ABG interpretation is one of the most testable NCLEX skills, and one of the most learnable, because it follows a fixed method. Work through these 10 practice questions on the four acid-base disorders, the ROME method, compensation levels, and common causes, each with the full interpretation worked out step by step.
Acid-Base & ABGs: Normals, ROME & Compensation
Start every ABG with the normal ranges: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L, and PaO2 80-100 mmHg. A pH below 7.35 is acidosis; above 7.45 is alkalosis. Once you know the pH is off, decide whether the lungs or the kidneys caused it. The fastest tool is ROME: Respiratory is Opposite (pH and PaCO2 move in opposite directions) and Metabolic is Equal (pH and HCO3 move in the same direction). CO2 is your acid/respiratory value; HCO3 is your base/metabolic value.
The four primary disorders follow logically. Respiratory acidosis (low pH, high CO2) comes from hypoventilation and CO2 retention: COPD, opioid or sedative oversedation, airway obstruction, and chest trauma. Respiratory alkalosis (high pH, low CO2) comes from hyperventilation and CO2 blow-off: anxiety, pain, fever, early sepsis, and high altitude. Metabolic acidosis (low pH, low HCO3) comes from added acid or lost bicarbonate: DKA, lactic acidosis/shock, kidney failure, and severe diarrhea, with Kussmaul respirations as compensation. Metabolic alkalosis (high pH, high HCO3) comes from acid loss via vomiting or NG suction, excess antacids, diuretics, and hypokalemia.
Finally, read compensation. Uncompensated means the pH is abnormal and only the primary system is off. Partially compensated means the pH is still abnormal but BOTH CO2 and HCO3 are now abnormal. Fully compensated means the pH has returned to 7.35-7.45 while both CO2 and HCO3 remain abnormal. When the pH is fully compensated, the side of 7.40 it rests on names the primary problem: a pH of 7.35 to 7.39 points to a primary acidosis and 7.41 to 7.45 to a primary alkalosis. Remember the lungs compensate quickly by adjusting CO2, and the kidneys compensate slowly by adjusting HCO3.
Acid-Base & ABG Interpretation NCLEX Practice Questions
Question 1 · ABG interpretation
A client has the following arterial blood gas: pH 7.28, PaCO2 58 mmHg, HCO3 24 mEq/L. Which interpretation is correct?
- A Metabolic acidosis, uncompensated
- B Respiratory acidosis, uncompensated
- C Respiratory acidosis, partially compensated
- D Respiratory alkalosis, uncompensated
Show answer & rationale
Correct answer: B
Work the gas in order. The pH of 7.28 is below 7.35, so this is acidosis. The PaCO2 of 58 mmHg is above the normal 35-45, meaning CO2 is retained. Using ROME, Respiratory is Opposite: the pH went down while the CO2 went up, so the two values move in opposite directions and the lungs are the cause. That makes this respiratory acidosis. The HCO3 of 24 mEq/L is squarely within the normal 22-26 range, so the kidneys have not begun to compensate. When the pH is abnormal and only the primary (respiratory) system is abnormal, the disorder is uncompensated. Therefore respiratory acidosis, uncompensated is correct. Metabolic acidosis is wrong because HCO3 is normal, not low; in a metabolic problem the pH and HCO3 move together (Equal), and here HCO3 has not changed. Respiratory acidosis, partially compensated is wrong because partial compensation requires BOTH the CO2 and HCO3 to be abnormal while the pH remains off; the HCO3 here is still normal. Respiratory alkalosis is wrong because the pH is acidic, not alkalotic, and the CO2 is high rather than low. Clinical pearl: a rising CO2 with an acidic pH points to hypoventilation, seen in COPD, opioid oversedation, or airway obstruction, so assess the client's respiratory rate and level of consciousness first.
Question 2 · ABG interpretation
A client in diabetic ketoacidosis has this ABG: pH 7.30, PaCO2 38 mmHg, HCO3 16 mEq/L. How should the nurse interpret these results?
- A Metabolic acidosis, uncompensated
- B Respiratory acidosis, uncompensated
- C Metabolic alkalosis, uncompensated
- D Metabolic acidosis, fully compensated
Show answer & rationale
Correct answer: A
Begin with the pH of 7.30, which is below 7.35 and therefore acidotic. Next check the CO2: 38 mmHg falls within the normal 35-45 range, so the lungs are not the driver. Then check the HCO3: 16 mEq/L is below the normal 22-26, so bicarbonate is low. Using ROME, Metabolic is Equal, the pH and HCO3 both moved down together, confirming a metabolic problem. Low bicarbonate with an acidic pH is metabolic acidosis, which fits DKA perfectly because ketoacids consume bicarbonate. Because the CO2 is still normal, the respiratory system has not yet compensated, so this is uncompensated. Metabolic acidosis, uncompensated is correct. Respiratory acidosis is wrong because the CO2 is normal, not elevated; a respiratory cause would require an abnormal CO2 moving opposite to the pH. Metabolic alkalosis is wrong because the pH is acidic and the HCO3 is low, whereas alkalosis would show a high pH and high HCO3. Metabolic acidosis, fully compensated is wrong because full compensation requires the pH to return into the 7.35-7.45 range while both CO2 and HCO3 stay abnormal; here the pH is still abnormal at 7.30 and the CO2 is normal. Clinical pearl: watch for Kussmaul respirations, deep and rapid breathing, as the body's attempt to blow off CO2 and raise the pH; if you saw a low CO2 here, the client would be partially compensated.
Question 3 · Compensation
A client with long-standing COPD has this ABG: pH 7.36, PaCO2 55 mmHg, HCO3 32 mEq/L. Which interpretation is most accurate?
- A Metabolic alkalosis, partially compensated
- B Respiratory acidosis, partially compensated
- C Metabolic alkalosis, fully compensated
- D Respiratory acidosis, fully compensated
Show answer & rationale
Correct answer: D
The pH of 7.36 is within the normal 7.35-7.45 range, which immediately signals full compensation, because a normalized pH with abnormal gases means the compensating system has caught up. To find the primary disorder when the pH is normal, notice which side of 7.40 it sits on: 7.36 is on the acidic side, so the underlying problem is an acidosis. Now check the values. The PaCO2 of 55 mmHg is high (retained acid), and the HCO3 of 32 mEq/L is high (retained base). The high CO2 explains an acidosis (respiratory), so this is the primary disorder, and the high HCO3 is the kidneys' slow compensation pulling the pH back to normal. That makes it respiratory acidosis, fully compensated, the classic chronic COPD picture. Respiratory acidosis, partially compensated is wrong because partial compensation leaves the pH abnormal; here the pH is normal. Metabolic alkalosis, fully compensated is wrong because although both values are high, the normalized pH sits on the acidic side of 7.40, indicating the primary disorder is an acidosis, not an alkalosis. Metabolic alkalosis, partially compensated is wrong for the same reason and because the pH is normal, not abnormal. Clinical pearl: when the pH is normal but the gases are abnormal, split 7.40, below it means the primary disorder is acidosis, above it means alkalosis. Chronic CO2 retainers rely on this renal compensation, so avoid over-oxygenating and blunting their drive to breathe.
Question 4 · Causes
Which condition is most likely to cause respiratory alkalosis?
- A Chronic COPD with CO2 retention
- B Opioid overdose with respiratory depression
- C Acute anxiety attack with hyperventilation
- D Prolonged nasogastric suctioning
Show answer & rationale
Correct answer: C
Respiratory alkalosis is defined by a high pH and a low PaCO2, and it is caused by hyperventilation that blows off CO2 faster than the body produces it. An acute anxiety attack with hyperventilation is the textbook trigger: rapid breathing drives CO2 down, the pH rises, and the client feels lightheaded and tingly. Therefore the anxiety attack is correct. Chronic COPD with CO2 retention is wrong because retaining CO2 raises the PaCO2 and lowers the pH, producing respiratory acidosis, the opposite disorder. Opioid overdose with respiratory depression is wrong because opioids slow the respiratory rate, causing hypoventilation and CO2 retention, which again yields respiratory acidosis. Prolonged nasogastric suctioning is wrong because it removes hydrochloric acid from the stomach, causing a metabolic problem, specifically metabolic alkalosis from acid loss, not a respiratory one. Clinical pearl: any state that speeds up breathing can cause respiratory alkalosis, remember that anxiety, pain, fever, early sepsis, and high altitude all make you breathe faster and blow off CO2. The management focus is not oxygen but slowing the respiratory rate so CO2 can normalize. Contrast this with the acidosis causes, where the common thread is anything that slows or obstructs breathing.
Question 5 · Interventions
A client in the emergency department is hyperventilating from a panic attack. The ABG shows pH 7.52, PaCO2 28 mmHg, HCO3 24 mEq/L. Which nursing intervention is the priority?
- A Coach the client to breathe slowly to retain CO2
- B Administer high-flow oxygen by non-rebreather mask
- C Prepare the client for immediate intubation
- D Administer intravenous sodium bicarbonate
Show answer & rationale
Correct answer: A
The ABG confirms respiratory alkalosis: the pH of 7.52 is alkalotic and the PaCO2 of 28 mmHg is low, meaning the client is blowing off too much CO2 through hyperventilation, while the HCO3 of 24 is normal (uncompensated). The problem is behavioral overbreathing, so the fix is to slow the breathing down and let CO2 build back toward normal. Coaching the client to breathe slowly, using calm reassurance and a slow paced count, is the priority and directly reverses the cause. Administering high-flow oxygen is wrong because oxygenation is not the problem; the low value here is CO2, and the PaO2 is not reported as deficient, so extra oxygen does nothing to raise the CO2 or lower the pH. Preparing for immediate intubation is wrong and excessive; the client is breathing too fast, not failing to breathe, and there is no respiratory failure or airway compromise requiring mechanical ventilation. Administering IV sodium bicarbonate is wrong and dangerous because bicarbonate is a base and would worsen the alkalosis; bicarbonate is reserved for severe metabolic acidosis, the opposite scenario. Clinical pearl: for anxiety-driven respiratory alkalosis, the evidence-based approach is calm coaching and slowed breathing; older paper-bag rebreathing is discouraged because it can dangerously lower oxygen in a client whose symptoms are actually caused by another condition. Treat the anxiety and the gas normalizes.
Question 6 · ABG interpretation
A client with 3 days of persistent vomiting has this ABG: pH 7.50, PaCO2 42 mmHg, HCO3 32 mEq/L. Which interpretation is correct?
- A Respiratory alkalosis, uncompensated
- B Metabolic alkalosis, uncompensated
- C Metabolic acidosis, uncompensated
- D Metabolic alkalosis, partially compensated
Show answer & rationale
Correct answer: B
Start with the pH of 7.50, which is above 7.45 and therefore alkalotic. Check the CO2: 42 mmHg is within the normal 35-45 range, so the lungs are not responsible. Check the HCO3: 32 mEq/L is above the normal 22-26, so bicarbonate is elevated. Using ROME, Metabolic is Equal, the pH and HCO3 both moved up together, confirming a metabolic origin. High bicarbonate with an alkalotic pH is metabolic alkalosis, which fits persistent vomiting because losing gastric hydrochloric acid leaves the blood relatively more basic. Because the CO2 is still normal, the respiratory system has not yet begun to compensate, so this is uncompensated. Metabolic alkalosis, uncompensated is correct. Respiratory alkalosis is wrong because the CO2 is normal, not low; a respiratory alkalosis requires a low CO2 moving opposite to the pH. Metabolic acidosis is wrong because both the pH and HCO3 are high, not low. Metabolic alkalosis, partially compensated is wrong because partial compensation requires the CO2 to also be abnormal (the lungs would hypoventilate, raising CO2 to retain acid); here the CO2 is still normal. Clinical pearl: vomiting and nasogastric suction are the classic acid-loss causes of metabolic alkalosis, and these clients are frequently hypokalemic and hypochloremic, so monitor potassium closely and expect slow, shallow compensatory breathing if the body starts to retain CO2.
Question 7 · Compensation
A client in septic shock has this ABG: pH 7.32, PaCO2 30 mmHg, HCO3 16 mEq/L. Which interpretation is most accurate?
- A Metabolic acidosis, uncompensated
- B Respiratory alkalosis, partially compensated
- C Metabolic acidosis, partially compensated
- D Respiratory acidosis, fully compensated
Show answer & rationale
Correct answer: C
The pH of 7.32 is below 7.35, so the client is acidotic, and because the pH is still abnormal this is not fully compensated. Now identify the primary disorder. The HCO3 of 16 mEq/L is low (below 22-26), and the CO2 of 30 mmHg is also low (below 35-45). Two clues point to which is primary: first, ROME says Metabolic is Equal, and the pH and HCO3 both moved down together, matching a metabolic acidosis; second, the low CO2 actually opposes acidosis (a low CO2 would raise pH), so the lungs are compensating, not causing. This is the lactic acidosis of shock, where the low HCO3 is the problem and the fast breathing (low CO2) is compensation. Because BOTH values are abnormal but the pH is still off, this is partially compensated. Metabolic acidosis, partially compensated is correct. Metabolic acidosis, uncompensated is wrong because the CO2 is not normal; the respiratory system has already started to compensate. Respiratory alkalosis, partially compensated is wrong because the pH is acidic, not alkalotic; the low CO2 is a compensatory response, not the primary disorder. Respiratory acidosis, fully compensated is wrong on two counts: the CO2 is low rather than high, and full compensation would require a normal pH. Clinical pearl: when both the CO2 and HCO3 are abnormal, decide which one matches the pH direction, that value names the primary disorder, and the other is compensation.
Question 8 · Causes
A nurse is monitoring four clients. Which client is at greatest risk for developing respiratory acidosis?
- A A client with a kidney stone who is hyperventilating from acute pain
- B A client with type 1 diabetes who is in untreated diabetic ketoacidosis
- C A client with a paralytic ileus who is on prolonged nasogastric suction
- D A postoperative client on high-dose IV opioids with respirations of 8
Show answer & rationale
Correct answer: D
Respiratory acidosis develops when the client hypoventilates and retains CO2, raising the PaCO2 and lowering the pH. A postoperative client on high-dose IV opioids with respirations of only 8 is the clearest risk: opioids depress the respiratory drive, breathing slows, CO2 accumulates, and the pH falls. This is the correct answer. A client with a kidney stone who is hyperventilating from acute pain is wrong because rapid breathing blows off CO2 and produces respiratory alkalosis, the opposite disorder. A client with type 1 diabetes who is in untreated diabetic ketoacidosis is wrong because DKA causes a metabolic acidosis from ketoacid accumulation and lost bicarbonate, not a respiratory one; in fact these clients hyperventilate with Kussmaul respirations to compensate. A client with a paralytic ileus who is on prolonged nasogastric suction is wrong because removing gastric acid causes metabolic alkalosis, again not a respiratory acidosis. Clinical pearl: the common denominator for respiratory acidosis is anything that reduces ventilation, opioid or sedative oversedation, COPD, airway obstruction, chest trauma, or neuromuscular weakness. For the opioid client, monitor the respiratory rate and sedation level closely, and have naloxone available; a rate of 8 is a red flag warranting prompt intervention before CO2 climbs further.
Question 9 · Causes (SATA)
The nurse is reviewing conditions that can cause metabolic acidosis. Which of the following should the nurse identify as causes of metabolic acidosis? Select all that apply.
- A Prolonged vomiting
- B Diabetic ketoacidosis
- C Acute anxiety with hyperventilation
- D Septic shock with lactic acidosis
- E End-stage kidney failure
- F Severe, prolonged diarrhea
Show answer & rationale
Correct answer: B, D, E, F
Metabolic acidosis is a low pH with a low HCO3, caused either by adding acid or by losing bicarbonate. Diabetic ketoacidosis qualifies because ketoacids accumulate and consume bicarbonate. Septic shock with lactic acidosis qualifies because poor tissue perfusion drives anaerobic metabolism, producing lactic acid. End-stage kidney failure qualifies because the failing kidneys cannot excrete hydrogen ions or regenerate bicarbonate, so acids build up. Severe, prolonged diarrhea qualifies because intestinal and pancreatic secretions are bicarbonate-rich, and losing them from below depletes the body's base. These four are all correct. Prolonged vomiting is NOT a cause of metabolic acidosis; losing gastric hydrochloric acid from above produces the opposite disorder, metabolic alkalosis. Acute anxiety with hyperventilation is also NOT a cause of metabolic acidosis; blowing off CO2 raises the pH and produces respiratory alkalosis, a respiratory rather than a metabolic problem. Clinical pearl: a quick memory aid for metabolic acidosis causes is to think ketones, lactate, kidneys, and diarrhea. The direction of fluid loss matters, losing acid-rich fluid from the top (vomiting, NG suction) causes alkalosis, while losing base-rich fluid from the bottom (diarrhea) causes acidosis. Watch these clients for Kussmaul respirations, the deep rapid breathing that blows off CO2 to compensate for the falling pH.
Question 10 · Normal values (SATA)
A nursing instructor asks students to identify the normal reference ranges for arterial blood gas values. Which of the following are correct normal values? Select all that apply.
- A pH 7.35 to 7.45
- B PaCO2 50 to 60 mmHg
- C PaCO2 35 to 45 mmHg
- D HCO3 22 to 26 mEq/L
- E HCO3 15 to 20 mEq/L
- F PaO2 80 to 100 mmHg
Show answer & rationale
Correct answer: A, C, D, F
Knowing the normal ABG reference ranges cold is the foundation of every interpretation. The normal arterial pH is 7.35 to 7.45; below 7.35 is acidosis and above 7.45 is alkalosis, so this is correct. The normal PaCO2 is 35 to 45 mmHg, the respiratory or acid parameter controlled by the lungs, so this is correct. The normal HCO3 (bicarbonate) is 22 to 26 mEq/L, the metabolic or base parameter regulated by the kidneys, so this is correct. The normal PaO2 is 80 to 100 mmHg, reflecting oxygenation, so this is correct. PaCO2 50 to 60 mmHg is NOT normal; a value that high indicates CO2 retention, which would contribute to respiratory acidosis. HCO3 15 to 20 mEq/L is NOT normal; that range is below the lower limit of 22 and would indicate the low bicarbonate seen in metabolic acidosis. Clinical pearl: commit these four ranges to memory as a set, pH 7.35-7.45, CO2 35-45, HCO3 22-26, and PaO2 80-100, because you cannot classify a gas as acidotic, alkalotic, or compensated without them. A useful trick: the CO2 range (35-45) and the last two digits of the pH range (7.35-7.45) both share 35 and 45, which makes the CO2 easy to recall. Always interpret the pH first, then CO2, then HCO3.
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Acid-Base & ABG FAQ
What is the fastest way to interpret an ABG on the NCLEX?
Use a simple order and the ROME method. First look at the pH: below 7.35 is acidosis, above 7.45 is alkalosis. Next look at the PaCO2 (normal 35-45), your respiratory/acid value, then the HCO3 (normal 22-26), your metabolic/base value. Apply ROME: Respiratory is Opposite (pH and CO2 move in opposite directions) and Metabolic is Equal (pH and HCO3 move in the same direction). Then name the disorder by which value fits its ROME rule: if the CO2 is abnormal and moves opposite the pH, it is respiratory; if the HCO3 is abnormal and moves the same direction as the pH, it is metabolic.
How do I tell uncompensated, partial, and full compensation apart?
Look at the pH and how many systems are abnormal. Uncompensated means the pH is abnormal and only the primary system (CO2 or HCO3) is off. Partially compensated means the pH is still abnormal but BOTH the CO2 and HCO3 are now abnormal because the other system is responding. Fully (completely) compensated means the pH has returned into the normal 7.35-7.45 range while both the CO2 and HCO3 remain abnormal. Remember the lungs compensate fast and the kidneys compensate slowly.
What are the causes of the four primary acid-base disorders?
Respiratory acidosis (low pH, high CO2) comes from hypoventilation: COPD, opioid oversedation, airway obstruction, chest trauma. Respiratory alkalosis (high pH, low CO2) comes from hyperventilation: anxiety, pain, fever, early sepsis, high altitude. Metabolic acidosis (low pH, low HCO3) comes from DKA, lactic acidosis/shock, kidney failure, and severe diarrhea. Metabolic alkalosis (high pH, high HCO3) comes from vomiting or NG suction, excess antacids, diuretics, and hypokalemia.
Why does vomiting cause alkalosis but diarrhea causes acidosis?
It comes down to what fluid you lose and from where. Vomiting and nasogastric suction remove acidic gastric contents (hydrochloric acid) from the top of the GI tract, leaving the blood relatively more basic, which produces metabolic alkalosis. Diarrhea removes bicarbonate-rich intestinal and pancreatic secretions from the bottom, depleting the body's base and producing metabolic acidosis. A quick rule: lose acid from the top means alkalosis, lose base from the bottom means acidosis.