Anion Gap

Anion gap:
The anion gap is a calculation that is used to account for unmeasured cations like potassium, magnesium, and calcium, as well as anions such as protein, SO₄2-, and H₂PO₄2-. The calculation is as follows:

Na+ - (Cl- + HCO₃-)

Potassium can be put on the Na+ side but some equations leave it out and use a slightly different reference range. The normal range for the calculation above is 7-15 mmol/L. A decreased anion gap can indicate an increase in unmeasured cations like magnesium and calcium. An increased anion gap is a sign of metabolic acidosis.

Study tip
There are a lot of different things that can cause an increased anion gap, to remember them use the phrase MUDPILES.

M: Methanol
U: Uremia
D: Diabetes
P: Propylene glycol
I: Isoniazid
L: Lactate
E: Ethylene glycol
S: Salicylates

Another way to account for unmeasured cations and anions is osmolality.

Osmolality can be measured in the lab or it can be roughly calculated with the following equation:

Osmolality calculation:
2*Na + (Glu/20) + BUN/3

The osmolal gap is measured osmolality minus calculated osmolality; a value >12 is considered significant and is an indicator of unmeasured anions (MUDPILES). Osmolal gap is often used as a rapid test for ethanol, methanol, and isopropanol.

Blood urea nitrogen (BUN):
BUN measures the amount of nitrogen in the blood coming from the waste product urea. Increased BUN is associated with impaired renal function and/or a high protein diet.

Creatinine:
Creatinine is a breakdown product of creatine phosphate in muscle and is generally produced at a constant rate, so an increase is indicative of a glomerular filtration problem (renal impairment). Creatinine is filtered freely through the glomerulus and excreted entirely by the kidney so it is a good marker for renal disease in urine.

BUN-Creatinine ratio:
A typical BUN-Creatinine ratio is 10:1 to 20:1. Both BUN and creatinine exit through the kidneys but for the most part creatinine is not reabsorbed; whereas, BUN can be reabsorbed, so the ratio is mostly controlled by an increase or decrease in BUN. An increased ratio is indicative of a pre-renal acute condition such as congestive heart failure, dehydration, or gastrointestinal bleeding. A decreased ratio is indicative of chronic renal damage because the kidneys are unable to reabsorb BUN.