Osmolar gap

[strongboy2005]

So I am trying to do some reading on toxicology before residency starts and came across the familiar calculated osmolarity equation (which, when subtracted from the measured osmolarity, gives the osmolar gap):

2[Na+]+glucose/18+BUN/2.8

I understand the "2[Na+]" term to be inclusive of the anions that must be present to maintain electroneutrality (for example, if I put NaCl in water, I can simply double the concentration of Na+ to account for dissolved Cl- and calculate the number of osmoles in the solution). This is easier than trying to round up the concentrations of all the ions and add them together.

I noticed that K+ is missing from this equation. At first glance, with the normal calculated osmolarity being close to 300, I thought maybe the K+ was left out as a negligible factor. But if there is an anion for every cation, then the [K+] term should also be doubled. With a normal [K+] of 4 mEq/L in the blood, matching anions would bring the contribution to 8 mOsm/L. This is more than is contributed by a normal glucose (glucose of 108 mg/dL produces 6 mOsm/L) and by BUN (BUN of 20 mg/dL produces ~7 mOsm/L).

So what gives? Why isn't the equation 2([Na+]+[K+])+glucose/18+BUN/2.8 to account for the 8 mOsm/L produced by potassium salts dissolved in the blood?

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[Bostonredsox]

First off, you are calculating serum osmolality, not osmolar gap. Osmolar gap is the difference between the serum osmoLality (calculated) and the serum osmoLality (measured). It is generally present in ethylene glycol and methanol overdoses (had one intern year was most awesome and laborious patient to keep alive I have yet seen).

Second the relative amount of K+ that is extracellular is so small in comparison to Sodium, it played a neglible roll in the amount of solute in the serum (osmolality). Hope that helps.

EDIT: i misread your question I see now you explained how you were deriving osmolar gap in your first paragraph, thus you can ignore my first paragraph, sorry.

 

[Daiphon]

So I am trying to do some reading on toxicology before residency starts and came across the familiar calculated osmolarity equation (which, when subtracted from the measured osmolarity, gives the osmolar gap):

2[Na+]+glucose/18+BUN/2.8

I understand the "2[Na+]" term to be inclusive of the anions that must be present to maintain electroneutrality (for example, if I put NaCl in water, I can simply double the concentration of Na+ to account for dissolved Cl- and calculate the number of osmoles in the solution). This is easier than trying to round up the concentrations of all the ions and add them together.

I noticed that K+ is missing from this equation. At first glance, with the normal calculated osmolarity being close to 300, I thought maybe the K+ was left out as a negligible factor. But if there is an anion for every cation, then the [K+] term should also be doubled. With a normal [K+] of 4 mEq/L in the blood, matching anions would bring the contribution to 8 mOsm/L. This is more than is contributed by a normal glucose (glucose of 108 mg/dL produces 6 mOsm/L) and by BUN (BUN of 20 mg/dL produces ~7 mOsm/L).

So what gives? Why isn't the equation 2([Na+]+[K+])+glucose/18+BUN/2.8 to account for the 8 mOsm/L produced by potassium salts dissolved in the blood?

First & foremost, the osmole gap is a flawed concept. Kindergarten 101: you can't subtract apples from oranges, but that's exactly what the osmole gap requires.

Osmole Gap = measured osmoLality (osmoLality comes from the Lab) minus calculated osmolaRity (osmolaRity comes from 'Rithmatic)

That being said, it's still taught; and the major reason you double the sodium is because of electroneutrality - it's easier to do this than try to account for all the unmeasured anions in solution (i.e. why an anion gap normally exists). Potassium is ignored b/c its contribution is minimal.

Don't hang your hat on the OG to make/break a diagnosis though. The "mixed AG/OG" patient is already sick, and absence of an OG doesn't preclude a ToxOH ingestion, as a "normal" serum osmole load can vary from patient to patient.

Read this: http://www.ncbi.nlm.nih.gov/m/pubmed/12898493/?i=2&from=mycyk mountain

Cheers!
-d

Sent from my DROID BIONIC using Tapatalk

 

[BADMD]

I've found at least 6 different formulae for calculating the osmolarity. There are at least two published ranges for what the gap should be. the molecular weight of ethanol is 4.6 osm/mmol, but there is some evidence that due to solvation effect you should actually use 3.8. Ketones will give you unaccounted for osms. I'll spare the math, but you can have a level of methanol/ethylene glycol/diethylene glycol that would suggest the need for dialysis while still having an osmolar gap under 10.

The method is neither sensitive not specific. It is a throw back to a prior era where laboratory capabilities were far more limited.

Better rule of thumb: Anion gap you can't account for? Give antizol and send a sample for methanol/Ethylene glycol. Once the antizol is in, you have 12 hours to sort it out.