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.
These guidelines should be followed for submission of chemistry tests:
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.
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