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ABG practice questions for the RRT / TMC exam

Arterial blood gases show up all over the RRT, TMC and the new 2027 RTE. These 10 free practice questions cover the whole method: reading the pH, finding the primary disorder, naming the compensation, checking it with Winter's formula and the acute and chronic rules, spotting mixed disorders, the anion gap, and the A-a gradient. Each one has a step-by-step explanation and a reason why every wrong choice is wrong.

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1. A patient breathing room air has: pH 7.25, PaCO2 60 mm Hg, HCO3- 26 mEq/L. How should this ABG be interpreted?

  1. Partially compensated respiratory acidosis
  2. Acute (uncompensated) respiratory acidosis
  3. Uncompensated metabolic acidosis
  4. Fully compensated respiratory acidosis
Show answer and explanation

Answer: B. Acute (uncompensated) respiratory acidosis

Step 1, pH: 7.25 is below 7.35, so acidemia. Step 2, which value explains it: PaCO2 60 is high (acid side) and HCO3- 26 is normal, so the primary problem is respiratory. Step 3, compensation: HCO3- is still in the normal range (22-26), so the kidneys have not compensated yet. Acute check: HCO3- should rise only about 1 mEq/L per 10 mm Hg rise in PaCO2, so 24 + 2 = 26, exactly what is seen.

  • A: Partial compensation needs an HCO3- above 26 with the pH still abnormal. Here HCO3- is normal.
  • B: Correct. High PaCO2 explains the low pH, and HCO3- has not moved out of the normal range.
  • C: A metabolic acidosis needs a low HCO3-. HCO3- here is 26.
  • D: Full compensation would bring the pH back to 7.35-7.45. The pH is 7.25.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · StatPearls: Respiratory Acidosis

2. An anxious patient in the ED has: pH 7.50, PaCO2 30 mm Hg, HCO3- 23 mEq/L. What is the interpretation?

  1. Uncompensated respiratory alkalosis
  2. Uncompensated metabolic alkalosis
  3. Partially compensated respiratory alkalosis
  4. Fully compensated metabolic acidosis
Show answer and explanation

Answer: A. Uncompensated respiratory alkalosis

Step 1: pH 7.50 is above 7.45, so alkalemia. Step 2: PaCO2 30 is low (base side) and matches the high pH, so the primary problem is respiratory. Step 3: HCO3- 23 is normal, so there is no renal compensation yet. In acute respiratory alkalosis HCO3- falls only about 2 mEq/L per 10 mm Hg drop in PaCO2 (expected about 22), which fits.

  • A: Correct. Low PaCO2 explains the high pH; HCO3- is still normal.
  • B: Metabolic alkalosis needs a high HCO3-. HCO3- is 23.
  • C: Partial compensation would need HCO3- below 22 with the pH still high.
  • D: The pH is alkalemic, so the primary process cannot be an acidosis.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · StatPearls: Respiratory Alkalosis

3. A patient with a severe infection has: pH 7.32, PaCO2 28 mm Hg, HCO3- 14 mEq/L. Which interpretation is best?

  1. Uncompensated metabolic acidosis with a normal respiratory response
  2. Metabolic acidosis with a concurrent respiratory acidosis
  3. Partially compensated respiratory alkalosis
  4. Partially compensated metabolic acidosis, appropriately compensated
Show answer and explanation

Answer: D. Partially compensated metabolic acidosis, appropriately compensated

Step 1: pH 7.32 = acidemia. Step 2: HCO3- 14 is low (acid side) and matches, so the primary problem is metabolic acidosis. Step 3: PaCO2 28 has moved (lungs are blowing off CO2) but the pH is still abnormal, so it is partially compensated. Step 4, is the compensation appropriate? Winter's formula: expected PaCO2 = 1.5 x 14 + 8 = 29 +- 2 (27-31). The measured 28 is in range, so there is no second respiratory disorder.

  • A: Uncompensated would mean a normal PaCO2 (35-45). PaCO2 has dropped to 28.
  • B: A concurrent respiratory acidosis would leave PaCO2 above the Winter's range (above 31).
  • C: With acidemia, the primary process must be an acidosis. Low PaCO2 here is the compensation.
  • D: Correct. Low HCO3- is primary; PaCO2 28 is within the Winter's range of 27-31.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · Merck Manual: Winter's Formula for Expected PCO2

4. A patient with 2 days of vomiting has: pH 7.50, PaCO2 45 mm Hg, HCO3- 34 mEq/L. How should this be interpreted?

  1. Uncompensated respiratory alkalosis
  2. Metabolic alkalosis with a concurrent primary respiratory alkalosis
  3. Metabolic alkalosis with appropriate respiratory compensation
  4. Fully compensated respiratory acidosis
Show answer and explanation

Answer: C. Metabolic alkalosis with appropriate respiratory compensation

Step 1: pH 7.50 = alkalemia. Step 2: HCO3- 34 is high (base side) and matches, so the primary problem is metabolic alkalosis (loss of gastric acid). Step 3: PaCO2 45 is at the top of normal because the patient hypoventilates slightly to compensate. Step 4: expected PaCO2 = 0.7 x HCO3- + 20 = 0.7 x 34 + 20 = about 44 (+-5, so about 39-49). The measured 45 fits, so the compensation is appropriate.

  • A: A respiratory alkalosis needs a low PaCO2. PaCO2 is 45.
  • B: A concurrent respiratory alkalosis would push PaCO2 below the expected range (below about 39).
  • C: Correct. High HCO3- is primary, and PaCO2 45 is inside the expected 39-49 range.
  • D: A pH of 7.50 is outside the normal range, and the primary process is alkalosis, not acidosis.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · StatPearls: Metabolic Alkalosis

5. A patient with long-standing COPD, stable at home, has: pH 7.35, PaCO2 60 mm Hg, HCO3- 32 mEq/L. Using the classic RT classification, what is the interpretation?

  1. Partially compensated respiratory acidosis
  2. Fully compensated respiratory acidosis
  3. Fully compensated metabolic alkalosis
  4. Acute respiratory acidosis
Show answer and explanation

Answer: B. Fully compensated respiratory acidosis

Step 1: pH 7.35 is within 7.35-7.45, so the pH is normal. Step 2: with a normal pH, look at which side of 7.40 it sits on. 7.35 is on the acid side, so the primary problem is the acid-side value: PaCO2 60 (respiratory acidosis). Step 3: HCO3- 32 is high, which is the renal compensation, and since the pH is normal it is fully compensated. Check: chronic respiratory acidosis raises HCO3- about 3.5 mEq/L per 10 mm Hg, so 24 + 3.5 x 2 = 31, close to 32.

  • A: Partial compensation means the pH is still abnormal. Here it is 7.35, within normal.
  • B: Correct. Normal pH on the acid side, high PaCO2 primary, HCO3- raised by the kidneys.
  • C: A primary metabolic alkalosis would put the pH on the alkaline side of 7.40.
  • D: In an acute respiratory acidosis HCO3- would be about 26 and the pH clearly low.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · StatPearls: Respiratory Acidosis

6. At sea level on room air (PB 760 mm Hg, FiO2 0.21), a patient after an opioid overdose has PaO2 50 mm Hg and PaCO2 70 mm Hg. Using R = 0.8, what is the A-a gradient and the most likely cause of the hypoxemia?

  1. About 12 mm Hg; hypoventilation alone
  2. About 12 mm Hg; intrapulmonary shunt
  3. About 50 mm Hg; V/Q mismatch
  4. About 100 mm Hg; alveolar-capillary diffusion defect
Show answer and explanation

Answer: A. About 12 mm Hg; hypoventilation alone

Step 1, alveolar gas equation: PAO2 = FiO2 x (PB - 47) - PaCO2/0.8 = 0.21 x 713 - 70/0.8 = 149.7 - 87.5 = about 62 mm Hg. Step 2: A-a gradient = PAO2 - PaO2 = 62 - 50 = about 12 mm Hg, which is normal. Step 3: a low PaO2 with a NORMAL A-a gradient means the lungs are transferring oxygen normally; the alveolar PO2 is low because CO2 has displaced it. That is pure hypoventilation, which is fixed by restoring ventilation (reversal agent, support breathing).

  • A: Correct. PAO2 about 62, gradient about 12 (normal): hypoventilation is the cause.
  • B: A shunt widens the A-a gradient. A gradient of 12 is normal.
  • C: This uses 100 as PAO2 and ignores the high PaCO2. V/Q mismatch would widen the gradient.
  • D: The gradient is not 100; and a diffusion defect would widen it.

Sources: StatPearls: Physiology, Alveolar to Arterial Oxygen Gradient · American Thoracic Society: Interpretation of Arterial Blood Gases

7. During resuscitation from cardiac arrest, an ABG shows: pH 7.11, PaCO2 55 mm Hg, HCO3- 17 mEq/L. What is the interpretation?

  1. Partially compensated respiratory acidosis
  2. Partially compensated metabolic acidosis
  3. Metabolic acidosis with respiratory alkalosis
  4. Combined respiratory and metabolic acidosis
Show answer and explanation

Answer: D. Combined respiratory and metabolic acidosis

Step 1: pH 7.11 = severe acidemia. Step 2: PaCO2 55 is high (acid side) AND HCO3- 17 is low (acid side). Both values point toward acid, so both a respiratory and a metabolic acidosis are present. Neither is compensating the other: if one were compensating, it would move in the opposite direction. This pattern is typical of arrest (poor ventilation plus lactic acid).

  • A: Compensation for a respiratory acidosis would RAISE the HCO3-, not lower it.
  • B: Compensation for a metabolic acidosis would LOWER the PaCO2, not raise it.
  • C: A respiratory alkalosis needs a low PaCO2. PaCO2 is 55.
  • D: Correct. Both PaCO2 and HCO3- are on the acid side.

Sources: American Thoracic Society: Interpretation of Arterial Blood Gases · StatPearls: Arterial Blood Gas

8. A patient has: pH 7.29, PaCO2 26 mm Hg, HCO3- 12 mEq/L, Na+ 140 mEq/L, Cl- 100 mEq/L. What is the anion gap and the type of acidosis?

  1. 12 mEq/L; normal anion gap metabolic acidosis
  2. 40 mEq/L; high anion gap metabolic acidosis
  3. 28 mEq/L; high anion gap metabolic acidosis
  4. 28 mEq/L; respiratory acidosis
Show answer and explanation

Answer: C. 28 mEq/L; high anion gap metabolic acidosis

Step 1: pH 7.29 with HCO3- 12 = metabolic acidosis; PaCO2 26 is the respiratory compensation (Winter's: 1.5 x 12 + 8 = 26 +- 2, appropriate). Step 2, anion gap = Na+ - (Cl- + HCO3-) = 140 - (100 + 12) = 28 mEq/L. A typical reference gap is about 8-12 (check your lab), so this is a high anion gap acidosis: think ketoacids, lactate, toxic alcohols, salicylates, or kidney failure.

  • A: This is the HCO3-, not the gap.
  • B: This forgets to subtract the HCO3- (140 - 100).
  • C: Correct. 140 - 112 = 28: high anion gap metabolic acidosis.
  • D: PaCO2 is low, so the acidosis is metabolic, not respiratory.

Sources: StatPearls: Anion Gap and Non-Anion Gap Metabolic Acidosis · Merck Manual: Winter's Formula for Expected PCO2

9. A patient with diabetic ketoacidosis who is becoming drowsy has: pH 7.18, PaCO2 33 mm Hg, HCO3- 12 mEq/L. What does this gas show?

  1. Metabolic acidosis with appropriate respiratory compensation
  2. Metabolic acidosis with a concurrent respiratory acidosis
  3. Metabolic acidosis with a concurrent respiratory alkalosis
  4. Uncompensated respiratory acidosis
Show answer and explanation

Answer: B. Metabolic acidosis with a concurrent respiratory acidosis

Step 1: pH 7.18 with HCO3- 12 = primary metabolic acidosis. Step 2: PaCO2 33 is below normal, so it looks like compensation. Step 3, check it: Winter's formula expected PaCO2 = 1.5 x 12 + 8 = 26 +- 2 (24-28). The measured 33 is HIGHER than expected, so the patient is not breathing off enough CO2: a respiratory acidosis is also present. In a tiring or drowsy patient this is a warning sign of ventilatory failure.

  • A: Appropriate compensation would put PaCO2 at 24-28. 33 is too high.
  • B: Correct. PaCO2 33 is above the Winter's range of 24-28.
  • C: A concurrent respiratory alkalosis would put PaCO2 BELOW 24.
  • D: PaCO2 33 is low, and HCO3- 12 is the primary problem.

Sources: Merck Manual: Winter's Formula for Expected PCO2 · American Thoracic Society: Interpretation of Arterial Blood Gases

10. A patient has: pH 7.32, PaCO2 70 mm Hg, HCO3- 35 mEq/L. Which description fits best?

  1. Chronic, partially compensated respiratory acidosis
  2. Acute respiratory acidosis with only acute buffering of HCO3-
  3. Primary metabolic alkalosis with respiratory compensation
  4. Combined respiratory acidosis and metabolic acidosis
Show answer and explanation

Answer: A. Chronic, partially compensated respiratory acidosis

Step 1: pH 7.32 = acidemia; PaCO2 70 is high and matches, so the primary problem is respiratory acidosis. Step 2: HCO3- 35 is high, so the kidneys are compensating, but the pH is still low: partially compensated. Step 3, acute or chronic? PaCO2 is up 30 mm Hg. Acute expected HCO3- = 24 + 3 = 27. Chronic expected HCO3- = 24 + 3.5 x 3 = about 34.5. The measured 35 matches the chronic pattern, which takes days of renal adaptation.

  • A: Correct. HCO3- 35 matches the chronic estimate of about 34.5.
  • B: Acute compensation would give an HCO3- of only about 27.
  • C: With acidemia, the primary process must be an acidosis.
  • D: A metabolic acidosis would lower the HCO3-; here it is raised.

Sources: StatPearls: Respiratory Acidosis · Merck Manual: Primary Changes and Compensations in Simple Acid-Base Disorders

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Study aid only. Not clinical advice. Original practice items, not actual or recalled exam questions.

Written and reviewed by a practicing RRT for TidalPrep. Formulas checked against the cited sources on November 2, 2026.