Renal function tests

Question 1

Oliguria

Answer 1

Reduced urine output per day

Question 2

Polyuria

Answer 2

Excessive urine output per day

Question 3

Nocturia

Answer 3

Waking repeatedly at night to pee

Question 4

Oedema

Answer 4

Buildup of fluid that causes swelling

Question 5

Preliminary investigations to renal function tests

Answer 5

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

Question 6

3 classes of renal function tests

Answer 6

Glomerular function test
Renal plasma flow test
Tubular function test

Question 7

Tests under glomerular function tests

Answer 7

Urea clearance test
Endogenous creatinine clearance test
Inulin clearance test

Question 8

Tests under renal plasma flow

Answer 8

Para-amino hippurate (PAH) test
Filtration fraction test

Question 9

Tests under tubular function tests

Answer 9

Concentration and dilution tests
15 mins PSP excretion tests

Question 10

Clinical significance of Urea clearance test

Answer 10

Higher volume cleared per minute means normal renal function
Lower means impaired function

Question 11

Formula for calculating urea clearance rate (ml)

Answer 11

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)

Question 12

How to correct for normal urea clearance rate in children and those of abnormal stature

Answer 12

Multiply by 1.73/BS
where BS = The body surface area derived from their weight and height

Question 13

Procedure for carrying out urea clearance test

Answer 13

1. The patient is given a light breakfast with 2-3 glasses of water
2. The patient should urinate completely, throw it away. Note the time of urination.
3. One hour later, urine and blood are collected for checking urea concentration.
4. Another urine sample is collected one hour later. The average of the urea conc in the two urine samples is used.

Question 14

Precautions to note when carrying out urea clearance test

Answer 14

The test should be ran between breakfast and lunch

The patient should remain at rest throughout the duration of the test

Question 15

Result interpretation of the urea clearance test

Answer 15

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

Question 16

Three reasons why creatinine is preferred for estimating Glomerular filtration rate

Answer 16

It is metabolized in the body
It is mostly excreted at the glomerulus
Estimation of creatinine is relatively simple

Question 17

Procedure for Endogenous creatinine clearance test

Answer 17

1. Accurate volume of urine is collected over 24 hours, starting and ending at 7am
2. A blood sample is collected too to measure serum creatinine levels
3. Estimate the conc of creatinine in both using
4. Obtain creatinine clearance rate using the formula

Question 18

Formula for obtaining creatinine clearance rate

Answer 18

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)

Question 19

Normal value for creatinine clearance rate

Answer 19

95 - 105 ml/mt

Question 20

Why is inulin ideal for testing glomerular function?

Answer 20

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

Question 21

What is the procedure for carrying out the inulin clearance test

Answer 21

1. Very light breakfast given at 7:30 am (half glass of milk and one slice toast)
2. At 8:00 am, 10gm of Inulin is dissolved in 100ml of saline and administered via IV at 10ml per minute
3. One hour later, bladder is emptied and discarded. Note the time.
4. Collect two more urine samples one and two hours later respectively. Note the volume of each and measure the conc of inulin.
5. 30 mins before each urine sample is collected, collect a blood sample (10-15ml). This is to get inulin conc in blood.
6. 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.

Question 22

Precautions when carrying out the inulin clearance test

Answer 22

Test should be done in the morning, if possible, patient should be hospitalized the day before the test.

Question 23

Inulin clearance rate formula

Answer 23

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)

Question 24

Normal range of inulin clearance rate

Answer 24

100 - 150 ml/min

Question 25

What is renal plasma flow

Answer 25

The rate at which blood plasma passes through the kidneys

Question 26

Normal renal plasma flow value

Answer 26

574 ml/min

Question 27

What is filtration fraction

Answer 27

This is the portion of plasma filtered from the glomerulus relative to the renal plasma flow

Question 28

Formula for filtration fraction

Answer 28

Inulin clearance rate/ PAH clearance rate
OR
GFR/RPF

Question 29

Normal range of filtration fraction

Answer 29

0.16 - 0.21

Question 30

Clinical significance of filtration fraction

Answer 30

Malignant hypertension - increase in FF
Congestive heart failure - increase in FF
Glomerulonephritis - decrease in FF