Plasma versus Serum Potasium
On 10/14/2011 5:47 PM, Gilbert Hill wrote:
2010-10-14
Clem,
I am afraid that I have let down on two questions – “high sensitivity troponin” and “plasma
potassium”. I will try to deal with the latter in this letter and the former “soon”.
The plasma potassium topic has a very long history, with varying periods of high and low
interest, punctuated by the publication of reports of pseudohyperkalemia (apparently related
to the type of specimen presented for analysis), versus true hyperkalemia. The clinical
challenge is to distinguish between true hyperkalemia, which may benefit from active
treatment, and pseudohyperkalemia, which in principle should not be “treated”. The LOINC
challenge is how our existing model might be used to separate the two.
My thoughts proceed along these lines:
1)
There are numerous publications showing that under controlled circumstances, the
potassium (K) concentration in a sample that has been permitted to clot prior to analysis (i.e.
serum) will be higher than the K concentration in a sample that has not been permitted to clot
prior to analysis (i.e. plasma), and this is generally believed to be due to the release of K from
platelets, and possibly other cells, during clotting. The difference may be increased in the
presence of significant thrombocytosis or leukocytosis.
2)
However, there are also publications showing that for a specimen collected into a tube
containing an anticoagulant, (the product of which would be plasma), many factors can lead to
an increase in the K concentration of the plasma towards that found in serum. These factors
include (1) the collection process (use of tourniquet, type of specimen container, (2) the type of
anticoagulant), (3) the mode of specimen transportation, and (4) the interval between
collection and analysis. In other words, the act of collecting the sample into a tube containing
an anticoagulant is no guarantee that the final K concentration in the sample will reflect the K
concentration in plasma.
3)
I have not found a publication that describes a standardized method of collection and
post collection handling that will ensure the product truly represents “plasma”,
4)
It has been argued that given the vagaries of specimen collection and transport in the
real world, any attempt to distinguish between serum and plasma K would be futile and
potentially misleading, if not dangerous. It may be a very useful thing to do, but simply calling
one tube plasma and one tube serum is unlikely to work.
5)
The question is, given all of the other variables (not even considering patient variables,
but including collection technique, collection container, specimen transport, specimen
separation – each of which has its own set of variables, as well of course of the difficulty of
documenting any of the foregoing), whether in a particular patient we can be confident that an
apparent difference between serum and plasma K is real, or an artefact. If we could be
confident that the difference is real, then a unique LOINC code would be justified.
6)
If a new LOINC entry is created for potassium_plasma, a comment in the Method field
might be helpful, or the Status could be “Trial” but I would not see these measures protecting
the entry from potentially providing misinformation – specifically, leading the clinician to think
that the result actually represented the “in vivo” plasma concentration. I would think that a
carefully worded comment in the test request would be a better way to “communicate” the
clinical problem, rather than to depend on a remote LOINC code to convey the message.
So . . . you can see that while I understand the basis for the request and accept the logic behind
it, I have serious doubts about its practical value.
For the record, here at the Hospital For Sick Children we have used plasma samples for all
electrolyte measurements since at 1965, and have not been aware of significant differences in
reference ranges from other institutions - but then, our definition of a plasma sample was
simply one collected into a heparin containing tube, and did not imply any special care with
collection or transport (although of course, all samples are given special TLC!). I believe that
such a definition (probably widened to include other anticoagulants) represents common
practice and leads me to doubt whether LOINC can contribute to solving this particular clinical
challenge by creating a potassium_plasma code.
Gil