the apparent strong ion difference (SIDapp)
this is the electric charge difference between the known strong anions and the known strong kations, measured in mEq/l.
a "strong ion" is one that under pH conditions compatible with life is for all intents and purposes 100% ionised -
sodium and chlor are such substances. there is virtually not a single atom of metallic sodium in our bodies.
Na+ and Cl- are the main contributors, K+, Ca2+, Mg2+ play a secondary role.
our programmes require that you give values for Na+, K+ and Cl-; normal values are assumed for
Ca2+, Mg2+, if you do not specify these.
we are thus using all of the "strong" kations and all of the known "strong" anions. this is a virtually complete panorama on the kation side
of the balance, as significant (i.e. in the mEq/l range) amounts of kations other than the ones mentioned is a very rare ocurrence.
on the anion side, though, only chloride and lactate are routinely measured, and these are "strong" ions. the difference between the
two sides of the electrical balance is called the apparent Strong Ion Difference (SIDapp). the balance
is filled up with the "weak anions" and the "unknown anions".
in the literature you will often find a defintion of SID as:
[Na+]+[K+]
-[Cl-]-[lactate-]. this does, of course, disregard
[Ca2+] and [Mg2+],
which together use to amount to a charge of ~6mEq/l.
as this corresponds well to the normal amount of "unknown anions" in serum, this calculations is valid in metabolically normal patients.
BUT: the derived normal values are different by the mentioned 6mEq/l! our values for SIDapp are higher
by this amount: 40 to 46mEq/l.
