What does my ABG show?
Enter the pH, PaCO2, and bicarbonate from an arterial blood gas, and optionally sodium, chloride, and albumin, to see the acid-base pattern, the expected compensation, and the anion gap. For information only. This is not medical advice or a diagnosis. Do not use it to make treatment decisions without a clinician. No licensed clinician has reviewed this page.
- Primary disorder
- Metabolic acidosis
The blood gas shows: Metabolic acidosis.
- Compensation
- Expected PaCO2 24.0 to 28.0 mmHg (Winter's formula): appropriate respiratory compensation.
- Anion gap (mmol/L)
- 24
- Delta ratio
- 1
- PaCO2 (mmHg)
- 28
Primary disorder: Metabolic acidosis. The blood gas shows: Metabolic acidosis.
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
Reads an arterial blood gas (ABG): the primary acid-base disorder from pH, PaCO2, and bicarbonate, the expected compensation by Winter's formula and the Boston rules, and the anion gap and delta ratio.
Example with the default inputs (pH 7.25, PaCO2 unit mmHg, PaCO2 (mmHg) 28, HCO3 (mmol/L) 12, Sodium (mmol/L) 140, Chloride (mmol/L) 104): The blood gas shows: Metabolic acidosis.
Method: Primary disorder from pH (7.35 to 7.45), PaCO2 (35 to 45 mmHg), and HCO3 (22 to 26 mmol/L). Winter's formula: PaCO2 = 1.5 × HCO3 + 8 ± 2; metabolic alkalosis: PaCO2 = 40 + 0.7 × (HCO3 − 24) ± 2; respiratory acidosis: HCO3 = 24 + 0.1 (acute) or 0.4 (chronic) × (PaCO2 − 40); respiratory alkalosis: HCO3 = 24 − 0.2 (acute) or 0.4 (chronic) × (40 − PaCO2). Anion gap = Na − (Cl + HCO3), corrected + 2.5 × (4 − albumin).
- An arterial sample. Normal ranges: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mmol/L.
- For a respiratory disorder, an HCO3 more than 2 mmol/L outside the acute-to-chronic range suggests an added metabolic disorder.
- The delta ratio uses a normal anion gap of 12 and a normal HCO3 of 24, and is shown only for a metabolic acidosis with an anion gap over 12.
- This is a teaching aid: it does not replace a clinician reading the whole picture.
Worked examples
Each example is checked against the calculator on every build.
- pH 7.25, PaCO2 unit mmHg, PaCO2 (mmHg) 28, HCO3 (mmol/L) 12, Sodium (mmol/L) 140, Chloride (mmol/L) 104 gives Primary disorder Metabolic acidosis, Compensation Expected PaCO2 24.0 to 28.0 mmHg (Winter's formula): appropriate respiratory compensation., Anion gap (mmol/L) 24, Delta ratio 1.Source: Winter's formula (Albert 1967): 1.5 × 12 + 8 = 26 ± 2 mmHg, so 28 fits
- pH 7.25, PaCO2 unit mmHg, PaCO2 (mmHg) 60, HCO3 (mmol/L) 26 gives Primary disorder Respiratory acidosis, Compensation Expected HCO3 26.0 (acute) to 32.0 (chronic) mmol/L: HCO3 fits an acute, chronic, or in-between respiratory acidosis..Source: Berend et al., N Engl J Med 2014, Table 1: acute 1 mmol/L per 10 mmHg, 24 + 0.1 × 20 = 26
- pH 7.5, PaCO2 unit kPa, PaCO2 (kPa) 6.4, HCO3 (mmol/L) 36 gives Primary disorder Metabolic alkalosis, Compensation Expected PaCO2 46.4 to 50.4 mmHg: appropriate respiratory compensation., PaCO2 (mmHg) 48.003936.Source: Berend et al. 2014: 40 + 0.7 × (36 − 24) = 48.4 ± 2 mmHg
- pH 7.15, PaCO2 unit mmHg, PaCO2 (mmHg) 50, HCO3 (mmol/L) 17 gives Primary disorder Mixed metabolic and respiratory acidosis.
- pH 7.3, PaCO2 unit mmHg, PaCO2 (mmHg) 30, HCO3 (mmol/L) 14, Sodium (mmol/L) 138, Chloride (mmol/L) 110, Albumin (g/dL) 2 gives Primary disorder Metabolic acidosis, Anion gap (mmol/L) 14, Albumin-corrected anion gap (mmol/L) 19, Delta ratio 0.7.Source: Figge 1998: 14 + 2.5 × (4 − 2) = 19
Medical disclaimer
This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.
When this does not apply. The rules are for arterial blood from adults (Berend 2014). A venous gas has different normal values. The compensation rules are approximations, and a mixed disorder can hide behind values that look normal.
How the blood gas is read
PaCO2 is in mmHg; a value in kPa is divided by 0.1333224 (1 mmHg = 133.3224 Pa). HCO3, sodium, and chloride are in mmol/L (the same number as mEq/L). Albumin is in g/dL.
Step 1 and 2: the primary disorder
Normal ranges: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mmol/L. The checks run in this order, and the first match wins:
| pH | Condition | Primary disorder |
|---|---|---|
| under 7.35 | HCO3 under 22 and PaCO2 over 45 | Mixed metabolic and respiratory acidosis |
| over 7.45 | HCO3 over 26 and PaCO2 under 35 | Mixed metabolic and respiratory alkalosis |
| under 7.35 | HCO3 under 22 | Metabolic acidosis |
| over 7.45 | HCO3 over 26 | Metabolic alkalosis |
| under 7.35 | PaCO2 over 45 | Respiratory acidosis |
| over 7.45 | PaCO2 under 35 | Respiratory alkalosis |
| under 7.35 or over 7.45 | none of the above | No simple disorder fits: check the values |
| 7.35 to 7.45 | PaCO2 over 45 and HCO3 over 26, or PaCO2 under 35 and HCO3 under 22 | Normal pH with a possible mixed disorder |
| 7.35 to 7.45 | PaCO2 or HCO3 outside its range | Normal pH with PaCO2 or HCO3 outside the usual range |
| 7.35 to 7.45 | all in range | No acid-base disorder |
Step 3: compensation (single primary disorders only)
| Primary disorder | Expected value (Berend 2014; Albert 1967) | Reading |
|---|---|---|
| Metabolic acidosis | PaCO2 = 1.5 × HCO3 + 8, ± 2 mmHg | PaCO2 above the range: added respiratory acidosis; below: added respiratory alkalosis |
| Metabolic alkalosis | PaCO2 = 40 + 0.7 × (HCO3 − 24), ± 2 mmHg | same as above |
| Respiratory acidosis | HCO3 = 24 + 0.1 × (PaCO2 − 40) acute, 24 + 0.4 × (PaCO2 − 40) chronic | HCO3 more than 2 below the acute value: added metabolic acidosis; more than 2 above the chronic value: added metabolic alkalosis |
| Respiratory alkalosis | HCO3 = 24 − 0.2 × (40 − PaCO2) acute, 24 − 0.4 × (40 − PaCO2) chronic | HCO3 more than 2 above the acute value: added metabolic alkalosis; more than 2 below the chronic value: added metabolic acidosis |
A value exactly on a range edge counts as within the range, judged on the decimal values: for example PaCO2 36.9 with HCO3 20.6 sits on the lower edge (1.5 × 20.6 + 8 − 2 = 36.9), so the compensation is appropriate. The page shows the range to one decimal, halves rounded up from the decimal value (HCO3 12.1 gives 24.15 to 28.15, shown as 24.2 to 28.2). Berend 2014 (Table 1) gives the chronic respiratory change as 4 to 5 mmol/L per 10 mmHg; the page uses 4 (0.4), and the ± 2 margin then reaches 5 per 10 mmHg at a PaCO2 20 mmHg from 40. The ± 2 margin for the respiratory rules is this page's convention, to match the one for the metabolic rules. Berend 2014 uses tighter reference values (pH 7.38 to 7.42, PaCO2 38 to 42 mmHg); this page uses the wider ranges above, which most teaching texts use.
Step 4: anion gap and delta ratio
When both sodium and chloride are entered:
- Anion gap = Na − (Cl + HCO3)
- With albumin: corrected anion gap = anion gap + 2.5 × (4.0 − albumin) (Figge 1998)
- For a metabolic acidosis (alone or mixed) with the gap (corrected, if albumin is entered) over 12 and HCO3 under 24: delta ratio = (gap − 12) ÷ (24 − HCO3)
Normal anion gaps differ between labs (Kraut and Madias 2007); 12 is the value most teaching texts use.
Inputs the calculator accepts
pH 6.5 to 8.0, PaCO2 5 to 200 mmHg (0.67 to 26.7 kPa), HCO3 2 to 70, sodium 100 to 180, chloride 60 to 150 mmol/L, albumin 0.5 to 7 g/dL.
Worked examples by hand
The default. pH 7.25, PaCO2 28, HCO3 12, Na 140, Cl 104. pH under 7.35 and HCO3 under 22: metabolic acidosis. Winter's formula: 1.5 × 12 + 8 = 26, so 24.0 to 28.0 mmHg; 28 is within: appropriate compensation. Anion gap = 140 − (104 + 12) = 24. Delta ratio = (24 − 12) ÷ (24 − 12) = 1.00.
Respiratory acidosis. pH 7.25, PaCO2 60, HCO3 26. PaCO2 over 45: respiratory acidosis. Acute: 24 + 0.1 × 20 = 26.0; chronic: 24 + 0.4 × 20 = 32.0. HCO3 26 is not below 24.0 or above 34.0, so it fits.
Metabolic alkalosis in kPa. pH 7.50, PaCO2 6.4 kPa = 6.4 ÷ 0.1333224 = 48.0 mmHg, HCO3 36: metabolic alkalosis. Expected PaCO2 = 40 + 0.7 × 12 = 48.4, so 46.4 to 50.4 mmHg; 48.0 is within.
Mixed acidosis. pH 7.15, PaCO2 50, HCO3 17: mixed metabolic and respiratory acidosis.
Low albumin. pH 7.30, PaCO2 30, HCO3 14, Na 138, Cl 110, albumin 2.0: metabolic acidosis. Anion gap = 138 − 124 = 14; corrected = 14 + 2.5 × 2 = 19; delta ratio = (19 − 12) ÷ (24 − 14) = 0.70. Winter's formula: 1.5 × 14 + 8 = 29, so 27.0 to 31.0; 30 is within.
Other questions people ask
How do you read an ABG?
Step 1: is the pH low (acidemia, under 7.35) or high (alkalemia, over 7.45)? Step 2: which value explains it, the PaCO2 (respiratory) or the bicarbonate (metabolic)? Step 3: is the other value compensating as much as expected? Step 4: in a metabolic acidosis, work out the anion gap. This page follows those steps.
What is Winter's formula?
Winter's formula predicts the PaCO2 the lungs should reach to compensate for a metabolic acidosis: expected PaCO2 = 1.5 × HCO3 + 8, plus or minus 2 mmHg (Albert, Dell, and Winters, 1967). A PaCO2 above that range means an added respiratory acidosis; below it, an added respiratory alkalosis.
What is the anion gap?
The anion gap is sodium minus (chloride plus bicarbonate). It estimates acids in the blood that the panel does not measure. A high gap in a metabolic acidosis points to causes such as ketoacidosis, lactic acidosis, kidney failure, or some poisonings. Low albumin lowers the gap, so the page can correct it.
What is the delta ratio?
In a high anion gap metabolic acidosis, the delta ratio compares the rise in the anion gap above 12 with the fall in bicarbonate below 24: (anion gap − 12) ÷ (24 − HCO3). Berend (2014) notes that in ketoacidosis the two change about 1 to 1, and in lactic acidosis the bicarbonate falls about 0.6 times the rise in the gap. A fall in bicarbonate much larger than the rise in the gap suggests an added normal-gap acidosis; a much smaller fall suggests an added metabolic alkalosis.
Why are acute and chronic respiratory values different?
In the first hours, blood buffers change bicarbonate only a little. Over 3 to 5 days, the kidneys change it much more. So the expected bicarbonate for the same PaCO2 is different in an acute and a chronic respiratory disorder.