Arterial blood gas
mm Hg
Bicarbonate (HCO³⁻)
Anion gap values
Chloride (Cl⁻)
Sodium (Na⁺)
Anion gap
Corrected anion gap

This acid base calculator interprets a patient's arterial blood gas values and determines their anion gap. All you need is the results of the patient's blood sample.

In the article below we included some information on acid base balance, arterial blood gas interpretation, how to calculate the anion gap and a practical example of how use this calculator.

What is acid base balance?

If the body is functioning correctly, the acid-base balance in your blood is maintained automatically. The ability of the body to do this suggests that your lungs and kidneys are working fine and keeping the body healthy. Yet, even a slight decrease in their function can lead to significant illness, such as acidosis or alkalosis. If it is the lungs that fail, the acidosis/alkalosis is known as respiratory. On the contrary, if the kidneys deteriorate in their function, we name the acidosis/alkalosis metabolic.

By acidosis we mean a state where the pH of your arterial blood is below 7.35, and alkalosis is where the pH is higher than 7.45.

It is possible, however, for the person's arterial pH to stay within the correct range while the person has imbalanced acid-base levels. How? Our bodies are smart and try to compensate every time something begins to fail. Therefore, if the lungs stop working, one should present with metabolic compensation. Also, in the case of kidneys deterioration, there's respiratory compensation.

Arterial blood gas values

We often use arterial blood gas (ABG) in a hospital to estimate a patient's oxygenation. Unlike saturation (with a norm SatO2 > 92%), ABG gives a direct result, providing a quick and accurate clinical evaluation of a patient's status.

A doctor begins by taking a blood sample from an artery, most commonly the radial artery, or sometimes the femoral and brachial arteries. It can also be taken from veins, although that's less reliable and oxygenation data cannot be estimated from it - it solely determines pH and CO2 levels.

An ABG gives information on the level of oxygenation, the patient's ability to remove carbon dioxide, and general acidity. More specifically, it provides medical personnel with the following data:

  • pH;
  • Partial pressure of oxygen (PaO2);
  • Partial pressure of carbon dioxide (PaCO2); and
  • Bicarbonate (HCO3).

Blood gas interpretation

The norms of each component of arterial blood gas values are strictly determined and are as follows:

Normal value
7.35 - 7.45
75 - 100 mmHg
35 - 45 mmHg
Bicarbonate (HCO3)
22 - 26 mEq/L

How to calculate anion gap

The anion gap is the difference between specific positively charged ions (cations) and particular negatively charged ions (anions). We can measure those ions either from the serum, plasma, or urine.

We commonly calculate it without the potassium value, i.e., just with the sodium, chloride, and bicarbonate value. Anion gap, AG, equals the difference between most prevalent cation, sodium, Na+, and the sum of most common anions (chloride Cl- and bicarbonate HCO3-).

AG = Na+ - (Cl- + HCO3-)

The standard value of AG is between 8 - 16 mEq/L.

Acid base calculator - an explanation

This acid base calculator estimates both the anion gap and provides you with an arterial blood gas interpretation. Altogether, they help to determine the status of the patient - their acid-base balance. All you need to know is a few parameters:

  • pH (Norm: 7.35 - 7.45);
  • PCO2 - partial pressure of carbon dioxide (Norm: 35 - 45 mmHg);
  • Bicarbonates (Norm: 22 - 26 mEq/L);
  • Chloride - Cl- level;
  • Sodium - Na+ level; and
  • Albumin level.

The acid-base calculator estimates the level of oxygenation from the first three values. The result is whether the patient is in acidosis, alkalosis, or within the normal range. This tool also suggests if those values are due to metabolic, respiratory, or combined deficiencies.

How to calculate anion gap? Take the ion levels and put them into the equation specified above: AG = Na+ - (Cl- + HCO3-). The normal values should be between 8 and 16 mEq/L.

A practical example

It's time for us to explain how we can use this acid-base calculator in practice. Ian is a 86 year-old man with severe COPD (Chronic Obstructive Pulmonary Disease) and a couple more health conditions. Recently, he showed up at the ER (Emergency room), presenting fatigue and shortness of breath. His saturation was around 85%, so the doctor immediately ordered ABG (arterial blood gas test) and ion test.

Here are the test's results:

  1. pH: 7.1;
  2. pCO2: 56 mmHg; and
  3. Bicarbonates: 24 mEq/L.

Therefore, his pH is low and indicates acidosis. To be more specific, uncompensated respiratory acidosis.

As for the ions in blood:

  1. Chloride: 108 mEq/L;
  2. Sodium: 140 mEq/L; and
  3. Albumin: 3.3 g/dL.

It means that the patient's anion gap equals 8 mEq/L, which is within the normal range. His albumin is a bit low, with correct values being between 3.4 - 5.4 g/L.

Dominika Miszewska, PhD candidate
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