View source for Base excess

{{Short description|Excess or deficit in amount of base present in blood}}
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In [[physiology]], '''base excess''' and '''base deficit''' refer to an excess or deficit, respectively, in the amount of [[base (chemistry)|base]] present in the blood. The value is usually reported as a concentration in units of mEq/L (mmol/L), with positive numbers indicating an excess of base and negative a deficit. A typical [[Reference_ranges_for_blood_tests#Other electrolytes and metabolites|reference range]] for base excess is −2 to +2 mEq/L.<ref>{{citation|title=A Manual of Laboratory and Diagnostic Tests|author=Frances Talaska Fischbach|author2=Marshall Barnett Dunning|edition=8th|year=2008|page=973|isbn=978-0-7817-7194-8|postscript=.}}</ref>

Comparison of the base excess with the reference range assists in determining whether an [[acid–base homeostasis|acid/base disturbance]] is caused by a respiratory, metabolic, or mixed metabolic/respiratory problem. While [[carbon dioxide]] defines the respiratory component of acid–base balance, base excess defines the metabolic component. Accordingly, measurement of base excess is defined, under a standardized pressure of carbon dioxide, by [[acid–base titration|titrating]] back to a standardized blood [[pH]] of 7.40.

The predominant base contributing to base excess is [[bicarbonate]]. Thus, a deviation of serum bicarbonate from the reference range is ordinarily mirrored by a deviation in base excess. However, base excess is a more comprehensive measurement, encompassing all metabolic contributions.

==Definition==
<div style="float:right;border:solid 10px white;">{{Test sample values}}</div>
Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO<sub>2</sub> of {{convert|40|mmHg|abbr=on}}.<ref name="kibble">{{citation|author=Jonathan D. Kibble|author2=Colby R. Halsey|title=Medical Physiology: The Big Picture|year=2009|page=249|isbn=978-0-07-164302-3|postscript=.}}</ref> A base deficit (i.e., a negative base excess) can be correspondingly defined by the amount of strong base that must be added.

A further distinction can be made between actual and standard base excess: '''actual''' base excess is that present in the blood, while '''standard''' base excess is the value when the [[hemoglobin]] is at 5 g/dl. The latter gives a better view of the base excess of the entire [[extracellular fluid]].<ref>[http://www.acid-base.com/terminology.php Acid-Base Tutorial — Terminology]</ref>

Base excess (or deficit) is one of several values typically reported with arterial blood gas analysis that is derived from other measured data.<ref name="kibble" />

The term and concept of base excess were first introduced by [[Poul Bjørndahl Astrup|Poul Astrup]] and [[Ole Siggaard-Andersen]] in 1958.

==Estimation==
Base excess can be estimated from the [[bicarbonate]] concentration ([HCO<sub>3</sub><sup>−</sup>]) and [[pH]] by the equation:<ref name="cornell">[http://www-users.med.cornell.edu/~spon/picu/calc/basecalc.htm Medical Calculators > Calculated Bicarbonate & Base Excess] Steven Pon, MD, Weill Medical College of Cornell University</ref>

<math> Base~excess = 0.93 \times \left ( \left [ HCO_3^- \right ] - 24.4 + 14.8 \times \left ( pH - 7.4 \right ) \right )</math>

with units of mEq/L. The same can be alternatively expressed as

<math> Base~excess = 0.93 \times [HCO_3^-] + 13.77 \times pH - 124.58 </math>
----
Calculations are based on the [[Henderson-Hasselbalch]] equation:
:<math> pH = pK + log \frac{[HCO_3^-]}{[CO_2]}</math>

Ultimately the end result is:
:<math>BE = 0.02786 \times PaCO_2 \times 10^{(pH - 6.1)} + 13.77 \times pH - 124.58</math>

==Interpretation==
Base excess beyond the reference range indicates
* [[metabolic alkalosis]], or [[respiratory acidosis]] with renal compensation if too high (more than +2 mEq/L)
* [[metabolic acidosis]], or [[respiratory alkalosis]] with renal compensation if too low (less than −2 mEq/L)

Blood pH is determined by both a metabolic component, measured by base excess, and a respiratory component, measured by PaCO<sub>2</sub> (partial pressure of [[carbon dioxide]]). Often a disturbance in one triggers a partial compensation in the other. A secondary (compensatory) process can be readily identified because it ''opposes'' the observed deviation in blood pH.

For example, inadequate ventilation, a respiratory problem, causes a buildup of CO<sub>2</sub>, hence respiratory acidosis; the kidneys then attempt to compensate for the low pH by raising blood bicarbonate. The kidneys only partially compensate, so the patient may still have a low blood pH, i.e. acidemia. In summary, the kidneys partially compensate for respiratory acidosis by raising blood bicarbonate.

A high base excess, thus [[metabolic alkalosis]], usually involves an excess of [[bicarbonate]]. It can be caused by
* Compensation for primary [[respiratory acidosis]]
* Excessive loss of HCl in gastric acid by vomiting
* Renal overproduction of bicarbonate, in either [[contraction alkalosis]] or [[Cushing's disease]]

A base deficit (a below-normal base excess), thus [[metabolic acidosis]], usually involves either excretion of bicarbonate or neutralization of bicarbonate by excess organic acids. Common causes include
* Compensation for primary [[respiratory alkalosis]]
* [[Diabetic ketoacidosis]], in which high levels of acidic [[ketone bodies]] are produced
* [[Lactic acidosis]], due to [[anaerobic metabolism]] during heavy exercise or [[Hypoxia (medical)|hypoxia]]
* [[Chronic kidney failure]], preventing excretion of acid and resorption and production of bicarbonate
* [[Diarrhea]], in which large amounts of bicarbonate are excreted
* Ingestion of poisons such as [[methanol]], [[ethylene glycol]], or excessive [[aspirin]]

The serum [[anion gap]] is useful for determining whether a base deficit is caused by addition of acid or loss of bicarbonate. 
* Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis).
* Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea). The anion gap is maintained because bicarbonate is exchanged for [[chloride]] during excretion.

==See==
* [[Acid–base homeostasis]]
* [[Metabolic acidosis]] / [[Metabolic alkalosis]]
* [[Arterial blood gas]]

==References==
<references/>

==External links==
* [http://www.acid-base.com/ acid-base.com]
* [http://www.siggaard-andersen.dk/OsaAnthologyOnBE.htm Anthology on Base Excess (O.Siggaard-Andersen)]
* [http://emedicine.medscape.com/article/768159-overview Emedicine: Lactic Acidosis]

{{Renal physiology}}
{{Blood tests}}

[[Category:Chemical pathology]]
[[Category:Diagnostic intensive care medicine]]