{{Short description|Excess or deficit in amount of base present in blood}} {{Infobox diagnostic | Name = Base excess | Image = | Alt = | Caption = | DiseasesDB = 1267 | ICD10 = | ICD9 = | ICDO = | MedlinePlus = | eMedicine = | MeshID = | LOINC = {{LOINC|11555-0}} | HCPCSlevel2 = | Reference_range = }} In physiology, '''base excess''' and '''base deficit''' refer to an excess or deficit, respectively, in the amount of 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 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 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 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 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">[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 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 * 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:Clinical chemistry Category:Diagnostic intensive care medicine