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Clinical Pathology Laboratory - Sample Submission
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Samples for Chemistry
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| 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:
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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). |
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For heparinized samples (especially microtainers), 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!! |
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Microtainers should be avoided at all costs. There is no excuse for
submitting a microtainer from a large dog. 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 microtainers.
These should be full (approximately 600 µL of blood/microtainer).
We need at least 2 microtainers to perform all our chemistry panels.
1 microtainer is acceptable if only performing a few tests, e.g. creatinine
only. Red top microtainers are not acceptable under any circumstances.
Do not label microtainers with sticky adhesive tape. It sticks to
our centrifuges and we cannot get the tubes out without affecting
the plasma. |
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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. |
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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 (see below). |
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An example of an appropriately filled out request form for chemistry
tests. |
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Mailed in samples or samples collected after hours |
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| 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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