Lecture 8 : Renal Function Tests

• Clearance Tests

• Creatinine

• Cystatin C

• Beta2-Microlobulin (B2M)

• Radionucleotides

• Urea

Glomerular Filtration Tests

Clearance Tests

• standard tests used to measure the filtering capacity of the glomeruli

• Measures rate in milliliters per minute at which kidneys are able to remove a filterable substance from the blood.

• must be neither reabsorbed nor secreted by the tubules

• Stability in urine during a 24hr collection

• Consistency of the plasma level

• Availability to the body and tests to analyze

Factors to be considered in selecting a clearance test substance

• Exogenous Procedure

• Endogenous Procedure

Two categories of clearance tests

Exogenous Procedure

Test that requires an infused substance (foreign to the body)

Endogenous Procedure

Suitable substance is already in the body

• Creatinine

• Beta2-Microglobulin

• Cystatin C

• Radioisotopes

Primary substances used in clearance tests

To assess glomerular filtration rate (GFR) using serum creatinine and specific equations.

What is the purpose of estimating kidney function?

• Cockcroft-Gault (1973)

• MDRD (1999)

• CKD-EPI (2009)

• CKD-EPI (2021)

What are the four main equations used to estimate kidney function?

Two measurements of 24-hour creatinine excretion per kg (n = 236).

What is the Cockcroft-Gault (1973) study design?

18–92 years old, all white men.

What population was used in the Cockcroft-Gault equation?

CrCl = (140 − age) × weight / (72 × SCr)

Multiply by 0.85 if female

What is the Cockcroft-Gault equation?

Uses weight; needs adjustment for BSA and BMI > 30

What are the limitations of the Cockcroft-Gault formula

• Cross-sectional study (n = 1628).

• Non-diabetic CKD population, 18–70 years, ~80% white.

What is the MDRD (1999) study design and population?

eGFR = 186.3 × (SCr)^−1.154 × (Age)^−0.203

Multiply by 0.742 if female and 1.21 if Black.

What is the MDRD equation?

Underestimates measured GFR at higher levels.

What is a limitation of the MDRD formula?

Cross-sectional validation (n = 3896), estimation of GFR using creatinine.

What is the CKD-EPI (2009) study design?

31.5% Black, median age 47, mean GFR 67.6

What population was used in CKD-EPI 2009?

eGFR = 141 × min(SCr/k,1)^a × max(SCr/k,1)^−1.209 × 0.993^Age

Multiply by 1.018 if female, 1.159 if Black.

What is the CKD-EPI (2009) equation?

Limited inclusion of elderly, racial, and ethnic minorities.

What are the limitations of CKD-EPI 2009?

Cross-sectional validation (n = 4050), estimation of GFR using creatinine.

What is the CKD-EPI (2021) study design?

14.3% Black, 10 years older on average, with mGFR 9 points higher than 2009 dataset.

What population was used in CKD-EPI 2021?

eGFR = 142 × min(SCr/k,1)^a × max(SCr/k,1)^−1.200 × 0.9938^Age

Multiply by 1.012 if female. (No race variable)

What is the CKD-EPI (2021) equation?

Limited number of Black patients with low GFR; combining creatinine and cystatin C gives more accurate results.

What are the limitations of CKD-EPI 2021?

• Strassinger → MDRD-IDMS traceable formula

• NKDEP & National Kidney Foundation → CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) Formula

Recommended Formula for Estimating GFR

CKD - EPI Formula (2009 - 2021 Update)

Serum Creatinine

What does SCr stand for?

Constant depending on sex

What does K stand for?

• 0.7

• 0.9

In CKD - EPI formula , what is the constant for :

• females

• Males

• - 0.329

• - 0.411

Alpha 2009 CKD - EPI values

• females

• Males

• - 0.241

• - 0. 302

Alpha 2021 CKD - EPI values

• females

• Males

If SCr/k < 1 → use computed value

If SCr/k > or equal to 1 → assign 1

In CKD - EPI formula, How to determine min?

• min (SCr/K , 1)

• If SCr/κ > 1 → use computed value

• If SCr/κ ≤ 1 → assign 1

In CKD - EPI formula, How to determine max?

• max(SCr/κ, 1)

Creatinine

• waste product of muscle metabolism

• Normally found at a relatively constant level in blood

Creatine phosphokinase

Creatinine is produce by what enzyme ?

phosphocreatine

Whenever the body wants to store energy, creatine accepts the donated phosphate from ATP and forms

_____________.

ATP

Whenever the body needs energy, phosphocreatine can donate back the phosphate group to ADP and form _______.

some creatinine is secreted by the tubules, and secretion increases as blood levels rise, potentially overestimating GFR.

Why is creatinine secretion by the tubules important to consider in kidney function tests?

react chemically during analysis, can falsely elevate creatinine values

What is the effect of plasma chromogens on creatinine testing?

Gentamicin, cephalosporins, and cimetidine

Which medications can inhibit tubular secretion of creatinine and cause falsely high serum levels?

Bacteria break down urinary creatinine, leading to falsely low results.

What happens if urine specimens are kept at room temperature for too long?

Increased meat intake raises creatinine levels in both urine and plasma during a 24-hour collection.

How does diet affect creatinine levels?

muscle mass and supplementation affect creatinine production, leading to inaccurate estimates of GFR

Why is creatinine clearance unreliable in patients with muscle-wasting diseases or athletes?

Incomplete collection causes underestimation of clearance results.

Why is the completeness of a 24-hour urine collection important in creatinine clearance tests?

To ensure accuracy and comparability between individuals of different sizes; correction is especially important in children.

Why should creatinine clearance values be corrected for body surface area (BSA)?

CP = UV and C = UV/P

GFR formula

• Milliliters of plasma cleared per minute

• mg/dL of plasma creatinine

• mg/dL of urine creatinine

• urine Volume in mL/min

In GFR formula , what do these stand for?

• C

• P

• U

• V

Using urine creatinine of 120 mg/dL (U) , plasma creatinine of 1.0 mg/dL (P) , and urine volume of 1440 mL obtained from a 24-hour specimen (V), calculate the GFR.

Calculate the urine volume (V) for a 2-hour specimen measuring 249 mL.

Estimated Glomerular Filtration Rates

• Results of newer methods that do not require the collection of timed (24hr) urine specimens

• Equations are superior to serum creatinine alone

Modification of Diet in Renal Disease (MDRD) Formula

Most frequently used formula

MDRD-IDMS-Traceable Formula

• recommended by the National Kidney Education Program (NKDEP)

• Designed to equal the results that compare to the reference body size of 1.73m²

• Not accurate for pediatric patients

Lower than 60mL /min

MDRD-IDMS-Traceable Formula Is most accurate when results are?

MDRD-IDMS-Traceable Formula

Older formula for estimating GFR

What type of formula is the Cockcroft-Gault?

Serum Creatinine (SCr)

What parameter does Cockcroft-Gault use to estimate GFR?

Sex and age

What factors does Cockcroft-Gault formula adjust for?

Variability due to muscle mass differences

What does Cockcroft-Gault formula aim to reduce in GFR estimation?

• Disease-related muscle mass changes

• Extra-renal elimination

• Tubular secretion of creatinine

What factors are not accounted for in the formula?

50 kg + 2.3 kg per inch over 5 ft

How do you calculate Ideal Body Weight (IBW) for males?

45.5 kg + 2.3 kg per inch over 5 ft

How do you calculate Ideal Body Weight (IBW) for females?

Obese, edematous, and chronically ill patients

In which patients does Cockcroft-Gault overestimate GFR?

Ignores disease-related muscle mass, extra-renal elimination, tubular secretion

What are the main weaknesses of Cockcroft-Gault formula?

Schwartz Formula

Used for estimating creatinine clearance for children

Modified Schwartz Formula

Used to estimate GFR more accurately in children

• Requires knowing serum creatinine value, height, BUN , and serum Cystatin C

KDIGO stages of Chronic Kidney Disease

The use of GFR or eGFR Values are interpreted by using?

KDIGO stages of Chronic Kidney Disease, Stage 1

≥90 ml/min/1.73m² (normal or high; with evidence of kidney damage)

KDIGO stages of Chronic Kidney Disease, Stage 2

60–89 ml/min/1.73m² (mild decrease)

KDIGO stages of Chronic Kidney Disease, Stage 3a

45–59 ml/min/1.73m²

(mild to moderate decrease)

KDIGO stages of Chronic Kidney Disease, Stage 3b

30–44 ml/min/1.73m² (moderate to severe decrease)

KDIGO stages of Chronic Kidney Disease, Stage 4

15–29 ml/min/1.73m² (severe decrease)

KDIGO stages of Chronic Kidney Disease, Stage 5

<15 ml/min/1.73m² (kidney failure / end-stage renal disease)

• Patients usually require dialysis

• Dialysis indication:

- Not based on GFR alone

- Consider: Presence of symptoms (e.g., uremia

→ nausea, confusion) And Metabolic

complications

Cystatin C

• Size: 13 kDa, 122 amino acids

• Function: Cystatin proteinase inhibitor

• Produced by all nucleated cells

Cystatin C Formula

Proximal tubular Injury

Cystatin C in urine is a marker for ?

Cystatin C production

• Relatively constant from 4 months to 70 years

• Note affected by: muscle mass, sex, race

• Considered better than creatinine as a GFR marker

High IL-10

inhibits cystatin C production

Glucocorticoids

• Reduce cystatin C production

• Can lead to improper renal function assessment in renal transplant patients

• Measurements are difficult and expensive

• Limited availability compared to creatinine

assays

Why is Cystatin C not widely used?

• Normally completely reabsorbed by proximal tubules

• No urinary excretion

Why the Renal Clearance of Cystatin C cannot be directly measured ?

Indirect estimate of GFR

Change in serum Cystatin C concentration are used for what clinical use?

11.6 kilodaltons,99 amino acids

Size of Beta2-Microglobulin

MHC class I molecule

Components Of Beta2-Microglobulin

Present in all nucleated cells

Location Of Beta2-Microglobulin

Plays a role in CD8 (killer T-cell) development

Function Of Beta2-Microglobulin

• Elevated levels in multiple myeloma

• Elevated levels in lymphoma

Clinical Associations (non-renal) of Beta2-Microglobulin

True

True or false:

B2M is freely filtered at the glomerulus

Fully reabsorbed and degraded

What happens to B2M in the proximal tubule normally?

In renal failure (like cystatin C)

When does plasma B2M concentration rise?

When proximal tubular reabsorption is impaired (e.g., acute kidney injury)

When is B2M excreted in urine?

Sensitive enzyme immunoassays

How is B2M measured?

Sensitive indicator of decreased GFR (more sensitive than creatinine clearance)

What does a rise in plasma B2M indicate?

Not reliable in patients with Immunologic disorders and Malignancy

Limitations of Beta2-Microglobulin

Radionucleotides

Method for determining glomerular filtration (plasma disappearance of the radioactive material)

Exogenous procedure (requires injection)

Radionucleotide Procedures Type?

Radioactive material (e.g., Technetium)

What substance is injected in Radionucleotides ?

Disappearance of the radioactive substance from circulation

What is observed after injection in Radionucleotide Procedures?

• 125l-iothalamate

• Chromium-51 ethylene-diamine-tetra-acetic acid (51 CRP-EDTA)

• Technetium-99-labeled di ethylene-training-pentaacetate (99-Tc-DTPA)

• Iohexol-non-radioactive agent used for children

markers used in Radionucleotide procedures

• visualization of filtration (one or both kidneys)

• Assess the viability of transplanted kidney

Clinical Applications for Radionucleotides

Urea

Main waste product of nitrogen-containing compounds in the body.