# Base excess

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{{Short description|Excess or deficit in amount of base present in blood}}
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In [physiology](/source/physiology), '''base excess''' and '''base deficit''' refer to an excess or deficit, respectively, in the amount of [base](/source/base_(chemistry)) 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 range](/source/Reference_ranges_for_blood_tests) 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 disturbance](/source/acid%E2%80%93base_homeostasis) is caused by a respiratory, metabolic, or mixed metabolic/respiratory problem. While [carbon dioxide](/source/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 [titrating](/source/acid%E2%80%93base_titration) back to a standardized blood [pH](/source/pH) of 7.40.

The predominant base contributing to base excess is [bicarbonate](/source/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](/source/hemoglobin) is at 5 g/dl. The latter gives a better view of the base excess of the entire [extracellular fluid](/source/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 Astrup](/source/Poul_Bj%C3%B8rndahl_Astrup) and [Ole Siggaard-Andersen](/source/Ole_Siggaard-Andersen) in 1958.

==Estimation==
Base excess can be estimated from the [bicarbonate](/source/bicarbonate) concentration ([HCO<sub>3</sub><sup>−</sup>]) and [pH](/source/pH) by the equation:<ref name="cornell">[https://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](/source/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](/source/metabolic_alkalosis), or [respiratory acidosis](/source/respiratory_acidosis) with renal compensation if too high (more than +2 mEq/L)
* [metabolic acidosis](/source/metabolic_acidosis), or [respiratory alkalosis](/source/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](/source/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](/source/metabolic_alkalosis), usually involves an excess of [bicarbonate](/source/bicarbonate). It can be caused by
* Compensation for primary [respiratory acidosis](/source/respiratory_acidosis)
* Excessive loss of HCl in gastric acid by vomiting
* Renal overproduction of bicarbonate, in either [contraction alkalosis](/source/contraction_alkalosis) or [Cushing's disease](/source/Cushing's_disease)

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

The serum [anion gap](/source/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](/source/chloride) during excretion.

==See==
* [Acid–base homeostasis](/source/Acid%E2%80%93base_homeostasis)
* [Metabolic acidosis](/source/Metabolic_acidosis) / [Metabolic alkalosis](/source/Metabolic_alkalosis)
* [Arterial blood gas](/source/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:Clinical chemistry
Category:Diagnostic intensive care medicine

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Adapted from the Wikipedia article [Base excess](https://en.wikipedia.org/wiki/Base_excess) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Base_excess?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.