Samples for Chemistry
In general, serum samples (red top tubes) are preferred for chemistry testing. This is because our chemistry reference intervals are based on serum not plasma. In general, there is little difference between serum and plasma, except for certain analytes. For example, LDH, potassium and phosphate are higher in serum than plasma, because of release of these constituents from cells during clotting. Protein and globulins are higher in plasma than serum, because plasma contains fibrinogen. The disadvantage with serum is that the samples can take a while to clot, therefore for late afternoon samples (after 3 pm Monday to Friday, after 12 pm Saturday), collection into heparin (green top tube) is advised to expedite sample handling. All stats for chemistry should be submitted in heparin so that we do not have to wait for the blood to clot. Citrate (blue top) and EDTA (lavender top) cannot be used for chemistry panels because they chelate minerals (e.g. calcium) and interfere with the tests. Furthermore, citrate dilutes the sample.
We recommend that from an individual patient, samples for chemistry should always be collected into the same tube (heparin or red top) for the duration of the patient's stay in hospital. This will ensure that changes in analytes are patient- or disease- and not anticoagulant-related. For example, if the first chemistry panel is submitted in heparin, all subsequent chemistry tests should be submitted in heparin to allow more valid comparison between sequential results.
Chemistry Test Submission Guidelines
- A full clot tube should be submitted whenever possible. Heparin tubes should always be more than half full. Underfilling of heparin will artefactually increase total bilirubin values. By providing us with as much blood as possible, we can perform additional tests as required, e.g. sample dilution etc. If we do not have enough blood, we cannot perform all the required tests. If you know that you did not collect enough blood (we need at least 1 ml of serum), you will need to prioritise which tests you want run, otherwise the important tests or tests you want may be QNS (quantity insufficient).
- For heparinized samples (especially Microtainer® tubes), ensure that the blood is mixed promptly with the heparin to avoid sample clotting. This should be done by rolling it between your palms or gentle inversion several times. Do not shake the tube!!
- Microtainer® tubes should be avoided. However, if only a small amount of blood can be collected, e.g. from a young dog or cat, or very sick animals in which multiple, sequential samples are going to be collected, the blood should be collected into 2 heparin Microtainer® tubes. These should be full (approximately 600 µL of blood/tube). We need at least 2 Microtainer® tubes to perform all our chemistry panels. 1 Microtainer® tube is acceptable if only performing a few tests, e.g. creatinine only. Red top Microtainer® tubes are not acceptable under any circumstances. Do not label Microtainer® tubes with sticky adhesive tape. It sticks to our centrifuges and we cannot get the tubes out without affecting the plasma.
- The tubes should be labeled with the patient identification and owner name at the minimum. A request form with pertinent history details should be submitted concurrently with the sample, e.g. dog administered oxyglobin. If you have separated serum or plasma from cells, please inform us if the sample is separated serum or heparinized plasma.
- If there is going to be a delay between sample collection and submission, the sample should be centrifuged and the serum or plasma separated from cells.
- For ionized calcium, serum should be submitted in a plain red top tube. EDTA or citrate samples cannot be used (these anticoagulants chelate calcium). Heparinized plasma (green top tube) also results in a falsely decreased iCa, and is not recommended. The sample should be refrigerated and shipped overnight in an insulated container with freezer packs. If more than a 48 hour delay, the serum should be frozen and shipped on dry ice. For more information, see eClinPath.
Mailed in samples or samples collected after hours
When there is going to be a delay between sample collection and submission, e.g. samples shipped to the laboratory or collected after hours, always separate serum or plasma from cells. This also applies to corvac (serum separator tubes). Constituents, e.g. AST, potassium, leak from cells with storage and will result in artefactually high values, complicating result interpretation. Furthermore, cells utilize glucose, resulting in low concentrations of this analyte with storage. This will also happen in unseparated samples collected into corvac tubes. Most analytes are stable for up to 48 hours if kept refrigerated, therefore refrigerate and mail in on a cool pack.
Microtainer® is a registered trademark of Becton, Dickinson and Company.