Lessons · Medicine foundations · the anion gap
The anion gap: what the measured ions do not account for
The anion gap is the sodium minus the sum of the chloride and the bicarbonate, all in millimoles per litre.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A set of electrolytes appears on the screen and somebody asks for the gap before you have finished reading the numbers off it. The arithmetic is an addition and a subtraction, and the only thing that trips people is which two go inside the bracket.
How to think about it
Write the formula first, brackets and all. Add the chloride and the bicarbonate inside the bracket, on their own line. Subtract that single total from the sodium. One number goes in front of the minus sign, and it is always the sodium.
Worked example
Anion gap = Na − (Cl + HCO3)Sodium alone on the left; the other two added together on the right.
Sodium 140, chloride 104, bicarbonate 24 mmol/LAll three in millimoles per litre, all from the question.
104 + 24 = 128 mmol/LThe bracket first.
140 − 128 = 12 mmol/LTwelve millimoles per litre.
Your turn
Sodium is 138, chloride is 100 and bicarbonate is 18 mmol/L. Write the line that gives the anion gap.
138 − (100 + 18) = mmol/L
Solve one, graded on the server
The trap
Subtracting only the chloride and forgetting the bicarbonate, which leaves a gap in the thirties. Both ions go inside the bracket; the bracket is the whole point of writing the formula down first.