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USMLE Step 1Renal & acid–baseEP 21 of 21

Acid-base practice cases: three ABG vignettes, one algorithm

The same six steps solve one, two or three acid-base disorders. Work through two ABG vignettes in full, then try the practice case yourself.

  • 5:44 video
  • 3 min read
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  • Published 2 Nov 2026
Video lesson · 5:44 · Watch on YouTubeSeries: Acid–Base Disorders & ABG Interpretation · @medservatory
Chapters (8)
By the end of this lesson

Integrate the stepwise algorithm to solve three original acid-base vignettes (single, mixed and triple disorders).

The short version

  1. Run all six steps, in order: pH, primary disorder, compensation, anion gap, delta ratio, then whether the story explains every finding.
  2. A missed compensation window points to another disorder.
  3. A normal anion gap skips the delta ratio; a high gap always runs it.
  4. Delta ratio below 1 adds a normal gap acidosis, 1 to 2 is pure, and above 2 adds a metabolic alkalosis.
  5. A normal bicarbonate can hide a metabolic alkalosis behind a high-gap acidosis.

A blood gas that looks perfect

Start with a gas that looks flawless: pH 7.40, PaCO₂ 40 mm Hg, HCO₃⁻ 24 mEq/L. Now add the electrolytes: sodium 141, chloride 95. Does it still look normal?

These acid-base practice cases apply the full algorithm to three original ABG vignettes, with one disorder, then two, then three. The first two are solved in full below; the third is the practice question.[1][7]

The six steps of the acid-base algorithm

  1. pHBelow 7.35 is acidemia; above 7.45 is alkalemia.[1]
  2. Primary disorderThe primary disorder is the change (PaCO₂ or HCO₃⁻) that explains the pH.[1][7]
  3. CompensationCheck the expected compensation. A miss points to a second disorder.[1][7]
  4. Anion gapNa⁺ − (Cl⁻ + HCO₃⁻), with K⁺ left out. Normal is about 12.[1]
  5. Delta ratioOnly if the gap is high: the rise in the gap divided by the fall in bicarbonate.[6]
  6. The storyDoes the clinical story explain every finding?[1]
Compensation rules used in these cases [1] [7]
Primary disorderExpected compensation
Metabolic acidosisWinters' formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8, ± 2
Respiratory alkalosisHCO₃⁻ falls 2 per 10 mm Hg fall in PaCO₂ when acute, 4 when chronic
Respiratory acidosisHCO₃⁻ rises 1 per 10 mm Hg rise in PaCO₂ when acute, 4 when chronic

Compensation constants and delta ratio cut-offs differ between sources. This lesson uses one convention throughout and treats the ratio as a strong clue, not proof.[1][7]

The delta ratio: what the number adds

Delta ratio
Delta ratio = (anion gap − 12) / (24 − HCO₃⁻)

the rise in the gap divided by the fall in bicarbonate

Reading the delta ratio when the anion gap is high (lesson convention)
Delta ratioMeaning
Below 1Adds a normal gap acidosis
1 to 2Pure high-gap acidosis
Above 2Adds a metabolic alkalosis

Case 1: an ABG vignette with a single disorder

A 22-year-old woman has tingling around her mouth, thirty minutes into a panic attack. Sodium is 139, chloride 105, bicarbonate 22. The pH is 7.52 and the PaCO₂ is 28.[5][7]

Case 1, step by step
  1. Step 1: pH 7.52 is above 7.45, so alkalemia
  2. Step 2: the PaCO₂ is low, so respiratory alkalosis
  3. Step 3: PaCO₂ fell by 12 (40 → 28), which is 1.2 tens
  4. Acute rule: 2 × 1.2 = 2.4, so HCO₃⁻ ≈ 24 − 2.4 ≈ 22
  5. Chronic rule: 4 × 1.2 = 4.8, so HCO₃⁻ ≈ 24 − 4.8 ≈ 19
  6. Measured HCO₃⁻ is 22, so this is acute
  7. Step 4: 139 − (105 + 22) = 12, a normal gap, so skip step 5
  8. Step 6: panic explains it; one disorder

Case 2: ABG vignette with two disorders

A 52-year-old man is in septic shock and has just had a large volume of saline. His lactate is 5.5 mmol/L. Sodium is 140, chloride 108, bicarbonate 13. The pH is 7.30 and the PaCO₂ is 27.[2]

Case 2, step by step
  1. Step 1: pH 7.30 is below 7.35, so acidemia
  2. Step 2: bicarbonate is low, so metabolic acidosis
  3. Step 3: Winters: 1.5 × 13 + 8 = 27.5, a window of 25.5 to 29.5; the PaCO₂ of 27 fits
  4. Step 4: 140 − (108 + 13) = 19, a high gap
  5. Step 5: the gap rose by 7 (19 − 12) and bicarbonate fell by 11 (24 − 13): 7 / 11 ≈ 0.6
  6. A ratio below 1 means a second, normal gap acidosis on top
  7. Step 6: lactate explains the gap, and saline explains the extra chloride

Compensation was appropriate here, yet there are still two disorders. The delta ratio is what finds the second one.[2][6][7]

The opening gas again: a hidden metabolic alkalosis

Return to the gas that looked perfect. The anion gap is 141 − (95 + 24) = 22, which is 10 above normal. If only a high-gap acidosis were present, bicarbonate should have fallen by about 10, to about 14. It is still 24.[1][6]

Something is holding bicarbonate up: a hidden metabolic alkalosis. A normal bicarbonate can sit beside a high-gap acidosis and hide it, which is why a high gap always runs the delta ratio.[1][6]

Case 3: the practice question is your turn

The third vignette is the practice question below. It is a man with two days of repeated vomiting and colicky abdominal pain who then receives an intravenous sedative. Nothing here is solved for you; apply the same steps.

Practice question

Original · USMLE style
Your turnQB-MS-S1-REN-022-01

A 66-year-old man comes to the emergency department after 2 days of repeated vomiting and colicky abdominal pain. He has no history of lung disease. He is given an intravenous sedative for the pain and becomes drowsy over the next hour. Temperature is 37.2 C (99.0 F), pulse is 118/min, respirations are 8/min, and blood pressure is 96/60 mm Hg. The abdomen is distended and diffusely tender. Laboratory studies show:

Results
Serum Na⁺138 mEq/L
Serum Cl⁻92 mEq/L
Serum HCO₃⁻18 mEq/L
Serum albumin4.0 g/dL
Serum lactate12 mmol/L
Arterial pH (room air)7.18
Arterial PCO₂ (room air)50 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 six steps of the acid-base algorithm?

pH; the primary disorder; compensation; the anion gap; the delta ratio, only if the gap is high; and whether the clinical story explains every finding.[1][7]

How do you know an ABG has a second disorder?

If the measured PaCO₂ or HCO₃⁻ does not match the expected compensation, a second primary process should be suspected. In metabolic acidosis, the expected PaCO₂ comes from Winters' formula.[1][7]

When do you calculate the delta ratio?

Only when the anion gap is high. A normal gap skips the delta ratio; a high gap always runs it.[1][6]

What does the delta ratio tell you?

Below 1 adds a normal gap acidosis, 1 to 2 is a pure high-gap acidosis, and above 2 adds a metabolic alkalosis. Treat it as a strong clue, not proof.[6]

Transcript

Read the full narration, by chapter655 words

0:00A blood gas that looks perfect

Here is a blood gas that looks perfect. pH 7.40, PaCO2 40, bicarbonate 24. Now add the electrolytes: sodium 141, chloride 95. Today: the full algorithm on three original cases, with one disorder, then two, then three. One: the pH.

0:21The six steps in 70 seconds

Below 7.35 is acidemia, and above 7.45 is alkalemia. Two: the primary disorder is the change that explains the pH. Three: compensation. For metabolic acidosis, Winters' formula: 1.5 times bicarbonate plus eight, plus or minus two. In respiratory alkalosis, bicarbonate falls two for every 10 that PaCO2 falls when acute, and four when chronic. In respiratory acidosis, it rises one for every 10 when acute, and four when chronic. A miss means a second disorder. Four: the anion gap, sodium minus the sum of chloride and bicarbonate. Normal is about 12. Five, only if the gap is high: the delta ratio, the rise in the gap divided by the fall in bicarbonate. Below one adds a normal gap acidosis. One to two is pure. Above two adds a metabolic alkalosis. Six: does the story explain every finding?

1:32Case 1: a single disorder

Case one: a single disorder. A 22-year-old woman has tingling around her mouth, 30 minutes into a panic attack. Sodium 139, chloride 105, bicarbonate 22. pH 7.52, PaCO2 28. One: alkalemia. Two: the PaCO2 is low, so respiratory alkalosis. Three: PaCO2 fell by 12. The acute rule predicts a bicarbonate near 22, and the chronic rule near 19. It is 22, so this is acute. Four: the gap is 12, so we skip step five. Six: panic explains it. One disorder.

2:19Case 2: two disorders

Case two: two disorders. A 52-year-old man is in septic shock and has just had a large volume of saline. His lactate is 5.5 millimoles per litre. Sodium 140, chloride 108, bicarbonate 13. pH 7.30, PaCO2 27. One: acidemia. Two: low bicarbonate, so metabolic acidosis. Three: Winters gives 27 and a half, a window of 25 and a half to 29 and a half. Twenty-seven fits. Four: the gap is 19, high. Five: the gap rose by seven, bicarbonate fell by 11, a ratio of about 0.6. Below one: a second, normal gap acidosis on top. Six: lactate explains the gap, and saline explains the extra chloride.

3:17The opening gas again: a hidden alkalosis

Now the first gas again. The gap is 22, 10 above normal, so bicarbonate should have fallen by about 10. It is still 24. A hidden metabolic alkalosis. Case three is your turn.

3:31Case 3: the practice question

A 66-year-old man has had 2 days of repeated vomiting and colicky abdominal pain. He has no history of lung disease. After an intravenous sedative in the emergency department, he is drowsy, breathing eight times a minute. Lactate is 12 mmol/L. Sodium 138, chloride 92, bicarbonate 18, albumin normal. pH 7.18, PaCO2 50. Which best describes his acid-base status? A, high gap acidosis alone, with appropriate compensation. B, high gap acidosis with a metabolic alkalosis. C, high gap acidosis with a respiratory acidosis. D, high gap acidosis with a metabolic alkalosis and a respiratory acidosis. E, high gap acidosis with a normal gap acidosis and a respiratory acidosis.

4:34The answer: wrong options first

Pause here and decide. The pH is acid and the gap is 28, so there is a metabolic acidosis. Winters gives 35, a window of 33 to 37. His PaCO2 is 50, far above it. A and B are out: they ignore the extra carbon dioxide. The gap rose by 16 and bicarbonate fell by only six, a ratio of about 2.7. C is out: a ratio above two points to a metabolic alkalosis. E is out: a normal gap acidosis would give a ratio below one. So the answer is D: lactic acidosis, a metabolic alkalosis from vomiting,

5:19High-yield recap and next lesson

and a respiratory acidosis from sedation. Three things to keep. One: run all six steps, in order. Two: a missed compensation window means another disorder. Three: a normal gap skips the delta ratio, and a high gap always runs it. Next up: GFR and the Starling forces at the glomerulus.

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 Acidosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-acidosis
  3. Lewis JL III. Metabolic Alkalosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-alkalosis
  4. Lewis JL III. Respiratory Acidosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/respiratory-acidosis
  5. Lewis JL III. Respiratory Alkalosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/respiratory-alkalosis
  6. 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
  7. Brandis K. Acid-Base Physiology, 9.2 Systematic Evaluation of Acid-Base Status and 9.3 Bedside Rules for Assessment of Compensation. med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Acid-base_Physiology_(Brandis)/09%3A_Assessment_of_Acid-Base_Disorders/9.03%3A_Bedside_Rules_for_Assessment_of_Compensation

Compensation constants and delta ratio cut-offs differ between sources; this lesson uses one convention throughout and treats 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. All video diagrams are original; no third-party artwork is used in the video. Example lab values are illustrative. 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.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-022.

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

In this series

Playlist on YouTube
← Previous · EP 20ABG interpretation in 6 steps: the acid-base algorithm
  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