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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?
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?