The cations normally present in urine are Na+, K+, NH4+, Ca++ and Mg++.
The anions normally present are Cl-, HCO3-, sulphate, phosphate and some organic anions.
Only Na+, K+ and Cl- are commonly measured in urine so the other charged species are the unmeasured anions (UA) and cations (UC).
Because of the requirement for macroscopic electroneutrality, total anion charge always equals total cation charge, so:
Cl- + UA = Na+ + K+ + UC
It has been found experimentally that the Urinary Anion Gap (UAG) provides a rough index of urinary ammonium excretion. Ammonium is positively charged so a rise in its urinary concentration (ie increased unmeasured cations) will cause a fall in UAG as can be appreciated by inspection of the formula above.
How is this useful? Consider the following:
It is easy to calculate the anion gap so this differentiation is easy and indeed clinically useful.
Diagnosis between the above 3 groups of causes is usually clinically obvious, but occasionally it may be useful to have an extra aid to help in deciding between a loss of base via the kidneys or the bowel.
Does this work?
Experimentally, it has been found that patients with diarrhoea severe enough to cause hyperchloraemic acidosis have a negative UAG (average value -27 +/- 10 mmol/l) and patients with acidosis due to altered urinary acidification had a positive UAG. In many cases, the cause (gut or kidney) will be obvious, but occasionally calculation of the urinary anion gap can be useful.
In a patient with a hyperchloraemic metabolic acidosis:
As a memory aid, remember ‘neGUTive’ - negative UAG in bowel causes. For more details of the use of the UAG in differentiating causes of distal urinary acidification, see Batlle et al (1989).
Remember that is most cases the diagnosis may be clinically obvious (eg severe diarrhoea is hard to miss) and consideration of the urinary anion gap is not necessary.