Chapter 2 - Renal Function Tests Flashcards
Broad range of tests that investigate the normal functions performed by the kidney
Together with careful history, physical examination and imaging can contribute to making an accurate diagnosis of specific Kidney disease
Renal Functional Test
Clinical Application of Renal Function Tests
- To asses 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).
When to assess renal function?
- Kidney Disorders
- Vulnerable Groups
- Systemic disease
- Drug toxicity
- Preoperative
Tests evaluated in Renal Functional Tests
Glomerular filtration
Tubular reabsorption
Tubular secretion
Renal blood flow
is widely accepted as one of best overall measure of kidney function
Glomerular Filtration Rate
A decrease in GFR precedes kidney failure in most forms of
progressive renal parenchymal diseases
GFR is measured by using the -
clearance of a substance present in plasma that is filtered freely at the glomerulus, but is neither reabsorbed nor secreted by the tubule
GFR is useful in:
- 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
To measure GFR, most methods involve the kidney’s ability to clear (CLEARANCE) either _______________
an exogenous or endogenous marker.
Stage 1 GFR Value
> 90 (Normal GFR)
Stage 2 GFR Value
60-89 (Mild decreased GFR)
Stage 3 GFR Value
30-59 (Moderate decreased GFR)
Stage 4 GFR Value
15-29 (Severe decreased GFR)
Stage 5 GFR Value
<15 (Renal Failure
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
Concept of Clearance
Characteristics of an Ideal Substance for Measuring GFR:
Stable
Inert
Freely Filtered
Neither Secreted Nor Reabsorbed
Characteristics (GFR) - : The substance should have a constant concentration in the blood.
Stable
Characteristics (GFR) - It should be physiologically inert, meaning it does not affect or alter kidney function.
Inert
Characteristics (GFR) - The substance should be able to pass through the glomeruli freely, without any restrictions.
Freely Filtered
Characteristics (GFR) - After filtration, the substance should not be reabsorbed back into the bloodstream or secreted by the renal tubules.
Neither Secreted Nor Reabsorbed
Markers for Exogenous Markers
Inulin
Iohexol and Iothalamate
Endogenous Markers
Creatinine
Cystatin C
Urea
considered the gold standard for measuring the glomerular filtration rate (GFR)
very accurate and reliable indicator of kidney function.
not commonly used in routine clinical practice
Inulin Clearance Test
is the current routine test substance
Creatinine Clearance
Advantages of Creatinine Clearance
Waste product of muscle destruction found at relatively constant plasma level
Automated chemical tests
Methods for Creatinine Clearance Test
- 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
Disadvantages of Creatinine Clearance Test
- 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
Normal values for Creatinine Clearance
Men: 107-139 mL/min
Women: 87-107 mL/min
Creatinine is produced as a result of __________; therefore, normal values are based on size; the larger the person, the more creatinine produced
muscle destruction
