Renal Function Tests Flashcards by Cza S.

Q

Broad range of tests that investigate the normal functions performed by the kidney

A

RENAL FUNCTION TEST

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Q

CLINICAL APPLICATIONS OF RENAL FUNCTION TESTS

A

To assess the functional capacity of kidney
Screening & Diagnosis of kidney disease.
Follow up the progression of kidney disease
Monitor the response to treatment
As a routine preoperative investigation.
As a routine precaution after starting certain drugs to check if kidney is damaged (as a side-effect).

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Q

When to assess renal function?

A

Kidney disorders
Vulnerable groups
Systemic diseases
Drug toxicity
Preoperative

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Q

● widely accepted as one of best overall measure of kidney function
● A statement of the complex function of the kidney in a single numerical expression

A

GLOMERULAR FILTRATION TESTS

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Q

GFR is useful in?

A

➔ In targeting treatment
➔ Monitoring progression
➔ Predicting the point at which renal replacement therapy will be required
➔ Used as a guide to dosage of renally excreted drugs to prevent potential drug toxicity

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Q

Classification of Chronic kidney diseases based on GFR:

Stage -
Disease - kidney disease with
GFR - Normal
Value - >90

A

Stage I

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Q

Classification of Chronic kidney diseases based on GFR:

Stage -
Disease - kidney disease with
GFR - mild decreased
Value - 60-89

A

Stage II

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Q

Classification of Chronic kidney diseases based on GFR:

Stage -
Disease - kidney disease with
GFR - moderate decrease
Value - 30-59

A

Stage III

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Q

Classification of Chronic kidney diseases based on GFR:

Stage -
Disease - kidney disease with
GFR - severe
Value - 15-29

A

Stage IV

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Q

Classification of Chronic kidney diseases based on GFR:

Stage -
Disease - renal failure
Value - <15

A

Stage V

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Q

CONCEPT OF CLEARANCE

A

“The volume of plasma from which the substance is completely cleared by the kidneys per unit time.”
● Measures the rate at which the kidneys are able to remove or clear a filterable substance from the blood

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Q

CHARACTERISTICS OF AN IDEAL SUBSTANCE FOR MEASURING GFR:

A

Stable
Inert
Freely filtered
Neither secreted nor reabsorbed

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Q

STANDARD CLEARANCE FORMULA

A

C= U×V/P

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Q

EXOGENOUS MARKERS

A

○ Inulin
○ Iohexol and Iothalamate

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Q

EXOGENOUS MARKERS

A

○ Inulin
○ Iohexol and Iothalamate

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Q

ENDOGENOUS MARKERS

A

○ Creatinine
○ Cystatin C
○ Urea

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Q

considered the gold standard for measuring the glomerular filtration rate (GFR)

A

Inulin

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Q

● very accurate and reliable indicator of kidney function.
● not commonly used in routine clinical practice

A

INULIN CLEARANCE TEST

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Q

the current routine test substance

A

Creatinine

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Q

Advantages of creatinine clearance

A

○ Waste product of muscle destruction found at relatively constant plasma level
○ Automated chemical tests

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Q

CREATININE CLEARANCE method

A

24 hour urine sample is preferred
First voided sample is discarded
Subsequently all urine passed is collected in
containers
Next morning voided sample is collected and all
containers are sent to laboratory
A blood sample is obtained at midpoint of urine collection
Cimetidine which blocks renal secretion can be used
to prevent overestimation
Final calculation is by the formula UV/P, with
adjustment of 10% for secretion

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Q

DISADVANTAGES of creatinine clearance

A

● small amounts of creatinine secreted by renal tubules can increase even further in advanced renal failure
● Creatinine level is affected by intake of meat and muscle mass
● collection of urine is incomplete often
● Creatinine levels are affected by drugs such as
cimetidine, probenecid and trimethoprim that block tubular secretion

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Q

Normal values of creatinine in Men

A

107-139 mL/min

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Q

Normal values of creatinine in Women

A

87-107 mL/min

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True or false: Normal values of creatinine in older people are low

True

Clinical significance of creatinine clearance

● Results are based on functioning nephrons ● Nephrons can double their workload if needed ● This is seen in persons with one kidney ● Creatinine does not detect early disease ● Monitor extent of known renal disease ● Determine feasibility of administering medications that may build up to toxic blood levels

METHODS NOT REQUIRING URINE COLLECTION

Beta2 microglobulin Cystatin C

● Small protein that dissociates from human leukocyte antigens at a constant rate ● Rapidly removed from the plasma by kidneys ● Measured by enzyme immunoassay ● Sensitive indicator of decrease in GFR ● Unreliable in patients with immunological disorders and malignancies

Beta2 microglobulin

● Small protein produced by all nucleated cells; filtered by glomerulus ● Absorbed by the renal tubules and broken down; no cystatin C is secreted ● Serum levels directly reflect GFR ● Monitors pediatric patients, diabetics, elderly, and critically ill patients ● Immunoassay procedures available

Cystatin C

is particularly useful in patients where creatinine measurements might be less accurate, such as those with low muscle mass, elderly individuals, or children.

Cystatin C-based GFR estimation

The most widely used equation is the CKD-EPI Cystatin C equation:

Check trans bish

These are radiographic contrast agents used in imaging studies

Iohexol and Iothalamate

These substances can be used as GFR markers in specialized settings. After administration, their plasma clearance can be measured to determine GFR.

RADIOISOTOPES

● Its less sensitivity ● Conc. Of urea affected by dietary protein, fluid intake, infection, surgery, etc. ● Approximately 40 % of the filtered urea is normally reabsorbed by the tubules.

UREA CLEARANCE TEST

● Normal value of urea

75 ml/min

● Good indicator of early renal disease ● Measure renal concentrating ability ● Often termed concentration tests ● Baseline for determining concentration is the 1.010 specific gravity of the original ultrafiltrate ● Control of fluid intake is necessary for accurate results

TUBULAR REABSORPTION TESTS

● Good indicator of early renal disease ● Measure renal concentrating ability ● Often termed concentration tests ● Baseline for determining concentration is the 1.010 specific gravity of the original ultrafiltrate ● Control of fluid intake is necessary for accurate results

TUBULAR REABSORPTION TESTS

TUBULAR REABSORPTION TESTS OLD TESTS

FISHBERG TEST MOSENTHAL TEST

TUBULAR REABSORPTION TESTS NEW TESTS

SPECIFIC GRAVITY OSMOLARITY

● The patient is deprived of fluid for 24 hours then measure urine SG ● SG should be > 1.026

FISHBERG TEST

● Compare day and night urine in terms of volume and SG

MOSENTHAL TEST

Influence by the number and density of particles in a solution

SPECIFIC GRAVITY

Influenced by the number of particles in a solution

OSMOLARITY

● Key parameter in a renal function test ● Measures the kidney’s ability to concentrate or dilute urine ● It reflects the density of urine compared to the water.

Specific Gravity

has replaced specific gravity as the test to assess renal concentration

Osmolarity

True or false Specific gravity includes number and size of molecules. Osmolarity only includes number of small molecules; Na and Cl are both equal to a large urea molecule

True

Clinical significance

● Evaluating renal concentrating ability ● Monitoring course of renal disease ● Monitoring fluid and electrolyte therapy ● Differential diagnosis of hyponatremia and hypernatremia ● Evaluating secretion of and response to ADH

Normal value of serum

275 – 300 mOsm

Normal value of urine

depends on fluid intake or exercise: 50 – 1400 mOsm

Ratio of serum to urine more accurate Normal

1:1

Ratio of serum to urine more accurate Controlled fluid intake

3:1

● Decreased ADH production ● Inability of tubules to respond to ADH

DIABETES INSIPIDUS

Ratio = no ADH receptors in CD

1:1

Ratio = inability to produce ADH

3:1

● Expands serum : urine ratio

FREE WATER CLEARANCE

● Tests are related because secretion is dependent on renal blood flow ● Interpretation requires attention to both functions and their tests ● To measure secretion, the blood flow must be adequate ● To measure blood flow, the secretion must be adequate

TUBULAR SECRETION RENAL BLOOD FLOW

● Test for renal blood flow ● secreted in proximal convoluted tubule, not by glomerular filtration ● loosely bound to plasma proteins ● completely removed from the blood each time it comes in contact with functional renal tissue ● Exogenous procedure

P-AMINOHIPPURIC ACID (PAH)

● Tests for tubular secretion of H+ and NH4+ ● Normal: 70 mEq/day of acid in form of H+, H2PO4-, NH4+ ● Alkaline tide = first morning, postprandial 2-8 p.m. Lowest pH at night ● Renal tubular acidosis is inability to produce an acid urine = metabolic acidosis ● PCT = secretion of H+ ● DCT = secretion NH3

Titratable Acidity/Urine Ammonia

Test procedure (PAH clearance test)

● Measurement of pH, titratable acidity, and ammonia ● Prime patients with acid load of ammonium chloride ● 2-hr urine specimens, fresh or toluene ● Titrate free H+ (titratable acidity) and total acidity ● Total acidity - titratable acidity = ammonia

Clinical Applications and Interpretation

● Diagnose Kidney Disease ● Monitor Disease Progression ● Guide Treatment Decisions ● Evaluate Risk