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USMLE Step 1Renal & acid–baseEP 20 of 21

ABG interpretation in 6 steps: the acid-base algorithm

One fixed sequence makes ABG interpretation systematic. A normal pH does not clear a gas, and a missed compensation window or a high delta ratio exposes the second disorder.

  • 5:32 video
  • 4 min read
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  • Published 31 Oct 2026
Video lesson · 5:32 · Watch on YouTubeSeries: Acid–Base Disorders & ABG Interpretation · @medservatory
Chapters (9)
By the end of this lesson

Apply a fixed stepwise sequence (pH, primary disorder, compensation, anion gap, delta ratio, clinical context) to interpret any arterial blood gas.

The short version

  1. Work in order: pH, primary disorder, compensation, anion gap, delta ratio, clinical context.
  2. A normal pH does not clear a gas, because opposing disorders can cancel out.
  3. A measured value that misses the expected compensation suggests a second disorder.
  4. Calculate the delta ratio when the anion gap is high: 1–2 is a pure high-gap acidosis, below 1 adds a normal-gap acidosis, above 2 adds a metabolic alkalosis.
  5. The story must explain every disorder found; if one has no cause, keep looking.

A normal pH and two abnormal numbers

ABG interpretation is easiest with a fixed order. Here is a blood gas: pH 7.42, PaCO₂ 24 mm Hg, bicarbonate 15 mmol/L. The pH is normal, and yet both other numbers are abnormal.

The algorithm has six steps, always in the same order. This lesson walks through each step, runs the algorithm on a worked case, and then returns to this opening gas.

The mental model: PaCO₂, HCO₃⁻ and the pH ratio

PaCO₂ is the lung side, and bicarbonate is the metabolic side. The pH follows their ratio, so a problem on one side is answered by a response from the other.[1][5]

That is why every later step asks the same thing: has the opposite side responded as much as it should?

Steps 1 and 2: the pH and the primary disorder

Step 1, the pH. Below 7.35 is acidemia, and above 7.45 is alkalemia. A normal pH does not clear the gas, because opposing disorders can cancel out.[1]

Step 2, the primary disorder. It is the change that explains the pH.[1][5]

For example, in acidemia a high PaCO₂ is a respiratory acidosis and a low bicarbonate is a metabolic acidosis. In alkalemia a low PaCO₂ is a respiratory alkalosis and a high bicarbonate is a metabolic alkalosis.[1][5]

Primary disorder by pH and the change that explains it
pHChange that explains itPrimary disorder
AcidemiaHigh PaCO₂Respiratory acidosis
AcidemiaLow HCO₃⁻Metabolic acidosis
AlkalemiaLow PaCO₂Respiratory alkalosis
AlkalemiaHigh HCO₃⁻Metabolic alkalosis

Step 3: is the compensation right?

Each primary disorder has an expected response from the other side. Compare the measured value with the expected one.[1][5]

The first question is always whether the other side has responded as it should. If the measured value is higher or lower than the rule predicts, suspect a second, separate disorder, which makes a mixed acid-base disorder.[1]

Expected compensation by primary disorder
Primary disorderExpected compensation
Metabolic acidosisWinters' formula: PaCO₂ = 1.5 × HCO₃⁻ + 8, ± 2
Metabolic alkalosisPaCO₂ = 40 + 0.7 × (rise in HCO₃⁻ above 24)
Respiratory acidosisHCO₃⁻ rises 1 per 10 mm Hg rise in PaCO₂ when acute, 4 when chronic
Respiratory alkalosisHCO₃⁻ falls 2 per 10 mm Hg fall in PaCO₂ when acute, 4 when chronic

Chronic means days, because the kidney is slow. Sources differ slightly on the constants: for example, one gives 5 rather than 4 per 10 mm Hg in chronic respiratory alkalosis, and the metabolic alkalosis slope is quoted as about 0.6–0.75 mm Hg of PaCO₂ per mmol/L of bicarbonate. Acute changes in respiratory acidosis are quoted as 1–2 in some sources.[1][5]

In metabolic alkalosis, PaCO₂ should not rise above 55 mm Hg as compensation.[1] For metabolic acidosis, the respiratory response reaches its maximum in about 12–24 hours.[5]

Steps 4 and 5: the anion gap and the delta ratio

Step 4: anion gap
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

The common normal value is 12 (K⁺ excluded).[1]

Step 5: delta ratio, when the gap is high
Delta ratio = (anion gap − 12) / (24 − HCO₃⁻)

The rise in the gap divided by the fall in bicarbonate from 24.[4]

The anion gap tells you whether a metabolic acidosis is a high anion gap metabolic acidosis. Only then does the delta ratio compare how far the gap has risen with how far bicarbonate has fallen from 24.[1][4]

Reading the delta ratio
Delta ratioMeaning
1–2Pure high-gap acidosis
Below 1Adds a normal-gap acidosis
Above 2Adds a metabolic alkalosis

A cross-check some sources use: add the rise in the gap (anion gap − 12) to the measured bicarbonate. A result above Merck's normal bicarbonate range of 22–29 suggests a concurrent metabolic alkalosis.[1]

These cut-offs differ between sources, so treat the ratio as a strong clue, not proof.[4]

Step 6 and a worked ABG interpretation

Step 6, the clinical context. Does the story explain every disorder you found? If one has no cause, keep looking. The six-step order is this lesson's own sequence; sources order the steps differently.[1][5]

The context step closes the loop: every disorder the numbers reveal should have a cause in the history, and a disorder with no cause is a prompt to look again. For example, vomiting loses gastric acid and causes metabolic alkalosis.[2]

Now run it on a patient in shock: sodium 140 mmol/L, chloride 100 mmol/L, bicarbonate 16 mmol/L, pH 7.31, PaCO₂ 33 mm Hg.

The algorithm on a patient in shock
  1. Step 1: pH 7.31 is below 7.35, so acidemia
  2. Step 2: bicarbonate is low, so metabolic acidosis
  3. Step 3: Winters gives 1.5 × 16 + 8 = 32, a window of 30–34; 33 fits
  4. Step 4: anion gap = 140 − (100 + 16) = 24, which is high
  5. Step 5: the gap rose by 12 (24 − 12), bicarbonate fell by 8 (24 − 16), a ratio of 1.5: pure high-gap acidosis
  6. Step 6: shock fits lactic acidosis. One disorder, one cause

Lactic acidosis and ketoacidosis are both high-gap acidoses, which is why the story and the numbers agree in this case.[3]

The opening gas again: a hidden respiratory alkalosis

Return to pH 7.42, PaCO₂ 24, bicarbonate 15. With a normal pH, start from the low bicarbonate of 15, which is a metabolic acidosis. Winters predicts a PaCO₂ near 30 (1.5 × 15 + 8 = 30.5, a window of 28.5–32.5), but the measured PaCO₂ is 24.[1][5]

The measured value misses the window, so suspect a second disorder: a hidden respiratory alkalosis, which is why the pH looked normal.

Practice question

Original · USMLE style
Your turnQB-MS-S1-REN-020-01

A 42-year-old man with a history of heavy alcohol use comes to the emergency department after 3 days of abdominal pain and repeated vomiting. He has eaten almost nothing during this time. Temperature is 37.0 C (98.6 F), pulse is 104/min, respirations are 18/min, and blood pressure is 118/74 mm Hg. Abdominal examination shows mild epigastric tenderness. Urine dipstick shows ketones. Laboratory studies show:

Results
Serum Na⁺138 mEq/L
Serum Cl⁻84 mEq/L
Serum HCO₃⁻21 mEq/L
Serum albumin4.0 g/dL
Serum glucose96 mg/dL
Arterial pH (room air)7.34
Arterial PCO₂ (room air)40 mm Hg
Which of the following best describes this patient's acid-base status?
Pick one option. Your answers stay on this device.

Common questions

What are the steps of ABG interpretation?

In order: the pH, the primary disorder, the compensation, the anion gap, the delta ratio (when the gap is high) and the clinical context.

Can an ABG be abnormal when the pH is normal?

Yes. A normal pH does not clear the gas, because opposing disorders can cancel out. Checking compensation is what exposes the second disorder.

How do you check compensation in metabolic acidosis?

Use Winters' formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8, plus or minus 2. If the measured PaCO₂ falls outside that window, suspect a second disorder.

What does the delta ratio tell you?

With a high anion gap, a ratio of 1–2 is a pure high-gap acidosis, below 1 adds a normal-gap acidosis, and above 2 adds a metabolic alkalosis. Cut-offs differ between sources, so treat it as a strong clue.

Transcript

Read the full narration, by chapter643 words

0:00A normal pH and two abnormal numbers

Here is a blood gas. pH 7.42, PaCO2 24, bicarbonate 15. The pH is normal, and yet both other numbers are abnormal. Today: one algorithm, six steps, that reads any arterial blood gas.

0:17The mental model: PCO2, HCO3- and the pH ratio

First, the mental model. PaCO2 is the lung side, and bicarbonate is the metabolic side. The pH follows their ratio, so a problem on one side is answered by a response from the other. Step one: the pH.

0:31Step 1: the pH

Below 7.35 is acidemia, and above 7.45 is alkalemia. A normal pH does not clear the gas, because opposing disorders can cancel out.

0:44Step 2: the primary disorder

Step two: the primary disorder. It is the change that explains the pH. In acidemia, a high PaCO2 is respiratory acidosis, and a low bicarbonate is metabolic acidosis. In alkalemia, a low PaCO2 is respiratory alkalosis, and a high bicarbonate is metabolic alkalosis. Step three: is the compensation right?

1:07Step 3: is the compensation right?

Metabolic acidosis: Winters' formula, 1.5 times bicarbonate plus eight, plus or minus two. Metabolic alkalosis: 40 plus 0.7 times the rise in bicarbonate above 24. Respiratory acidosis: bicarbonate rises one for every 10 that PaCO2 rises when acute, and four when chronic. Respiratory alkalosis: bicarbonate falls two for every 10 that PaCO2 falls when acute, and four when chronic. Chronic means days, because the kidney is slow. If the measured value misses the expected one, there is a second disorder.

1:52Steps 4 and 5: the anion gap and the delta ratio

Step four: the anion gap. Sodium minus the sum of chloride and bicarbonate. The common normal value is 12. Step five: the delta ratio, when the gap is high. The rise in the gap, divided by the fall in bicarbonate from 24. One to two is a pure high gap acidosis. Below one adds a normal gap acidosis. Above two adds a metabolic alkalosis.

2:22Step 6: clinical context, a worked example and the opening gas revisited

Step six: the clinical context. Does the story explain every disorder you found? If one has no cause, keep looking. Let's run it. A patient in shock: sodium 140, chloride 100, bicarbonate 16. pH 7.31, PaCO2 33. One: acidemia. Two: low bicarbonate, so metabolic acidosis. Three: Winters gives 32, so the window is 30 to 34. Thirty-three fits. Four: the gap is 24, which is high. Five: the gap rose by 12, bicarbonate fell by eight, a ratio of 1.5. A pure high gap acidosis. Six: shock fits lactic acidosis. One disorder, one cause. Now the first gas again. Bicarbonate 15 predicts a PaCO2 near 30, but it is 24. A hidden respiratory alkalosis.

3:25Practice question

Your turn. A 42-year-old man with heavy alcohol use has had 3 days of abdominal pain and repeated vomiting. His urine shows ketones. Sodium 138, chloride 84, bicarbonate 21, albumin normal. pH 7.34, PaCO2 40. Which best describes his acid-base status? A, high gap acidosis with appropriate compensation. B, high gap acidosis with a respiratory acidosis. C, high gap acidosis with a metabolic alkalosis. D, high gap acidosis with a normal gap acidosis. E, metabolic alkalosis with respiratory compensation. Pause here and decide. The pH is acid and bicarbonate is low, so the primary disorder is a metabolic acidosis. E is out: alkalosis cannot be primary when the pH is acid. Winters gives 39 and a half, a window of 37 and a half to 41 and a half. Forty fits. B is out: no extra carbon dioxide, so no respiratory acidosis. The gap is 33, and it rose by 21 while bicarbonate fell by only three. That is a ratio of seven. A is out: seven is far above one to two. D is out: a normal gap acidosis would give a ratio below one. So the answer is C: ketoacidosis, with a metabolic alkalosis from vomiting.

5:07High-yield recap and next lesson

Three things to keep. One: pH, primary disorder, compensation, gap, delta ratio, context, in that order. Two: a missed compensation window means a second disorder. Three: a high ratio means a hidden alkalosis. Next up: three original acid-base cases, to practise the whole algorithm.

Sources

Numbers in the text, e.g. [1], point here
  1. Lewis JL III. Acid-Base Disorders. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders
  2. Lewis JL III. Metabolic Alkalosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-alkalosis
  3. Lewis JL III. Metabolic Acidosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-acidosis
  4. Brandis K. Acid-Base Physiology, ch. 3 Acid-Base Disorders. med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Acid-base_Physiology_(Brandis)/03%3A_Acid-Base_Disorders
  5. Brandis K. Acid-Base Physiology, 9.2 Systematic Evaluation of Acid-Base Status and 9.3 compensation rules. med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Acid-base_Physiology_(Brandis)/09%3A_Assessment_of_Acid-Base_Disorders/9.02%3A_Systematic_Evaluation_of_Acid-Base_Status

Delta ratio cut-offs differ between sources; treat the ratio as a strong clue, not proof.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustration on thumbnail A: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): lungs. All video diagrams are original; no third-party artwork is used in the video. Example lab values are illustrative.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-020.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 19Acidosis and hyperkalemia: how pH shifts potassium
  1. EP 01ABG interpretation, made simple5:35
  2. EP 02Anion gap explained: high vs normal gap metabolic acidosis5:25
  3. EP 03Winters' formula: the expected PaCO₂ in metabolic acidosis5:10
  4. EP 04Delta ratio: find the hidden second acid–base disorder5:09
  5. EP 05New bicarbonate: how the kidney excretes acid with ammonium5:23
  6. EP 06High anion gap metabolic acidosis causes, sorted by the anion5:54
  7. EP 07Osmolar gap: how toxic alcohols hide from the anion gap5:31
  8. EP 08Urine anion gap: telling diarrhea from RTA5:33
  9. EP 09Renal tubular acidosis: tell RTA types 1, 2 and 4 apart5:24
  10. EP 10Metabolic alkalosis: generation, maintenance and urine chloride5:30
  11. EP 11Vomiting metabolic alkalosis: why the urine turns acid5:36
  12. EP 12Bartter vs Gitelman: NKCC2, NCC and urine calcium5:21
  13. EP 13Respiratory acidosis compensation: acute vs chronic5:22
  14. EP 14Respiratory alkalosis compensation: acute vs chronic5:33
  15. EP 15Metabolic alkalosis compensation: the expected PaCO₂5:12
  16. EP 16Salicylate poisoning acid-base: a classic mixed disorder5:31
  17. EP 17Type A vs type B lactic acidosis: split it by mechanism5:33
  18. EP 18DKA anion gap explained: from high gap to hyperchloraemic acidosis5:42
  19. EP 19Acidosis and hyperkalemia: how pH shifts potassium5:43
  20. EP 20ABG interpretation in 6 steps: the acid-base algorithm5:32
  21. EP 21Acid-base practice cases: three ABG vignettes, one algorithm5:44