1. Interpretation of renal biochemistry
Doc. Dr. Mine KUCUR

2. Overview Renal function tests Summary Tests for GFR Urinalysis
Tests for Renal Tubular Acidosis
Tests of Kidney Concentrating Ability
Summary

3. Renal function tests Detect renal damage Monitor functional damage
Help determine etiology
From a clinical perspective it is important to have test which would have these characteristics. No such test exists.
An early test to detect renal damage, for instance a simple strip test for haematuria is important in screening for heavy metal poisoning.
There is a clinical need to monitor a patient with renal disease and this is achieved by serial plasma measurements.
We need to know when to start dialysis in renal failure and laboratory tests assist the clinical decision making.
There are about a million nephrons in each kidney and this represents a considerable functional reserve. In renal disease about half the nephrons have to lose their functioning before the abnormality can be detected by conventional laboratory tests.

4. Laboratory tests of renal function
urine protein
urine glucose
hematuria
osmolality
glomerular filtration rate (GFR)
plasma creatinine
plasma urea
urine volume
urine urea
minerals in urine
I shall review the tests in the left column today. The measurement of urine protein is important in certain conditions, e.g.diabetes. The detection of substances such as red cells or glucose could be an early indicator of renal damage.

5. Tests of renal function
urine protein
urine glucose
hematuria
osmolality
glomerular filtration rate=GFR
plasma creatinine= Pcr
plasma urea-Purea
urine volume= V
urine urea- Uurea
cystatin C in plasma?
I shall review the tests in the left column today. The measurement of urine protein is important in certain conditions, e.g.diabetes. The detection of substances such as red cells or glucose could be an early indicator of renal damage.

6. Tests of Glomerular Filtration Rate
Urea
Creatinine
Creatinine Clearance
eGFR
Cystatin C

7. Glomerular Filtration Rate (GFR)
Volume of blood filtered across glomerulus per unit time
Best single measure of kidney function

8. GFR Normally 100-130 ml/min Determined by:
Net filtration pressure across glomerular basement membrane
Permeability and surface area of glomerular basement membrane

9. GFR
Patient’s remain asymptomatic until there has been a significant decline in GFR
Can be very accurately measured using “goldstandard” technique

10. GFR Ideal Marker Produced normally by the body
Produced at a constant rate
Filtered across glomerular membrane
Removed from the body only by the kidney filtered only, not reabsorbed or secreted

11. Candidate markers for GFR
Inulin
+ Filtered only
– Not made by body; must be injected
Creatinine
+ An endogenous product of muscle metabolism; near-constant production
– Filtered, but a bit secreted
Urea
+ An endogenous product of protein intake
– Filtered and absorbed; synthesis varies with diet0

12. Urea Used historically as marker of GFR
Freely filtered but both re-absorbed and excreted into the urine
Re-absorption into blood increased with volume depletion; therefore GFR underestimated
Diet, drugs, disease all significantly effect Urea production

13. Urea Product of protein catabolism Filtered
Reabsorbed in proximal tubule
If sodium is avidly reabsorbed, so is urea
Serum urea concentration measured as “Blood Urea Nitrogen (BUN)”

14. Urea Increase Decrease Volume depletion Volume Expansion
Dietary protein
Corticosteroids
Tetracyclines
Blood in G-I tract
Decrease
Volume Expansion
Liver disease
Severe malnutrition

15. Why does BUN increase?  GFR, but also: Increased renal reabsorption:
ECV depletion
Increased hepatic urea synthesis
High protein feeding
Corticosteroid treatment (Prednisone, etc.)
GI blood absorption

16. BUN: Uses  Proximal tubule Na and urea reabsorption!
Imperfect marker of  GFR
Marker for adequacy of protein intake
Marker for presence of uremic toxins in chronic renal failure
BUN:Cr ratio reflects ECV volume status:
10:1 = normal
>20:1 if ECV contracted. Why???
 Proximal tubule Na and urea reabsorption!

17. Creatinine Product of muscle metabolism
Some creatinine is of dietary origin
Freely filtered, but also actively secreted into urine
Secretion is affected by several drugs

18. Serum Creatinine Increase Decrease Male Age Meat in diet Female
Muscular body type
Cimetidine & some other medications
Decrease
Age
Female
Malnutrition
Muscle wasting
Amputation

19. Serum Creatinine Concentration
Normally mg/dl, depending on muscle mass
Inversely proportional to GFR
Good way to follow changes in GFR
BUT also elevated by  muscle mass,  tubular secretion

20. Creatinine Clearance
Measure serum and urine creatinine levels and urine volume and calculate serum volume cleared of creatinine
Same issues as with serum creatinine, except muscle mass
Requirements for 24 hour urine collection adds variability and inconvenience

21. Creatinine Clearance
Therefore, it represents the volume of serum completely cleared of creatinine per unit time
Since virtually all creatinine is cleared via glomerular filtration, it closely approximates the GFR

22. Creatinine Clearance UCr = 72 mg/dl SCr = 2.0 mg/dl V = 2 liters
EXAMPLE:
UCr = 72 mg/dl
SCr = 2.0 mg/dl
V = 2 liters
time = 24 hours

23. Limitations of Creatinine Clearance
Only valid at steady state—[Cr]serum must be stable
Trimethoprim, cimetidine lower tubular Cr secretion and lower CrCl without changing GFR:
Becomes more inaccurate at low GFR

24. Another Problem with Creatinine Clearance
Must be done on a properly collected, timed urine sample--patient error
How can we check accuracy of any timed urine collection?

25. Creatinine Excretion
The amount of creatinine excreted per day is stable for a given patient
It is function of muscle mass: generally higher in men vs. women, youth vs. elderly
expressed per kg lean body mass as the creatinine index

26. Quick formulae for estimating GFR Include some combination of sex, weight, serum creatinine, race, and age. Use only at steady state (stable SCr) Useful screens for decreased GFR, esp. in elderly and small people, where errors in drug dosing may be major

27. Creatinine Test Summary

28. Cystatin C
Cystatin C is a 13 KD protein produced by all cells at a constant rate
Freely filtered
Re-absorbed and catabolized by the kidney and does not appear in the urine

29. eGFR
Increasing requirements for dialysis and transplant (8 – 10% per year)
Shortage of transplantable kidneys
Large number at risk

30. eGFR

31. eGFR

32. Problem
Need an easy test to screen for early decreases in GFR that you can apply to a large, at-risk population
Can serum creatinine be made more sensitive by adding more information?

33. eGFR by MDRD Formula
Mathematically modified serum creatinine with additional information from patients age, sex and ethnicity
eGFR = x (serum creatinine) x (age)-0.203
(if female x (0.742))

34. eGFR
eGFR calculation has been recommended by National Kidney Foundation whenever a serum creatinine is performed in adults

35. Screen High Risk Groups
eGFR
Urinalysis
Albumin / Creatinine Ratio

36. Tests that predict kidney disease
eGFR
Albumin Creatinine Ratio
(aka ACR or Microalbumin)

37. Proteinuria In health:
High molecular weight proteins are retained in the circulation by the glomerular filter (Albumin, Immunoglobulins)
Low molecular weight proteins are filtered then reabsorbed by renal tubular cells

38. Proteinuria Glomerular: Tubular: Overflow:
Mostly albumin, because of its high concentration and therefore high filtered load
Tubular:
Low molecular weight proteins not reabsorbed by tubular cells (e.g. alpha-1 microglobulin)
Overflow:
Excessive filtration of one protein exceeds reabsorbtive capacity (Bence-Jones, myoglobin)

39. Albumin Creatinine Ratio (Microalbumin)
Normal albumin molecule
In health, there is very little or no albumin in the urine
Most dip sticks report albumin at greater than 150 mg/L

40. Urinary Albumin
Detection of low levels of albumin (even if below dipstick cut-off) is predictive of future kidney disease with diabetes
Very significant biologic variation usually requires repeat collections
Treatment usually based on timed urine albumin collections

41. Follow-up based on Screen Results
Kidney Ultrasound
Specialist Referral
Cardiovascular Risk Assessment
Diabetes Control
Smoking cessation
Hepatitis / Influenza Management

42. Creatinine Standardization in British Columbia
Based on Isotope dilution /mass spectrometry measurements of creatinine standards
Permits estimation and correction of creatinine and eGFR bias at the laboratory level

43. Importance of Standardization
Low bias creatinine:
Causes inappropriately increased eGFR
Patients will not receive the benefits of more intensive investigation of treatment
High bias creatinine:
Causes inappropriately decreased eGFR
Patients receive investigations and treatment which is not required. Wastes time, resources and increases anxiety.

44. Poor Creatinine Precision
Incorrect categorization of patients with both “normal” and decreased eGFR.

45. Total Error TE = % bias + 1.96 CV
Goal is <10% (requires bias ≤ 4% and CV ≤ 3%)

46. Kidney Functions
Selectively secretes into or re-absorbs from the filtrate to maintain
Water Balance
Tests: specific gravity, osmolarity, water deprivation testing, Antidiuretic hormone
Retention of nutrients
Tests: proteins, sugar, amino acids, phosphate
Secretes waste products
Tests: urate, oxalate, bile salts

47. Kidney Functions
Selectively secretes into or re-absorbs from the filtrate to maintain
Salt Balance
Tests: Na+, Cl-, K+ Aldosterone, Renin
Acid Base Balance
Tests: pH, HCO3-, NH4+ Acid loading, Urinary Anion Gap

48. Kidney Functions Target organ Production
Parathyroid hormone (Ca++, Mg++)
Aldosterone (salt balance)
ADH (water balance)
Production
Erythropoietin
1, 25 dihydroxycholecalciferol

49. Urinalysis Dipstick Protein Useful screening test
Dipstick more sensitive to albumin than other proteins
Large biologic variation

50. Urinalysis Dipstick – cont’d Hemoglobin
Glomerular, tubular or post-renal source
Reasonably sensitive
Positive dipstick and negative microscopy with lysed red cells

51. Urinalysis Dipstick – cont’d Glucose
Reasonable technically, however screening and monitoring programs for diabetes are now done by blood and Point-of-Care devices

52. Specific Gravity Approximate only
Measurement of osmolarity preferred when concentrating ability being assessed

53. pH pH changes with time in a collected urine
Calculations to determine urine ammonium levels and response to acid-loading generally required to assess for renal tubular acidosis

54. Microscopic Urinalysis
Epithelial Cells
Squamous, Transitional, Renal
All may be present in small numbers
Important to recognize possible malignancy
Comment on unusual numbers

55. Renal Tubular Epithelial

56. Red Cells May originate in any part of the urinary tract
Small numbers may be normal
There is provincial protocol for the investigation of persistent hematuria

57. White Blood Cells Neutrophils often present in small numbers
Lymphocytes and moncytes less often
Marker for infection or inflammation

58. Casts
Hyaline and granular casts not always pathologic, clinical correlation required
Red cell casts always significant, usually glomerular injury
WBC casts also always significant, usually infection, sometimes inflammation
Bacterial casts only found in pyelonephritis
Waxy casts found in significant kidney disease

59. Tests for Renal Tubular Acidosis
Urinary Anion Gap
(Na+ + K+) – Cl-
In acidosis the kidney should excrete NH4+ and the gap will be negative

60. RTA
If NH4 + is not present (or if HCO3 - is present) the gap will be neutral or positive, implying impaired kidney handling of acid load.
Urine Anion Gap = (Na+ + K+) –Cl-

61. RTA Ammonium Chloride Loading Load with ammonium chloride
Hourly measurements of urine pH
Normal at least one pH below 5.5

62. Tests of Kidney Concentrating Ability
To differentiate
Psychogenic polydipsia
Central diabetes insipidus
Nephrogenic diabetes insipidus

63. Overnight Water Deprivation Testing
(Serum osmolarity <295 monitor patient weight hourly)
Collect urine hourly from 0600 for osmolarity
Baseline serum osmolarity, Na+, ADH
When osmolarity plateaus repeat above tests and administer ADH

64. Interpretation
If urine concentrates (osmolarity >600 and serum osmolarity below <295)
Normal physiology (? Psychogenic polydipsia)

65. No Urine Concentration No Response to ADH
Nephrogenic diabetes insipidus

66. No Urine Concentration
Positive response to ADH
Central diabetes insipidus

67. To summarize:
1. Use the Creatinine Clearance as the best estimate of GFR
2. Use the Serum Creatinine to follow renal function over time
3. Use the Creatinine Index to check the adequacy of a urine collection
4. Use the BUN to help assess GFR, volume status, and protein intake