Renal function test summary, Cheat Sheet of Medicine

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Renal

Function

Tests

(RFTs)

Color Index:

● Main text

● Important

● Notes

● Boys

slides

● Girls

Lecture number 2

● To have a knowledge about functional units and

normal functions of the kidney.

● To have an idea about some examples of renal

disease.

● To know laboratory routine kidney function tests

(KFTs).

● To know other laboratory KFTs.

objectives

In Male slides Only

2- Proximal Convoluted Tubule

Factors facilitate filtration:

● high pressure in the

glomerular

capillaries, which is a

result of their

position between two

arterioles.

● the semipermeable

glomerular

basement membrane,

which has a

molecular size cutoff value

of

approximately 66,000 Da.

The volume of blood

filtered per minute is the

glomerular filtration rate

(GFR), and its

determination is essential

in evaluating renal

function.

Returns the bulk of each valuable

substance back to the blood

circulation.

• 75% of the water, sodium, and

chloride.

• 100% of the glucose (up to the

renal threshold).

• almost all of the amino acids,

vitamins, and proteins.

• varying amounts of urea , uric

acid, and ions, such as

magnesium , calcium and

potassium. (With the exception

of water and chloride ions

“because they reabsorbed

passively” , the process is active

that is the tubular epithelial cells

use energy to bind and transport

the substances across the

plasma membrane to the blood).

• Secretes products of kidney

Functions to filter incoming

blood.

1- Glomerulus

Each nephron is a complex apparatus comprised of

five basic parts:

In Male slides Only

3- Loop of Henle

Facilitates the reabsorption of

water, sodium, and chloride.

The osmolality in the

medulla in this portion of

the nephron increases

steadily from the

corticomedullary junction

inward.

4- Distal Convoluted

Tubule

● The filtrate entering this

section of

the nephron is close to its final

composition (which is urine).

● Effects small adjustments to

achieve

electrolyte and acid-base

homeostasis

(under the hormonal control of

both

antidiuretic hormone “ADH” and

aldosterone).

The distal convoluted tubule

is much shorter than the

proximal tubule, with two or

three coils that connect to a

collecting duct.

5- Collecting Duct

● The collecting ducts are the final site for either

concentrating or diluting urine.

● The hormones ADH and aldosterone act on this

segment of the nephron to control reabsorption

of water and sodium.

● Chloride and urea are also reabsorbed here

(partially).

● The collecting ducts in the medulla are highly

permeable to urea so urea will diffuse down its

concentration in gradient out of the tubule and into

the

medulla interstitium, increasing its osmolality.

Routine KFTs include the

measurement of:

Serum creatinine

(Cr).

Creatinine

clearance.

Serum urea.

Both serum Cr and creatinine clearance are used as

kidney function tests to :

Confirm the diagnosis of renal

disease.

Give an idea about the severity of the disease.

Follow up the treatment.

Electrolyte.

Serum creatinine (55-120 μmol/L in

adult):

● (^) Creatinine is the end product of

creatine catabolism.

● (^) 98% of the body creatine is present in

the muscles where it functions as store

of high energy in the form of creatine

phosphate.

● (^) About 1-2 % of total muscle creatine or

creatine phosphate pool is converted

daily to creatinine through the

spontaneous, non enzymatic loss of

water or phosphate.

● Creatinine in the plasma is filtered

freely at the glomerulus and secreted

by renal tubules ( 10 % of urinary

creatinine).

● Creatinine is NOT reabsorbed

by the renal tubules.

● (^) Plasma creatinine is an

endogenous substance not

affected by diet.

● Plasma creatinine remains fairly

constant throughout adult life.

Unit is very

important

We can ignore it

Accurate measurement of GFR by

clearance tests requires

determination of the concentration

in plasma and urine of a substance

that is:

● (^) Freely filtered at glomeruli.

● (^) Neither reabsorbed nor secreted by tubules.

● Its concentration in plasma needs to remains

constant throughout the period of urine

collection.

● (^) Better if the substance is present

endogenously.

● (^) Easily measured.

● (^) Creatinine meets most of these criteria.

Except that creatinine is secreted by 5-10%,

Creatinine clearance is

usually about 110 ml/min.

It falls slowly but

progressively to about 70

ml/min.? Decrease in muscle mass

Creatinine

clearance: Count. In the 20-40 year old

adults.

The GFR should be related to

surface area , when this is

done, results are similar to

those found in young adults.

In Children.

In individuals over 8o

years of age

Not costly

Cockcroft-Gault Formula for

Estimation of GFR

● As indicated above, the creatinine clearance is

measured by using a 24-hour urine collection, BUT this

does introduce the potential for errors in terms of

completion of the collection.

● (^) An alternative and convenient method is to employ

various formulae devised to calculate creatinine

clearance using parameters such as :

An example is the Cockcroft-

Gault Formula:

GFR = K x (140 – age) x Body

weight

Serum creatinine (μmol/L)

● K is a constant that varies with sex:

• (^) 1.23 for male
• (^) 1.04 for females.

● (^) The constant K is used as females have

a relatively lower muscle mass.

Cockcroft-Gault Formula for Estimation of

GFR:It should not be Limitations

used if : (^) We said that creatinine

is an endogenous

substance not affected

by diet.

BUT here it is affected

indirectly by causing a

decrease in muscle

• (^) serum creatinine level,
• (^) sex,
• (^) age,
• (^) weight of the subject.

1. Serum creatinine is changing

rapidly in bodybuilding

2. The diet is unusual, e.g., strict

vegetarian

3. Low muscle mass, e.g., muscle

How to convert

creatinine

concentration from

MG/DL to

micromol/L?

THE CONVERSION

FACTOR IS ( 88.4 )

e.g:

1mg/dl = 88.

micromol/l

First 3 points affect

(serum creatinine)

and forth point affect

body weight in the

equation

Serum Urea ( 2.5-6.6 mmol/L) in

adult:

● (^) Urea is formed in the liver from ammonia

released from deamination of amino acids.

● (^) High protein diet increases urea formation. Unlike

creatinine

● Any condition of high proteins catabolism (Cushing

syndrome, diabetes mellitus, starvation, thyrotoxicosis)

increase urea formation.

● (^) 50 % or more of urea filtered at the glomerulus is

passively reabsorbed by the renal tubules.

● As a kidney function test, serum urea is

inferior to serum creatinine because:

Amino

Acid

Ammon

ia

Urea

Ammonia has no charge, so it could defuse to the

brain from BBB. Thus it’s harmful and should be

converted to urea

Note that the serum urea is higher than

serum creatinine due to the unit mmol is

higher than μmol

SODIUM 135 to 145 mEq/L

POTASSIUM 3.5 to 5.5 mEq/L

CHLORIDES 100 to 110 mEq/L

BICARBONATE 24 to 26 mEq/L

CALCIUM 8.6 to 10 mg/dl

MAGNESIUM 1.6 to 2.4 mg/dl

PHOSPHORUS 3.0 to 5.0 mg/dl

URIC ACID 2.5 to 6.0 mg/dl

pH 7.

CREATININE 0.8 to 1.4 mg/dl

BUN (Blood Urea

Nitrogen)

15 to 20 mg/dl

Normal values of Internal

Chemical Environment controlled

by the Kidneys:

Examples of other KFTs:

● Cystatin C.

● (^) Microalbumin.

● (^) β2-Microglobulin (11,

Da).

● (^) Myoglobin (16,900 Da)

Numbers

are not

importan

t

Male’s Dr said: This

table is useful but

you don’t need to

memorize it

Remember that

microalbumin means a

small amount of

Albumin not another

type

Female’s Dr said: no

number have to be

memorized here

SAQs

Q1 1- confirm the diagnosis of renal disease.

2- give an idea about the severity of the disease.

3- follow up the treatment.

Q2 - Freely filtered at glomeruli.

• (^) Neither reabsorbed nor secreted by tubules.
• (^) ts concentration in plasma needs to remains constant throughout the period of urine

collection.

• (^) Better if the substance is present endogenously
• (^) Easily measured.

Q3 serum creatinine is changing rapidly / The diet is unusual, e.g., strict vegetarian

Low muscle mass, e.g., muscle wasting / Obesity

Q1/ both serum Cr and creatinine clearance are used as kidney function test

to?

Q2 / enumerate 4 requirements for accurate measurement of GFR by

clearance tests

Q3 / enumerate the limitations of Cockcroft-Gault Formula.

Sara Alotaebe

Mohammed Ibn Saqyan

Leaders

Our Team

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