Urinary Anion Gap = [Na+]+ [K+] – [Cl–]
- the cations normally present in urine are Na+, K+, NH4+, Ca2+,and Mg2+.
- 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).
- the urinary anion gap can help to differentiate between GIT and renal causes of a hyperchloraemic metabolic acidosis.
- 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
- hyperchloraemic acidosis can be caused by:
- (i) Loss of base via the kidney (eg renal tubular acidosis)
- (ii) Loss of base via the bowel (eg diarrhoea)
- (iii) Gain of mineral acid (eg HCl infusion)
- if the acidosis is due to loss of base via the bowel then the kidneys can respond appropriately by increasing ammonium excretion to cause a net loss of H+ from the body -> the UAG would tend to be decreased -> increased NH4+ (with presumably increased Cl-) => increased urinary cations => decreased UAG.
- if the acidosis is due to loss of base via the kidney, then as the problem is with the kidney it is not able to increase ammonium excretion and the UAG will not be increased.
- 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.
- low urinary AG = GI loss of base
- no change in urinary AG = renal loss of base
- negative urinary AG = severe diarrhea
- positive urinary AG = altered urinary acidification