Chemistry testing is usually performed on serum or plasma samples. To optimize results of testing, serum or plasma should be separated from cells as soon as possible after collection to minimize artifacts that occur with storage. Note that some chemistry tests can also be run on urine (e.g. protein to creatinine ratios, fractional excretion of electrolytes, urinary GGT measurement for renal injury) and ocular fluids (usually done as a post-mortem test).
View other sample collection pages specific for hematology, hemostasis, urinalysis and cytology.
Sample collection and handling guidelines
All samples should be kept cool (shipped on ice packs). If whole blood is shipped (not recommended due to storage artifacts; see below), the sample should be wrapped in paper towels to prevent direct contact of the tube with the ice, which will result in freezing and lysis of red blood cells. View guidelines for chemistry samples submission and general shipping recommended by the Animal Health Diagnostic Center of Cornell University.
Blood tubes
Serum
- Blood tube: Red top
- Contents: None.
- Uses: Serum is preferred to plasma for electrophoresis and testing of all chemistry analytes, except for potassium, which is higher in serum than in plasma due to release of potassium from cells during clotting.
- Processing:
- After the blood has clotted, rim the tube with a wooden applicator stick to loosen the clot (this may need to be performed several times in samples from horses and ruminants; their blood also takes a while to clot).
- Centrifuge at moderate speed (450 g).
- Remove the serum and place in another red top or plastic storage tube.
Heparinized plasma
- Blood tube: Green top
- Contents: Heparin. Heparin inhibits clotting by potentiating the action of antithrombin.
- Uses: It is the preferred anticoagulant for chemistry testing.
- Collection considerations: Tubes must be filled more than half full to avoid artifacts, such as a falsely increased total and indirect bilirubin.
- Processing:
- Centrifuge at moderate speed (450 g).
- Remove the plasma and place in a red top or plastic storage tube.
- Test interpretation points: Protein values are higher in plasma versus serum because fibrinogen is removed during clotting.
EDTA plasma
- Blood tube: Purple top
- Contents: Potassium EDTA. EDTA chelates calcium (and other divalent cations, such as iron), which is required for clotting.
- Uses: Can be used for testing of most analytes, with the exception of many enzymes, electrolytes (potassium), iron tests (iron, TIBC, % saturation) and minerals (calcium and magnesium). Many enzymes require divalent cations, such as calcium, for their function, which is chelated by the EDTA. EDTA plasma is mostly used for glucose measurement.
- Processing:
- Centrifuge at moderate speed (450 g)
- Remove the plasma and place in a red top or plastic storage tube.
- Test interpretation points: EDTA (even minor contamination) will markedly increase potassium and decrease calcium (and to a lesser extent, magnesium).
Citrate plasma
- Blood tube: Blue top
- Contents: Sodium citrate. This chelates calcium as well, but is more gentle than EDTA, so calcium can be added back for coagulation testing
- Usage: Citrate is used for clotting tests. This is generally not used for chemistry testing (will falsely increase sodium and decrease calcium and magnesium, similar to EDTA).
- Detailed information is available on the Animal Health Diagnostic Center of Cornell University’s citrate plasma sample page
Serum separator tubes (Corvac)
- Blood tube: Striped red top
- Contents: These tubes contain a gel, which facilitates separation of the clotted cells from serum.
- Usage: This is most useful in large animal blood, which takes a while to clot and can be difficult to obtain serum from. However, the gel does not prevent common artifacts associated with storage of whole blood for chemistry testing (see below) and the blood should still be centrifuged and serum placed in another tube.
- Processing:
- Centrifuge at moderate speed (450 g).
- Remove the plasma and place in a red top or plastic storage tube.
View the blood tube selection guide for different tests offered by the Animal Health Diagnostic Center of Cornell University.
Storage artifacts
Storage of serum or plasma on cells (not separating) can result in many false changes in chemistry results. These changes are minimized (not eliminated) by cold storage (refrigerated, shipping on ice packs) and still occur with serum separator tubes (the gel is not an impermeable barrier).
- Hypoglycemia: Glucose consumption by cells
- Hyperkalemia: In species with high potassium in their red blood cells (horses, some breeds of dogs such as Akitas, sheep, some breeds of cattle, including Holsteins, camelids).
- Hemolysis: This can affect many different test results in different ways. For more information, see interference indices, interferences, and common artifacts.
