Anion gap

The range depends on whether the lab includes potassium in the sum and on its analyser; some labs print 10 to 20 and others 3 to 11, so use the range printed.

The anion gap is not measured directly. It is a calculation the laboratory makes from sodium, chloride and bicarbonate (and in some labs potassium) that reflects the unmeasured acids and proteins in the blood. Doctors use it as a clue to why the body's acid balance has shifted.

It is mainly used in hospital when bicarbonate is low, to help doctors separate acid build-up from loss of bicarbonate through the gut or kidneys. It is rarely requested on its own in general practice.

• Uncontrolled diabetes with ketone build-up
• Reduced kidney filtering
• Lactic acid from a severe illness or very hard exercise
• Poisoning with methanol, ethylene glycol or aspirin in large amounts
• Dehydration, which concentrates the blood proteins

• Low albumin, which is the commonest reason in the general population
• Some blood-protein conditions
• Very high calcium or magnesium
• Lithium tablets

• It is calculated, so any small error in sodium, chloride or bicarbonate shows up in the gap
• Labs differ in the formula they use, so ranges are not comparable between labs
• A low albumin lowers the gap without any acid problem

Calculated from the same sample as the U&E; nothing extra is needed. It is only meaningful alongside the bicarbonate and albumin from the same sample.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.