INTRODUCTION TO CLINICAL CHEMISTRY

Prepared by: Miss Nada

Clinical chemistry is the measurement of the

amount of chemicals in body fluids.

Tests are often performed in groups of individual tests,

called panels. The Liver Function Test (LFT) is an

example of a panel. 

Proper sample collection and handling help ensure

that the results are valid.  Many different specimens

can be used, including whole blood, serum, plasma,

urine, faeces, synovial fluid and cerebrospinal fluid.

Blood plasma is the liquid component of blood, in which

the blood cells are suspended. It makes up about 55% of

total blood volume. It is composed of mostly water (90%

by volume), and contains dissolved proteins, glucose,

clotting factors, mineral ions, hormones and carbon

dioxide. Blood plasma is prepared simply by spinning a

tube of fresh blood in a centrifuge until the blood cells fall

to the bottom of the tube. The blood plasma is then

poured or drawn off.

Blood serum is blood plasma without fibrinogen or the

other clotting factors. Both plasma and serum are light

yellow in color.

Difference between Plasma & Serum

The preferred anticoagulant for chemistries is

heparin—in interferes with the fewest tests. 

It functions by inactivating prothrombin, one

of the clotting factors, and has a green cap. 

EDTA usually contains sodium or potassium

which inhibits the testing process and/or

changes the levels of some reagents. 

Type of anticoagulant:

Hemolysis is the rupture of red blood cells with the release of hemoglobin and the cellular constituents into the plasma, or liquid portion of whole blood. The release of hemoglobin causes the serum or plasma to appear pale red to cherry red in color.

Hemolysis & Lipemia:

Hemolysis can interfere with lab results due to

several processes:

Leakage of analytes from lysed erythrocytes may cause

a false increase in the amount of analyte measured in

serum if the analyte is normally present in a greater

amount inside the RBC than in plasma.  This can occur

when measuring the levels of potassium, creatine kinase

(CK), and alanine amino transferase (ALT).

If the analyte is normally present in greater amounts in

plasma than in the erythrocytes, the analyte will be

diluted by the lysing of the RBCs, causing a false

decrease. Sodium and chloride can both be increased.

Color interference (tinting the serum pink or red) can

cause false increases when using a

spectrophotometer. Hemoglobin, bilirubin and protein

are a few of the analytes affected by hemolysis.

Sometimes erythrocyte constituents can react with

analytes, causing a false decrease. This can occur

when testing for carbon dioxide, thyroxin and insulin. 

Hemolysis can cause increased turbidity when using

the refractometer to determine blood protein levels.

Lipemia: is the presence of an

abnormally high concentration of

lipid in the blood.

Lipemia is another interference

that can adversely affect test results. 

Hemolysis is enhanced by lipemia,

which increases erythrocyte fragility. 

Lipemia:

Lipids present in a specimen scatter light and can

cause either an increase or decrease in values,

depending upon the analytes being evaluated. 

Because electrolytes are in the aqueous phase of

blood, lipids may dilute their concentration. 

Lipemia can be minimized by fasting the patient

prior to blood collection, by ultracentrifuging the

sample or using polymers to precipitate the lipids

out of the sample.

Dry reagent strips require the comparison of a color

change on the reagent strip with a color chart. These

are most commonly used as quick screening tests,

and their accuracy is only low to moderate.

Wet and dry chemistry systems utilize a

spectrophotometer to mechanically measure color

change and are much more accurate than dry reagent

trips. The reagent is the difference between the two

systems—most chemistry systems today use dry

reagents which are mixed with a liquid specimen.  

CHEMISTRY SYSTEMS

Spectrophotometers use the Beer-Lambert (or

Beer’s) law to measure concentrations. This law

states that the amount of light absorbed by a

solution varies with the concentration of the

colored solute. Light is shown through a specimen

in a cuvette and the amount of light transmitted is

recorded by a photocell. This is then converted

into the amount of substance in the sample.

ଊ Know your glassware