Base Excess Calculator (Standard Base Excess)
Base Excess Calculator (Standard Base Excess)
Standard base excess from pH and bicarbonate by the Siggaard-Andersen form of the Van Slyke equation — the measure of how much metabolic acid or base is present independent of the respiratory component, which is exactly what bicarbonate alone cannot tell you.
Standard base excess
pH + HCO₃⁻ → SBEpH 7.28, bicarbonate 14 mmol/L
The Siggaard-Andersen form of the Van Slyke equation, at a haemoglobin of 5 g/dL
General form: BE = (1 − ctHb/43) × ((HCO₃⁻ − 24.4) + (2.3 × ctHb + 7.7) × (pH − 7.4)), ctHb in mmol/L
- HCO₃⁻ − 24.4
- how far bicarbonate is from its reference value. On its own this is not a metabolic measure, because bicarbonate moves with the pCO₂
- 14.83 × (pH − 7.4)
- the correction that removes the respiratory contribution. It is the buffer-line slope, β, and it is what turns a bicarbonate into a measure of metabolic derangement alone
- 0.9287
- 1 − ctHb/43, the factor that accounts for haemoglobin as a buffer, evaluated at ctHb = 3.1 mmol/L
- why 5 g/dL
- 3.1 mmol/L of haemoglobin is 5 g/dL, which is a normal whole-blood haemoglobin of 15 g/dL distributed through an extracellular volume three times that of blood. STANDARD base excess is base excess computed for that diluted haemoglobin, so it describes the whole extracellular fluid rather than the blood in the syringe — and, usefully, it is then almost independent of the patient’s actual haemoglobin
- 2.3 × ctHb + 7.7
- the general buffer-line slope. At ctHb 3.1 mmol/L it is 14.83; at the whole-blood haemoglobin of 9 mmol/L that Siggaard-Andersen works through, the same equation simplifies to 0.93 × (ΔHCO₃⁻ + 14.6 × ΔpH)
- the expanded form
- BE = 0.9287 × HCO₃⁻ + 13.77 × pH − 124.58 is the same equation multiplied out, with its constants rounded to four figures. It returns a slightly different answer for that reason — at pH 7.28 and HCO₃⁻ 14 it gives −11.33 against −11.31. Use either; do not be surprised when they differ in the first decimal place
- not the same as Zander’s
- the other form in wide use is BE = (1 − 0.0143 × cHb) × ((HCO₃⁻ − 24.26) + (9.5 + 1.63 × cHb) × (pH − 7.4)) − 0.2 × cHb × (1 − sO₂), with cHb in g/dL. It is a different parameterisation with a different buffer slope — 17.65 at 5 g/dL, not 14.83 — and the two should not be blended
- conditions
- base excess is defined as the acid or base needed to return one litre of fully oxygenated blood to pH 7.40 at 37 °C and a pCO₂ of 40 mmHg (5.3 kPa). Deoxygenated blood buffers differently, which is why Zander’s version carries an oxygen saturation term
Worked example
pH 7.28, bicarbonate 14 mmol/L
HCO₃⁻ − 24.4 = 14 − 24.4 = −10.4
pH − 7.4 = 7.28 − 7.4 = −0.12
14.83 × −0.12 = −1.78
−10.4 + (−1.78) = −12.18
0.9287 × −12.18 = −11.3 mmol/L — a severe base deficit
Note what the pH term did. A bicarbonate of 14 is 10.4 below normal, but the base deficit is 11.3, because the pH of 7.28 is higher than a bicarbonate of 14 alone would produce — the patient is hyperventilating, and that respiratory compensation has been removed from the answer
The same bicarbonate of 14 with a pH of 7.20, in a patient not compensating, gives 0.9287 × (−10.4 + 14.83 × −0.20) = −12.4 mmol/L. Same bicarbonate, more metabolic acid — which the bicarbonate could not have told you
By the expanded form: 0.9287 × 14 + 13.77 × 7.28 − 124.58 = 13.00 + 100.25 − 124.58 = −11.33, which differs in the second decimal place because its constants are rounded
What base excess adds to bicarbonate
| Gas | HCO₃⁻ | Standard base excess | What the pair shows |
|---|---|---|---|
| pH 7.28, pCO₂ 30, HCO₃⁻ 14 | 14 — low | −11.3 | A large metabolic acid load with respiratory compensation. Both numbers agree that something metabolic is wrong |
| pH 7.20, pCO₂ 35, HCO₃⁻ 14 | 14 — low | −12.4 | The same bicarbonate, more metabolic acid, because nothing is compensating. Bicarbonate cannot distinguish these two patients; base excess can |
| pH 7.25, pCO₂ 60, HCO₃⁻ 26 | 26 — normal | −0.6 | A pure acute respiratory acidosis. The bicarbonate is normal and the base excess confirms there is no metabolic component at all |
| pH 7.36, pCO₂ 70, HCO₃⁻ 38 | 38 — high | +12.1 | Chronic hypercapnia. The bicarbonate is high because the kidney has compensated, and the positive base excess is that compensation — not a primary alkalosis. Standard base excess does not subtract a chronic respiratory adaptation, which is the one situation where a large positive value is not a primary metabolic disorder |
| pH 7.55, pCO₂ 25, HCO₃⁻ 21 | 21 — low | −1.1 | An acute respiratory alkalosis with a bicarbonate below the reference interval. The base excess is normal: there is no metabolic disorder here, and reading the bicarbonate alone would invent one |
Base excess, base deficit and the words around them
| Term | Means |
|---|---|
| Base excess | The amount of strong acid needed to return the sample to pH 7.40 at 37 °C and a pCO₂ of 40 mmHg. A positive value means metabolic alkali is present |
| Base deficit | The same number with the sign reversed. A base excess of −8 is a base deficit of 8. Trauma literature uses base deficit; blood gas analysers print base excess |
| Actual base excess (ABE) | Computed with the patient’s measured haemoglobin. Describes the blood in the syringe |
| Standard base excess (SBE) | Computed at a haemoglobin of 5 g/dL, which models whole extracellular fluid. Almost independent of the patient’s own haemoglobin, and the one this page calculates |
| Buffer base | The total concentration of buffer anions. Base excess is the deviation of buffer base from its normal value |
| Strong ion difference | The Stewart-approach description of the same non-respiratory disturbance, from a different starting point |
The question bicarbonate cannot answer
Bicarbonate is not a measure of metabolic acid-base status, and that is the whole reason base excess exists. Bicarbonate sits in the middle of the carbon dioxide buffer system, so it moves whenever the pCO₂ moves. Hyperventilate a completely healthy person and their bicarbonate falls; let a patient retain CO₂ for a week and their bicarbonate rises by ten millimoles. In both cases the bicarbonate has left its reference interval and in neither case is there any metabolic disorder at all. Reading a low bicarbonate as a metabolic acidosis is one of the commonest errors in blood gas interpretation, and it is built into the measurement rather than into the reader.
Base excess removes the respiratory contribution arithmetically. The term 14.83 × (pH − 7.4) is a buffer line: it says how much of the bicarbonate’s displacement is explained by where the pH is, and subtracts it. What is left is the quantity of strong acid or base that would have to be added to return the sample to pH 7.40 at a pCO₂ of 40 mmHg — a number that does not change when the patient breathes differently. That is why a base excess of −11 and a base excess of −12 in two patients with identical bicarbonates of 14 is useful information, and why a base excess of −1.9 in a patient with a bicarbonate of 19 correctly reports no metabolic disorder.
The word standard carries specific meaning. Actual base excess is computed with the patient’s own haemoglobin and describes the blood in the syringe. Standard base excess is computed at a haemoglobin of 5 g/dL, and that figure is not arbitrary: it is a normal whole-blood haemoglobin of about 15 g/dL spread through an extracellular volume roughly three times that of blood. Because haemoglobin is a buffer, modelling the whole extracellular compartment rather than blood alone makes the answer nearly independent of the patient’s haemoglobin — so an anaemic and a polycythaemic patient with the same metabolic disturbance get the same number. Modern analysers report standard base excess for exactly that reason.
Which raises the obvious question of what this page is for, since the analyser already prints it. Two things. First, it explains a number most people read without knowing what was done to it, including why it is not simply the bicarbonate minus 24 and why two forms of the equation in circulation give answers that differ in the first decimal place. Second, it cross-checks: if the value here disagrees with the printed one, the pH and bicarbonate entered did not come from the same sample. What it cannot do is name a cause. A base deficit of 11 is equally consistent with lactic acidosis, ketoacidosis, uraemia, a toxic alcohol and a litre too much of 0.9% sodium chloride. Only the anion gap, the lactate and the patient separate them, and this number supports that reasoning rather than replacing it.
Frequently asked questions
What is the difference between base excess and bicarbonate?
Bicarbonate moves with the pCO₂, so it changes when a patient simply breathes differently — hyperventilation lowers it and chronic CO₂ retention raises it, with no metabolic disorder present. Base excess subtracts that respiratory contribution arithmetically and reports only the metabolic component. A patient with a respiratory alkalosis can have a bicarbonate of 19 and a base excess of −1.9, which is normal.
What is a normal base excess?
−2 to +2 mmol/L. A more negative value is a base deficit and indicates metabolic acid; a more positive value indicates metabolic alkali. A normal base excess does not exclude a mixed metabolic disorder, because a metabolic acidosis and a metabolic alkalosis of similar size cancel.
What is the difference between standard and actual base excess?
Actual base excess uses the patient’s measured haemoglobin and describes the blood sample. Standard base excess uses a haemoglobin of 5 g/dL — a normal whole-blood haemoglobin diluted into the extracellular fluid — and therefore describes the whole extracellular compartment and is nearly independent of the patient’s own haemoglobin. Modern analysers report standard base excess, and that is what this page calculates.
Why does my analyser’s base excess differ slightly from this one?
Usually because of the parameterisation. The Siggaard-Andersen form used here and Zander’s recommended form have different buffer-line slopes, and the fully expanded constant version of the same equation rounds its coefficients and shifts the answer in the first decimal place. Larger disagreements mean the pH and bicarbonate entered here did not come from the same gas, or the analyser used a measured haemoglobin rather than the standard 5 g/dL.
Does a large base deficit tell me what the acid is?
No. It quantifies the metabolic acid load and says nothing about its identity. Lactate, ketones, uraemic anions, toxic alcohols and chloride from resuscitation fluid all produce the same base deficit. The anion gap separates the organic causes from the hyperchloraemic ones, and the lactate and ketones name them.
Related calculators
References
- Siggaard-Andersen O. The van Slyke equation. Scand J Clin Lab Invest Suppl. 1977;146:15–20.
- Siggaard-Andersen O, Fogh-Andersen N. Base excess or buffer base (strong ion difference) as measure of a non-respiratory acid-base disturbance. Acta Anaesthesiol Scand Suppl. 1995;107:123–8.
- Lang W, Zander R. The accuracy of calculated base excess in blood. Clin Chem Lab Med. 2002;40(4):404–10.
- Berend K. Diagnostic use of base excess in acid-base disorders. N Engl J Med. 2018;378(15):1419–28.
- Morgan TJ. The Stewart approach — one clinician’s perspective. Clin Biochem Rev. 2009;30(2):41–54.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.