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Base Excess (Base Deficit)

A calculated number showing extra acid or base in your blood, apart from breathing. A negative value is a base deficit.

AI-verified against cited sources. Awaiting expert reviewAlso called BE, BD, BE(B)LOINC 1925-7

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What it is

Base excess is worked out by the analyzer from pH, pCO2 and hemoglobin rather than measured directly. It estimates how much acid or base would need to be added to bring the blood back to a pH of 7.40, if carbon dioxide were at its usual level. That makes it a way to look at the metabolic side of acid-base balance, the part that doesn't depend on breathing. Zero is the middle. A positive number means extra base. A negative number, called a base deficit and sometimes printed as BD, means extra acid or too little base.

Why it matters

Breathing changes pH by itself. Base excess helps your clinician see whether a change in acidity also comes from your body's chemistry, such as your kidneys or metabolism. In hospital studies of seriously ill people, a low base excess has been linked to worse outcomes, so care teams often follow it over time. For most results, it is one clue to read with pH, pCO2 and bicarbonate. It is not an answer on its own.

If it’s low

A base excess below the usual range, called a base deficit, means the blood has more acid than expected or too little base. Sometimes this is the body adapting. The kidneys let go of bicarbonate when carbon dioxide stays low for a long time, as at high altitude. It can also reflect acid building up or bicarbonate being lost.

Common causes

  • The kidneys adapting to long-term fast breathing, for example at high altitude
  • Large amounts of intravenous fluid, which can dilute the blood's bicarbonate
  • Severe diarrhea, which causes bicarbonate loss
  • Kidney problems
  • Diabetic ketoacidosis, or lactic acid building up when tissues are short of oxygen
  • Severe infection or poisoning

What can help

  • Ask your clinician what is driving the change, since the next steps depend on the cause
  • If you have diabetes or kidney disease, ask which warning signs should make you call right away
  • Ask whether lactate or electrolytes should be checked

If it’s high

A base excess above the usual range means the blood has more base than expected. Sometimes this is the body adapting. The kidneys hold on to bicarbonate when carbon dioxide stays high for a long time, as in some long-term lung conditions. It can also come from losing stomach acid or from certain medicines.

Common causes

  • The kidneys adapting to long-lasting high carbon dioxide, as in COPD
  • Vomiting, which removes stomach acid
  • Water pills (diuretics) and other medicines that change fluid and salt balance
  • Hormone conditions such as too much aldosterone

What can help

  • Ask your clinician whether this is the body adapting to a lung condition or something else
  • Review any water pills or other medicines with your clinician before changing anything
  • Ask whether electrolytes such as potassium and chloride should be checked

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: -2–2 mmol/L. Source: South African Family Practice, Turkish Journal of Anaesthesiology and Reanimation, Northwell Health Laboratories, Oregon Health & Science University (OHSU) Laboratory.

What can affect your result

  • Which version is printed: actual base excess uses your own hemoglobin, and standard base excess uses a fixed value
  • Sample handling, because the calculation depends on pH and pCO2
  • Unusual albumin or phosphate levels, which the calculation assumes are in the usual range
  • Large amounts of intravenous fluid in hospital
  • Long-lasting changes in breathing, which the kidneys offset over days
  • It is calculated from pH, pCO2 and bicarbonate, so air bubbles, delayed analysis or a mislabeled venous sample that shift those values also shift base excess.
  • The calculation assumes albumin and phosphate are in the usual range, so unusual levels can make it misleading.
  • Large amounts of intravenous fluid can dilute the blood and lower it.

Hours to days. Base excess reflects the body's chemistry rather than breathing, so it moves more slowly than pCO2.

When to talk to a clinician

There is no widely published urgent limit for base excess. It is read together with pH, pCO2 and your symptoms. It is usually measured in a clinic or hospital, where your care team sees the result right away. If you are not in a care setting and feel very short of breath, confused, unusually drowsy or faint, or have chest pain, call 911 or your local emergency number.

Questions you could ask

  • Is this the actual or the standard base excess on my report?
  • How does it fit with my pH, pCO2 and bicarbonate?
  • Could my base excess be related to a long-term lung condition or to a medicine I take?
  • Are my electrolytes worth checking?
  • Is my base deficit more likely linked to my kidneys, my metabolism or my hydration?
  • Should lactate or electrolytes be checked?

Questions people ask

What is the difference between base excess and bicarbonate?

Bicarbonate is one of the body's acid buffers, and its calculated value moves when your breathing changes. Base excess is calculated to remove the effect of breathing, so it shows the metabolic part on its own. Clinicians read the two together.

My report shows BD or a minus sign. What does that mean?

BD stands for base deficit. A base deficit is the same as a negative base excess, so a result of BD 5 means about the same as a base excess of -5.

Show the science and sources

Units

  • 1 mmol/L = 1 mmol/L
  • 1 mmol/L = 1 mEq/L

For clinicians and researchers

Base excess is the amount of strong acid or base needed to titrate a blood sample to pH 7.40 at pCO2 40 mmHg and 37 °C. Analyzers calculate it with the Van Slyke equation from pH, HCO3- and hemoglobin. Standard base excess (SBE, BE-ecf) uses a fixed, lower hemoglobin to better reflect the whole extracellular fluid, and is catalogued as its own marker. The usual interval for standard base excess is -2 to +2 mmol/L. The calculation assumes plasma albumin and phosphate are in the normal range; corrected formulas and the Stewart physicochemical approach account for them, and changes in standard base excess track changes in the strong ion difference when albumin and phosphate are stable. Observational studies link low base excess to mortality in critically ill patients with acute kidney injury, myocardial infarction and ischemic stroke, but reviews note that the clinical relevance of the modern base-excess framework is still uncertain. Venous base excess is similar to arterial.

LOINC 1925-7 (primary), 1927-3, 11555-0, 1926-5. Specimen: calculated.

Sources

  1. Mastering blood gas interpretation: A practical guide for primary care providers, South African Family Practice (2025)
  2. Base Excess and Beyond: Evolving Concepts in Acid-base Analysis, Turkish Journal of Anaesthesiology and Reanimation (2026)
  3. Blood Gas Arterial, Northwell Health Laboratories
  4. Blood Gases, Comprehensive, Oregon Health & Science University (OHSU) Laboratory

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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