Mike Addison wrote:
>
> This subject has probably been aired on the mailbase in the past
> but I would like to make an enquiry about how people correct
> calcium for serum albumin concentration (I do not want a lecture on
> the benefits of ionized calcium as I am fully aware of the
> arguments!).
>
> It has been my personal practice based on early literature not to
> correct calcium downward in the presence of high albumin
> concentrations. This was based on the fact that albumin
> concentrations can be acutely changed upwards during venous
> stasis and that this could temporarily disturb the equilibrium
> between albumin and calcium. My understanding could be wrong
> and discussions in my own lab have left us not knowing whether it
> is safe and valid to correct calcium down. Your thoughts and
> evidence will be much appreciated
Probably the best data on venous stasis came from Dent's laboratory many
years ago (Berry EM et al, BMJ 1973; 4: 640-3). They showed that during
prolonged venous stasis ionized calcium did not alter appreciably
because it is freely diffusible. On the basis of the consistency of the
changes induced in total calcium and albumin by tourniquet in healthy
volunteers, they recommended that total calcium should be adjusted
downwards for raised albumins as well as upwards for low albumins.
More recent evidence that this is valid in patients was given in Barth
JH et al, Ann Clin Biochem 1996; 33: 55-8. The Leeds General Infirmary
data base was searched for adult patients with discretionary requests
for total calcium analysis but with no other data suggestive of a
disturbance of calcium homostasis. A minimum of 100 data points were
collected at each albumin concentration from 20 to 55 g/L. There were
linear relationships between mean total calcium and albumin which
differed slightly over albumins from 20 to 37 g/L and 37 to 55 g/L.
However, the differences in the regression equations were not clinically
significant, so that adjustment ('correction') downwards of total
calcium for raised albumin seems justified.
Brian Payne
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