Anion gap

from Wikipedia, the free encyclopedia

The anion gap is a calculated metabolic parameter that is useful for medical professionals in the differential diagnosis of metabolic acidosis .

calculation

The anion gap is calculated by subtracting the blood serum concentrations of chloride plus bicarbonate ( anions ) from the concentrations of sodium plus potassium ( cations ):

= ([Na + ] + [K + ]) - ([Cl - ] + [HCO 3 - ])

In everyday practice, the potassium concentration is often neglected, which results in the following calculation formula:

= [Na + ] - ([Cl - ] + [HCO 3 - ])

application

The anion gap reflects the undetected anions in the plasma. These anions are affected differently depending on the type of metabolic acidosis. The primary benefit of calculating the anion gap is to narrow down the possible causes of metabolic acidosis in a patient. For example, in a patient who has a normal anion gap, causes can be excluded that would cause an enlarged anion gap.

Normal values

Modern analytical devices use ion-selective electrodes . With this measurement method, the normal range is between 3 and 11 mmol / l. Accordingly, there is an enlarged anion gap if the calculated value is above 11 mmol / l; a reduced anion gap is present at a calculated value of less than 3 mmol / l.

Interpretation and causes

The anion gap can either be enlarged, normal or rarely reduced.

On the one hand, an enlarged anion gap can indicate that there is a loss of HCO 3 - which is not compensated for by Cl - as is normally the case , but by other, unmeasured anions such as ketone bodies , lactate , phosphate and sulfate . On the other hand, it can also arise from the pathological increase in other anions (ketone bodies, lactate, toxins, see below).

In patients with metabolic acidosis and a normal anion gap, the HCO 3 - loss is compensated for by an increase in the Cl - concentration, which is why this form of metabolic acidosis is also known as hyperchloremic acidosis .

Enlarged anion gap ( addition acidosis )

The bicarbonate consumption is balanced by unmeasured anions; This creates an enlarged anion gap (the metabolic acidosis caused by increased supply of acids or reduced excretion via the kidneys and caused by the accumulation of hydrogen ions is called addition acidosis ).

Normal anion gap ( hyperchloremic acidosis , subtraction acidosis )

Usually the loss of HCO 3 - is made up by chloride. Then there is a normal anion gap (with increased chloride concentration).

Reduced anion gap

A reduced anion gap is rare. However, it can arise from the presence of abnormally positively charged proteins, such as in multiple myeloma or decreased serum albumin levels .

Anion gap in the urine

The urinary anion gap (UAG) is used to differentiate hyperchloremic metabolic acidosis. In chronic metabolic acidosis, the kidneys are able to acidify the urine through ammonium secretion. However, in the calculation are the measured urinary Na + -, K + - and Cl - - concentrations included.

= ([Na + ] + [K + ]) - ([Cl - ])

The normal range here is 20 to 60 mmol / l. If the kidney tries to compensate for an extrarenal acidosis (e.g. diarrhea with loss of HCO 3 - ), the anion gap in the urine becomes negative because the Na + and K + concentrations in favor of the NH, which is not included in the calculation 4 + remove cations. In the case of renal tubular acidosis , however, the gap remains positive.

literature

  • Thomas Fehr, University Hospital, 2009, Acid Base Disorders - A Practical Approach to Differential Diagnosis.

Individual evidence

  1. ↑ U. Gessler, D. Seybold: Disorders of the water, electrolyte and acid-base balance. In: Rudolf Gross , Paul Schölmerich , Wolfgang Gerok (Eds.): 1000 memoranda of internal medicine. Schattauer, Stuttgart / New York 1971; 4th, completely revised edition, ibid. 1989 (= UTB für Wissenschaft / Uni-Taschenbücher. Volume 522), ISBN 3-7945-1282-0 , pp. 199–202, here: p. 759.