the unknown anions, XA
the unknown anions are a diverse group of pathophysiologically significant substances.
they represent diverse substances, some being physiologically occurring metabolites like lactate
(if not measured separately),
citrate and acetate (related to the tricarboxylic acid cycle) or non-organic anions like sulfate,
others of pathologic origin like ketoacids, toxic metabolites of poisons like glycole,
others metabolic waste products normally taken away by the liver or kidneys.
their value is calculated as the difference between the SID based on
measuring the known strong ions and the SID based on measuring the known weak ions.
this is called the "strong ion gap", and is very similar to the more commonly known
anion gap.