastrointestinal haemorrhage or tissue hematoma.

32. Methods for estimation of BUN-
 Diacetyl monoxime urea method- Direct method.
Urea + Diacetyl monoxime Yellow diazine
derivative
• Urease-Berthelot reaction- Indirect method
Urea
Ammonia+CO2
Hydrolysis in presence of urease
Ammonia + Alkaline hypochlorite + Phenol
Indophenol

33.  Reference range of BUN in adults is 7-18 mg/dl.
 In adults > 60 years ,level is 8-21 mg/dl.

34. Serum creatinine-
 More sensitive and specific marker of renal
function as compared to BUN because-
Level is not affected by diet,protein catabolism or
other exogenous factors.
 Not a sensitive marker for early renal impairment
because significant increase in serum creatinine
does not occur until about 50% of kidney function is
lost.

35. Reference range of serum creatinine-
 Adult males- 0.7 – 1.3 mg/dl
 Adult females - 0.6-1.1 mg/dl
 Causes of increase in serum creatinine-
Pre renal,renal and post renal azotemia
Large amount of dietary meat
Active acromegaly and gigantism

36. Causes of decreased serum creatinine-
 Pregnancy
 Increasing age
 Methods of estimation of Serum creatinine-
Jaffe’s reaction
Enzymatic methods

37. Jaffe’s reaction ( Alkaline picrate
reaction )-
 Creatinine reacts with picrate in alkaline solution to
produce a yellow red colour.
 Non creatinine chromogens-
o Glucose,
o Fructose,
o Protein,
o Ascorbic acid,
o Acetoacetate,
o Acetone,
o Cephalosporin
Cause false elevation of serum creatinine level.

38. BUN/Serum Creatinine ratio-
 Normal ratio is 12:1 to 20:1
 Used to discriminate pre renal and post renal
azotemia from renal azotemia.

39. Microalbuminuria -
 Albuminuria in the range of 30-300 mg/ 24 hours.
 Earliest evidence of glomerular damage.
Macroalbuminuria –
• Albuminuria >300 mg/24 hours.
• Indicates significant glomerular damage.

40. Tests to evaluate proximal tubular
function-
 Glycosuria
 Generalized aminoaciduria
 Tubular proteinuria( normally low molecular weight
proteins are completely reabsorbed by proximal
renal tubule ).
 Urinary concentration of sodium ( Increases in acute
tubular necrosis).
 Fractional excretion of sodium ( FENa)

41. FENa =
Urine sodium x Plasma creatinine x 100
Plasma sodium x Urine creatinine
Values above 3% are strongly suggestive of acute
tubular necrosis.
(Refers to percentage of filtered sodium that has been
absorbed and percentage that has been excreted.)

42. Tests to assess distal tubular function-
 Urine specific gravity
 Urine osmolality
 Water deprivation test
 Water loading antidiuretic hormone suppression test
 Ammonium chloride loading test

43. Urine specific gravity --
 Normal specific gravity is 1.003 to 1.030
- It depends on amount of solutes in solution.
- It reflects the relative degree of concentration
or dilution of a urine specimen.
- It helps in evaluating the concentrating and
diluting abilities of the kidneys
- Urea( 20 %) ,Na Cl (25%), Sulphate ,
Phosphate – major contributors

44.  Increase in specific gravity-
Diabetes mellitus
Nephrotic syndrome
Fever
Congestive heart failure
 Decrease in specific gravity-
Diabetes insipidus
Compulsive water drinking
• Isosthenuria -Specific gravity is fixed (1.010 )

45. Measurement of specific gravity-
1. Urinometer method-
Based on principle of buoyancy
2. Refractometer method-
Measures refractive index of dissolved
solids
3. Reagent strip method-
indirect method .
3 main ingredients : polyelectrolyte ,
indicator and a buffer.

46. Urinometer-

47.  Urinometer is a hydrometer that is calibrated to
measure the specific gravity of urine at a specific
temperature, usually at 200C.
 Based on principle of buoyancy so the urinometer
will float higher in urine than in water, because urine
is denser.
 Thus higher the specific gravity of a specimen, the
higher the urinometer will float.

48.  Specific gravity is affected by presence of dense
molecules, protein and glucose.
Temperature correction-
 For every 30C below 200C, subtract 0.001 from the
reading and for every 30C above 200C, add 0.001.
 Subtract 0.003 from specific gravity after
temperature correction for each 1 g/dl of protein and
0.004 for each 1g/dl of glucose.

49. Urine Osmolality---
 Most sensitive method for determination of ability of
concentration by renal tubule.
 Measures number of dissolved particles in a
solution.
 Expressed as milliosmol/kg of water.
 Measured by osmometer.
 Urine/Plasma osmolality ratio is helpful in
distinguishing pre renal azotemia from acute tubular
nacrosis. ( Higher in pre renal azotemia ).

50. Water Deprivation Test-
 Water intake is restricted for specified period of time
followed by measurement of specific gravity or
osmolality.
 Normally urine osmolality should rise.
 If it fails to rise ,then desmopressin is administered
to differentiate between central diabetes insipidus
and nephrogenic diabetes insipidus.

51.  If urine osmolality is >800 mOsm/kg of water or
specific gravity is more than or equal to 1.025
following dehydration,concentrating ability of renal
tubules is normal.

52. Water loading Antidiuretic Hormone
suppression test--
 This test assesses capacity of kidney to make urine
dilute after water loading.
 After overnight fast ,patient empties the bladder and
drinks 20ml/kg of water in 15-30 minutes.
 Urine is collected at hourly intervals for next 4 hours
for measurement of urine volume,specific gravity
and osmolality.
 Plasma level of ADH and serum osmolality should
be measured at hourly intervals.

53.  Normally >90% of water should be excreted in 4
hours.
 Specific gravity should fall to 1.003 and osmolality
should fall to <100 mOsm/kg.
 In renal function impairment urine volume is
reduced(<80% of fluid intake is excreted ).
 Specific gravity and osmolality fail to decrease.

54. Ammonium Chloride Loading Test—
(Acid Load Test)
 Gold standard for the diagnosis of distal or type 1
Renal tubular acidosis.
 Urine pH and plasma bicarbonate are measured
after overnight fasting.
 If pH is < 5.4 ,acidifying ability of renal tubules is
normal.
 If pH is > 5.4 and plasma bicarbonate is low
Diagnosis of RTA is confirmed.

55.  If neither of above results is obtained patient is
given ammonium chloride orally ( 0.1 gm/kg )over
one hour after overnight fast and urine samples are
collected for next 6-8 hours
 Ammonium chloride makes blood acidic.
 If pH is <5.4 in any one of the samples,acidifying
ability of renal tubules is normal.

56. Renal Biopsy--
 Refers to obtaining a small piece of kidney tissue for
microscopic examination.
 Helpful for-
Establishing the diagnosis.
Assess severity and activity of disease.
Assess prognosis.
To plan treatment and monitor response to therapy.

57. Indications for Renal Biopsy --
 Nephrotic syndrome in adults
 Nephrotic syndrome not responding to corticosteroids in
children.
 Acute nephritic syndrome.
 Unexplained renal insufficiency
 Asymptomatic hematuria
 Isolated non nephrotic range proteinuria with renal
impairment.
 Impaired function of renal graft
 Involvement of kidney in systemic diseases like SLE
and amyloidosis.

58. Contraindications--
 Uncontrolled severe hypertension
 Haemorrhagic diathesis
 Solitary kidney
 Renal neoplasm
 Large and multiple renal cysts
 Small shrunken kidney
 Acute urinary tract infection like pyelonephritis.
 Urinary tract obstruction.

59. Complications--
 Haemorrhage
 Arteriovenous fistula
 Infection
 Perforation of viscus
 Death (rare )

60. After taking biopsy sections are stained
by-
 Hematoxyline and eosine – For general architecture
of kidney and cellularity.
 Periodic acid Schiff – To highlight basement
membrane and connective tissue matrix.
 Congo red – For amyloid.