Renal function tests Flashcards
Oliguria
Reduced urine output per day
Polyuria
Excessive urine output per day
Nocturia
Waking repeatedly at night to pee
Oedema
Buildup of fluid that causes swelling
Preliminary investigations to renal function tests
Assessing patient’s urine history e.g urine habits and things like appearance of oedema
Physical urine analysis. Colour, smell, volume, pH, specific gravity and microscopic analysis for abnormalities
Biochemical urinalysis . Looking for things like glucose, ketones, albumin, bilirubin and excess urobilinogen
Biochemical serum compound analysis. Specifically, an increase in uric acid, urea and creatinine. Also electrolytes such as potassium, sodium and bicarbonate
3 classes of renal function tests
Glomerular function test
Renal plasma flow test
Tubular function test
Tests under glomerular function tests
Urea clearance test
Endogenous creatinine clearance test
Inulin clearance test
Tests under renal plasma flow
Para-amino hippurate (PAH) test
Filtration fraction test
Tests under tubular function tests
Concentration and dilution tests
15 mins PSP excretion tests
Clinical significance of Urea clearance test
Higher volume cleared per minute means normal renal function
Lower means impaired function
Formula for calculating urea clearance rate (ml)
UV/P
where U = conc of urea in urine (mg/100ml)
V = volume of urine (ml/minute)
P = conc of urea in blood (mg/100ml)
How to correct for normal urea clearance rate in children and those of abnormal stature
Multiply by 1.73/BS
where BS = The body surface area derived from their weight and height
Procedure for carrying out urea clearance test
- The patient is given a light breakfast with 2-3 glasses of water
- The patient should urinate completely, throw it away. Note the time of urination.
- One hour later, urine and blood are collected for checking urea concentration.
- Another urine sample is collected one hour later. The average of the urea conc in the two urine samples is used.
Precautions to note when carrying out urea clearance test
The test should be ran between breakfast and lunch
The patient should remain at rest throughout the duration of the test
Result interpretation of the urea clearance test
Clearance rate of 70% or more indicates normal function
Between 40 - 70 means mild impairment
Between 20 - 40 means moderate impairment
Below 20 means severe impairment
Three reasons why creatinine is preferred for estimating Glomerular filtration rate
It is metabolized in the body
It is mostly excreted at the glomerulus
Estimation of creatinine is relatively simple
Procedure for Endogenous creatinine clearance test
- Accurate volume of urine is collected over 24 hours, starting and ending at 7am
- A blood sample is collected too to measure serum creatinine levels
- Estimate the conc of creatinine in both using
- Obtain creatinine clearance rate using the formula
Formula for obtaining creatinine clearance rate
UV/P
where U = conc of creatinine in urine (mg/dl)
V = volume of urine over 24 hours (ml/mt)
P = conc of creatinine in serum (mg/dl)
Normal value for creatinine clearance rate
95 - 105 ml/mt
Why is inulin ideal for testing glomerular function?
It is not metabolized in the body and it is excreted in the urine, entirely through the glomerulus. Also isn’t reabsorbed or secreted at any point along the renal tubule.
As such it is a perfect test for glomerular filtrate rate
What is the procedure for carrying out the inulin clearance test
- Very light breakfast given at 7:30 am (half glass of milk and one slice toast)
- At 8:00 am, 10gm of Inulin is dissolved in 100ml of saline and administered via IV at 10ml per minute
- One hour later, bladder is emptied and discarded. Note the time.
- Collect two more urine samples one and two hours later respectively. Note the volume of each and measure the conc of inulin.
- 30 mins before each urine sample is collected, collect a blood sample (10-15ml). This is to get inulin conc in blood.
- Separately get the inulin clearance rate using the formula for the two blood samples then find the average of each. This is the inulin clearance rate.
Precautions when carrying out the inulin clearance test
Test should be done in the morning, if possible, patient should be hospitalized the day before the test.
Inulin clearance rate formula
UV/P
where U = conc of inulin in urine (mg/dl)
V = volume of urine (ml/minute)
P = conc of inulin in serum (mg/dl)
Normal range of inulin clearance rate
100 - 150 ml/min
What is renal plasma flow
The rate at which blood plasma passes through the kidneys
Normal renal plasma flow value
574 ml/min
What is filtration fraction
This is the portion of plasma filtered from the glomerulus relative to the renal plasma flow
Formula for filtration fraction
Inulin clearance rate/ PAH clearance rate
OR
GFR/RPF
Normal range of filtration fraction
0.16 - 0.21
Clinical significance of filtration fraction
Malignant hypertension - increase in FF
Congestive heart failure - increase in FF
Glomerulonephritis - decrease in FF
