# What does my ABG show?

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.

- Page: https://www.acalculator.org/health/abg-calculator
- JSON spec: https://www.acalculator.org/health/abg-calculator.json
- Version: 7ebee2aee585

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| ph | pH | Arterial blood pH. |
| units | PaCO2 unit | PaCO2 in mmHg (US) or kPa (SI). |
| paco2 | PaCO2 (mmHg) | Partial pressure of carbon dioxide in arterial blood, in mmHg. |
| paco2Si | PaCO2 (kPa) | Partial pressure of carbon dioxide in arterial blood, in kPa. |
| hco3 | HCO3 (mmol/L) | Bicarbonate, in mmol/L (the same number as mEq/L). |
| na | Sodium (mmol/L) | Serum sodium, for the anion gap. Optional. |
| cl | Chloride (mmol/L) | Serum chloride, for the anion gap. Optional. |
| alb | Albumin (g/dL) | Serum albumin in g/dL, to correct the anion gap. Optional. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| primary | Primary disorder | The acid-base pattern of the pH, PaCO2, and HCO3. |
| compensation | Compensation | The expected compensation for a single primary disorder, and whether the measured value fits it. |
| ag | Anion gap (mmol/L) | Sodium − (chloride + HCO3). |
| agCorrected | Albumin-corrected anion gap (mmol/L) | Anion gap + 2.5 × (4.0 − albumin in g/dL). |
| deltaRatio | Delta ratio | (anion gap − 12) ÷ (24 − HCO3), for a metabolic acidosis with a high anion gap. |
| paco2Mmhg | PaCO2 (mmHg) | The PaCO2 used, in mmHg. |

## 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).

## Assumptions

- 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

1. ph = 7.25, units = us, paco2 = 28, hco3 = 12, na = 140, cl = 104 gives primary = Metabolic acidosis, compensation = Expected PaCO2 24.0 to 28.0 mmHg (Winter's formula): appropriate respiratory compensation., ag = 24, deltaRatio = 1. Source: Winter's formula (Albert 1967): 1.5 × 12 + 8 = 26 ± 2 mmHg, so 28 fits.
2. ph = 7.25, units = us, paco2 = 60, hco3 = 26 gives primary = 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.
3. ph = 7.5, units = si, paco2Si = 6.4, hco3 = 36 gives primary = Metabolic alkalosis, compensation = Expected PaCO2 46.4 to 50.4 mmHg: appropriate respiratory compensation., paco2Mmhg = 48.003936. Source: Berend et al. 2014: 40 + 0.7 × (36 − 24) = 48.4 ± 2 mmHg.
4. ph = 7.15, units = us, paco2 = 50, hco3 = 17 gives primary = Mixed metabolic and respiratory acidosis.
5. ph = 7.3, units = us, paco2 = 30, hco3 = 14, na = 138, cl = 110, alb = 2 gives primary = Metabolic acidosis, ag = 14, agCorrected = 19, deltaRatio = 0.7. Source: Figge 1998: 14 + 2.5 × (4 − 2) = 19.

## FAQ

### 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.

## Sources

- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445. https://doi.org/10.1056/NEJMra1003327
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322. https://doi.org/10.7326/0003-4819-66-2-312
- Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore). 1980;59(3):161-182. https://doi.org/10.1097/00005792-198005000-00001
- Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-1810. https://doi.org/10.1097/00003246-199811000-00019
- Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2(1):162-174. https://doi.org/10.2215/CJN.03020906
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B (1 mmHg = 133.3224 Pa). https://www.nist.gov/pml/special-publication-811