Introduction to Clinical Chemistry

 Reference:                     Veterinary Clinical Chemistry (Cornell)

 

INTRODUCTION:            

Clinical chemistry is the measurement of the amount of chemicals in body fluids.  In order to accurately perform the test and to ensure valid results, it is essential that the Veterinary Technician understand the principle of the test, the physiology of the body system(s) involved, and the medical use (interpretation) of the test.

 Tests are often performed in groups of individual tests, called panels.  Panels provide a broad data base, demonstrating multisystemic involvement.  Panels can also be used for screening an animal for problems, especially in geriatric animals.  The CBC is an example of a panel.  The presurgical panels that you will do in Surgical Nursing evaluate renal, hepatic and pancreatic function and help ensure that the animal will survive the surgery.

 Proper sample collection and handling help ensure that the results are valid.  Many different specimens can be used, including whole blood, serum, plasma, urine, feces, synovial fluid, cerebrospinal fluid and effusions.

The emphasis in this course will be on blood, serum, plasma and urine, with a little feces added.

Any time you add an anticoagulant to blood or another body fluid, you may alter its chemical make up.  It is best, therefore, to use serum when running blood chemistry tests.  If you need to do other lab evaluations, such as a CBC, you need to determine the requirements for these tests.  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 inhibit the testing process and/or change the levels of some reagents.  Fluoride can be used as a glucose preservative.

The basic rule of thumb when drawing blood is to collect enough to run the test three times, while ensuring that you do not take so much blood that you compromise the health of the animal.  Serum yield can be determined from the PCV.  If the PCV is 50%, for example, most of the remainder of the tube is serum.  If you collect 3 ml of blood that is 50% red blood cells, you can harvest approximately 1.5 ml of serum.

Hemolysis can interfere with lab results due to several processes.

  1. 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).
  2. 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.
  3. 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.
  4. Sometimes erythrocyte constituents can react with analytes, causing a false decrease.  This can occur when testing for carbon dioxide, thyroxin and insulin. 
  5. Hemolysis can cause increased turbidity when using the refractometer to determine blood protein levels.

Lipemia is another interference that can adversely affect test results.  Hemolysis is enhanced by lipemia, which increases erythrocyte fragility.  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 an animal prior to blood collection, by ultracentrifuging the sample (100,00g) or using polymers to precipitate the lipids out of the sample. 

 

ENZYMOLOGY:

Enzymes are biologically synthesized proteins that catalyze reactions by decreasing the energy of activation.  Each enzyme acts on a specific substrate.  A small of amount of many enzymes is present in serum and reflect normal cell necrosis and leakage. 

Enzymes are classed as leakage or induced.  Leakage enzymes are normally present within the cells.  If the cell is damaged, the enzymes leak across the cell membrane.  Induced enzymes are produced by cells that are irritated by inflammatory or toxic chemicals.  Some enzymes are manufactured by several different organs, such as alkaline phosphatase (AlkP).  These enzymes function identically regardless of site of production, but have slight variations of structure.  These variations are called isoenzymes, and commercial labs can differentiate between liver-based AlkP and bone-based AlkP. 

In the United States, enzymes are measured in IU/L, or international units per liter.  This is the amount of enzyme that will catalyze the conversion of one micromole of substrate per minute under standardized conditions (don’t worry…I won’t ask you to spit this definition back at me!).  The current international standard of measurement is he Katal; it measures moles per second, but it’s like the gray or sievert in radiology…we don’t really use it.

                       

CHEMISTRY SYSTEMS:

There are several different methods that can be used to determine the chemical composition of a fluid. 

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.  Some reagent strips do include a machine that read the color change, and these are more accurate than the color change measured by looking a colors.

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.  

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.