TOPIC DISCUSSION: Low or negative Anion gap!

 We often encounter positive anion gaps and know very well the causes of + anion gap.

In certain clinical settings,one can also see a low, zero or negative anion gap.
What are the causes that one has to consider in a low anion gap?
If one considers Na, K, Mg, Ca and immunoglobulins (IgG) as + cations and Cl, Bicarb, Phos, IgA and Phos as negative anions then the anion gap is the balance of these substances. Usually normal is 10-12.
So if one increases the + cations or decreases the - anions, you can have a negative or low anion gap.

Hence the causes are obvious
1. Hyperkalemia, Mg and Calcemia
2. elevated paraproteins( igG usually)-- one of the classic causes and whenever you see a low or negative anion gap, think paraproteinemias
3. decreased albumin or phosphorus
4. Lithium( increases the Cationic side)
5. Unmeasured cations like bromide or iodine or triglycerides can also do it
6. Lab error( most common cause)
7. Severe hypernatremia
8. Spurious elevation of HCo3 if cells are not separated from the sera
9. Over estimation of Cl ion

Out of these the ones to cause a negative anion gap more than low anion gap are lab errors, paraproteins, bromide and iodine intoxication.

Something to keep in mind while we walk the wards!
A nice review is here

http://www.ncbi.nlm.nih.gov/pubmed/17699401
http://www.pbfluids.com/2009/08/high-osmolar-gap-and-low-anion-gap.html