A 23-year-old dancer is referred for fatigue and leg cramps. Her blood pressure is 104/68 mm Hg, and she denies vomiting and any diuretic or laxative use. Results: sodium 137, potassium 2.7, chloride 89 and total CO2 36 mmol/L; magnesium normal. Arterial pH 7.49, pCO2 47 mm Hg [6.3 kPa], HCO3- 35 mmol/L. Spot urine: sodium 52 mmol/L, potassium 38 mmol/L, chloride 6 mmol/L, pH 7.5. A urine diuretic screen is negative on two separate samples, and plasma renin and aldosterone are both raised. The most likely cause of her metabolic alkalosis is:

Concealed self-induced vomiting

Gitelman syndrome (a salt-losing tubulopathy)

A 76-year-old man with severe heart failure has taken furosemide 160 mg daily for several months. He is admitted in cardiogenic shock with cold, mottled limbs. Results: sodium 133, potassium 3.2, chloride 88 and HCO3- 23 mmol/L; lactate 9.0 mmol/L [81 mg/dL] (reference 0.5-2.2 mmol/L); albumin 4.0 g/dL [40 g/L]; creatinine 1.6 mg/dL [141 µmol/L]. Arterial pH 7.37, pCO2 41 mm Hg [5.5 kPa]. Take the normal anion gap as 12 mmol/L (reference 8-16) and the normal HCO3- as 24 mmol/L. Which interpretation fits these results best?

High anion gap acidosis plus metabolic alkalosis

High anion gap acidosis plus respiratory acidosis

No significant acid-base disorder

High and normal anion gap acidosis together

A 63-year-old woman with amyotrophic lateral sclerosis has chronic hypoventilation. At a clinic visit a month ago, when she was stable, her arterial results were pH 7.36, pCO2 55 mm Hg [7.3 kPa], HCO3- 30 mmol/L. Tonight she is drowsy after taking a sleeping tablet. Arterial results: pH 7.24, pCO2 78 mm Hg [10.4 kPa], HCO3- 32 mmol/L, pO2 58 mm Hg [7.7 kPa]; anion gap and lactate are normal. Assume that bicarbonate rises by about 1 mmol/L per 10 mm Hg of pCO2 in acute, and about 3.5 mmol/L per 10 mm Hg in chronic, respiratory acidosis. Tonight's results are best classified as:

Acute-on-chronic respiratory acidosis

Respiratory acidosis plus metabolic acidosis

Chronic respiratory acidosis, compensated

Acute respiratory acidosis alone

Acetazolamide causes a metabolic acidosis because it:

Blocks bicarbonate reabsorption in the proximal tubule

Depresses breathing so that CO2 builds up in blood

Adds an unmeasured organic acid to the plasma

Blocks the action of aldosterone in the collecting duct

What do arterial blood gases usually show in the third trimester of a healthy pregnancy?

Mild respiratory alkalosis with a lower bicarbonate

Metabolic alkalosis with a higher pCO2

Respiratory acidosis with a higher bicarbonate

Metabolic acidosis with a high anion gap

Acid-base interpretation and disease correlation

30 ASCP MLS practice questions on Acid-base interpretation and disease correlation, part of Chemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 23-year-old dancer is referred for fatigue and leg cramps. Her blood pressure is 104/68 mm Hg, and she denies vomiting and any diuretic or laxative use. Results: sodium 137, potassium 2.7, chloride 89 and total CO2 36 mmol/L; magnesium normal. Arterial pH 7.49, pCO2 47 mm Hg [6.3 kPa], HCO3- 35 mmol/L. Spot urine: sodium 52 mmol/L, potassium 38 mmol/L, chloride 6 mmol/L, pH 7.5. A urine diuretic screen is negative on two separate samples, and plasma renin and aldosterone are both raised. The most likely cause of her metabolic alkalosis is:
1. ASurreptitious diuretic use
2. BGitelman syndrome (a salt-losing tubulopathy)
3. CConcealed self-induced vomiting
4. DPrimary hyperaldosteronism
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Answer: C · Concealed self-induced vomiting
A hypokalemic metabolic alkalosis with normal blood pressure and a high renin points to chloride loss, either from the stomach or into the urine. The urine chloride separates the two: a value below about 20 mmol/L, as here, means the kidney is conserving chloride, as it does after vomiting. Urine sodium is misleadingly high because the kidney is excreting the excess bicarbonate with sodium and potassium, which also makes the urine alkaline. Gitelman syndrome and active diuretic use both keep urine chloride high, and two negative diuretic screens make hidden diuretic use unlikely. Primary hyperaldosteronism causes hypertension with a suppressed renin.
- A: Active diuretic use keeps urine chloride high, and two screens were negative.
- B: Gitelman syndrome wastes chloride in the urine and usually lowers magnesium.
- D: Primary hyperaldosteronism causes hypertension with a suppressed renin.
Question 2 · Hard
A 76-year-old man with severe heart failure has taken furosemide 160 mg daily for several months. He is admitted in cardiogenic shock with cold, mottled limbs. Results: sodium 133, potassium 3.2, chloride 88 and HCO3- 23 mmol/L; lactate 9.0 mmol/L [81 mg/dL] (reference 0.5-2.2 mmol/L); albumin 4.0 g/dL [40 g/L]; creatinine 1.6 mg/dL [141 µmol/L]. Arterial pH 7.37, pCO2 41 mm Hg [5.5 kPa]. Take the normal anion gap as 12 mmol/L (reference 8-16) and the normal HCO3- as 24 mmol/L. Which interpretation fits these results best?
1. AHigh anion gap acidosis plus respiratory acidosis
2. BNo significant acid-base disorder
3. CHigh anion gap acidosis plus metabolic alkalosis
4. DHigh and normal anion gap acidosis together
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Answer: C · High anion gap acidosis plus metabolic alkalosis
The anion gap is 133 - (88 + 23) = 22 mmol/L, a rise of 10 above normal that matches the lactate of 9 mmol/L. A lactic acidosis of that size on its own should have lowered bicarbonate by about 10, to near 14 mmol/L, yet it is 23. Bicarbonate is higher than expected because months of loop diuretic have caused a chloride-depleted (contraction) metabolic alkalosis, shown by the low chloride and potassium. The pCO2 of 41 mm Hg fits Winter's formula for a bicarbonate of 23 (1.5 x 23 + 8 = 42.5 ± 2), so there is no added respiratory disorder, and a normal anion gap acidosis would have made bicarbonate fall more than the gap rose. A near-normal pH and bicarbonate never exclude a disorder when the anion gap is raised.
- A: The pCO2 of 41 mm Hg lies within Winter's expected range of about 40-45 mm Hg.
- B: An anion gap of 22 mmol/L with a lactate of 9 mmol/L is a real acidosis, hidden by an alkalosis.
- D: A normal anion gap acidosis would make bicarbonate fall more than the gap rose, not less.
Question 3 · Hard
A 63-year-old woman with amyotrophic lateral sclerosis has chronic hypoventilation. At a clinic visit a month ago, when she was stable, her arterial results were pH 7.36, pCO2 55 mm Hg [7.3 kPa], HCO3- 30 mmol/L. Tonight she is drowsy after taking a sleeping tablet. Arterial results: pH 7.24, pCO2 78 mm Hg [10.4 kPa], HCO3- 32 mmol/L, pO2 58 mm Hg [7.7 kPa]; anion gap and lactate are normal. Assume that bicarbonate rises by about 1 mmol/L per 10 mm Hg of pCO2 in acute, and about 3.5 mmol/L per 10 mm Hg in chronic, respiratory acidosis. Tonight's results are best classified as:
1. ARespiratory acidosis plus metabolic acidosis
2. BChronic respiratory acidosis, compensated
3. CAcute respiratory acidosis alone
4. DAcute-on-chronic respiratory acidosis
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Answer: D · Acute-on-chronic respiratory acidosis
Her clinic results show a chronic respiratory acidosis in which the kidneys had already raised bicarbonate to 30 mmol/L. Tonight pCO2 has risen a further 23 mm Hg, and the acute rule predicts only a 2-3 mmol/L rise in bicarbonate, to about 32 mmol/L, which is what is seen. Applying the chronic rule to the whole rise from 40 mm Hg would predict a bicarbonate near 37 mmol/L and a pH near 7.30, so the low pH could be misread as an added metabolic acidosis, which the normal anion gap and lactate exclude. Calling it acute alone ignores her raised baseline bicarbonate, and a pH of 7.24 is far from compensated. The sedative has cut her ventilatory drive on top of chronic neuromuscular weakness.
- A: Anion gap and lactate are normal, and bicarbonate fits an acute rise from her baseline.
- B: A pH of 7.24 is far from compensated, and pCO2 has just risen sharply.
- C: Her bicarbonate was already 30 mmol/L before tonight, so this is not acute alone.
Question 4 · Medium
Acetazolamide causes a metabolic acidosis because it:
1. ADepresses breathing so that CO2 builds up in blood
2. BAdds an unmeasured organic acid to the plasma
3. CBlocks bicarbonate reabsorption in the proximal tubule
4. DBlocks the action of aldosterone in the collecting duct
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Answer: C · Blocks bicarbonate reabsorption in the proximal tubule
Acetazolamide inhibits carbonic anhydrase in the proximal tubule, so filtered bicarbonate is not reclaimed and passes into the urine. Plasma bicarbonate falls and chloride rises, giving a normal anion gap (hyperchloremic) acidosis like proximal renal tubular acidosis. The urine is alkaline while the drug acts. The drug is used for glaucoma, altitude sickness and edema.
- A: Acetazolamide does not suppress breathing; it is used to stimulate it at altitude.
- B: No unmeasured acid is added, so the anion gap stays normal.
- D: Aldosterone blockade, as with spironolactone, causes a hyperkalemic acidosis by a different mechanism.
Question 5 · Medium
What do arterial blood gases usually show in the third trimester of a healthy pregnancy?
1. AMild respiratory alkalosis with a lower bicarbonate
2. BMetabolic alkalosis with a higher pCO2
3. CRespiratory acidosis with a higher bicarbonate
4. DMetabolic acidosis with a high anion gap
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Answer: A · Mild respiratory alkalosis with a lower bicarbonate
Progesterone stimulates the respiratory center, so pregnant women breathe more and their pCO2 falls to about 30 mm Hg (4.0 kPa). The kidneys compensate by excreting bicarbonate, which falls to about 18 to 22 mmol/L. The pH stays normal or slightly high. Results should be read against pregnancy values, since a pCO2 of 40 mm Hg can mean hypoventilation in late pregnancy.
- B: Pregnancy lowers pCO2; it does not raise it.
- C: Pregnancy increases breathing, so pCO2 falls rather than rises.
- D: A healthy pregnancy does not add unmeasured acids.
Question 6 · Medium
A 52-year-old with hypertension has K+ 2.9 mmol/L, HCO3- 34 mmol/L and urine chloride 60 mmol/L. Saline does not correct the alkalosis. The most likely cause is:
1. APrimary hyperaldosteronism
2. BNasogastric suction
3. CProlonged vomiting
4. DChronic diarrhea
Answer and explanation in LearnFree account
Question 7 · Easy
Poor tissue perfusion in shock most often causes which acid-base disorder?
1. AHigh anion gap metabolic acidosis
2. BNormal anion gap metabolic acidosis
3. CHypochloremic metabolic alkalosis
4. DAcute respiratory acidosis
Answer and explanation in LearnFree account
Question 8 · Hard
A patient with COPD who takes a loop diuretic has arterial pH 7.41, pCO2 60 mm Hg (8.0 kPa) and HCO3- 38 mmol/L. The best interpretation is:
1. ARespiratory acidosis with metabolic alkalosis
2. BMetabolic alkalosis with respiratory compensation
3. CNormal acid-base status for a COPD patient
4. DFully compensated chronic respiratory acidosis
Answer and explanation in LearnFree account
Question 9 · Hard
In acute respiratory acidosis, pCO2 rises from 40 to 70 mm Hg (5.3 to 9.3 kPa) within an hour. Starting from HCO3- 24 mmol/L, the expected bicarbonate is about:
1. A30 mmol/L
2. B24 mmol/L
3. C27 mmol/L
4. D34 mmol/L
Answer and explanation in LearnFree account
Question 10 · Hard
A climber has spent 3 weeks at high altitude. pCO2 is in mm Hg and HCO3- in mmol/L. Which arterial result set fits his acid-base status?
1. ApH 7.51, pCO2 28, HCO3- 22
2. BpH 7.31, pCO2 28, HCO3- 14
3. CpH 7.44, pCO2 28, HCO3- 19
4. DpH 7.48, pCO2 46, HCO3- 34
Answer and explanation in LearnFree account
Question 11 · Easy
What is the expected compensation for a primary metabolic acidosis?
1. ARenal excretion of more bicarbonate
2. BHypoventilation that raises pCO2
3. CRenal retention of more chloride
4. DHyperventilation that lowers pCO2
Answer and explanation in LearnFree account
Question 12 · Medium
pCO2 is in mm Hg and HCO3- in mmol/L. Which set shows a respiratory acidosis with partial renal compensation?
1. ApH 7.47, pCO2 50, HCO3- 36
2. BpH 7.23, pCO2 60, HCO3- 25
3. CpH 7.33, pCO2 60, HCO3- 31
4. DpH 7.28, pCO2 32, HCO3- 15
Answer and explanation in LearnFree account
Question 13 · Medium
Why does bicarbonate rise more per 10 mm Hg of pCO2 in chronic than in acute respiratory acidosis?
1. AThe lungs clear more CO2 as the disease progresses
2. BHemoglobin buffering grows stronger over several days
3. CRenal compensation takes 3 to 5 days to develop
4. DChloride is retained first and bicarbonate later
Answer and explanation in LearnFree account
Question 14 · Easy
An opioid overdose that slows breathing causes which primary acid-base disorder?
1. ARespiratory acidosis
2. BRespiratory alkalosis
3. CMetabolic acidosis
4. DMetabolic alkalosis
Answer and explanation in LearnFree account
Question 15 · Medium
A patient has a metabolic alkalosis after five days of vomiting. Which urine result best predicts that saline will correct it?
1. AA low urine sodium
2. BA high urine potassium
3. CA low urine chloride
4. DA low urine pH
Answer and explanation in LearnFree account
Question 16 · Hard
A 30-year-old with kidney stones has a normal anion gap metabolic acidosis, K+ 2.9 mmol/L and no diarrhea. Urine pH stays above 6.0 on repeated tests, and imaging shows nephrocalcinosis. The most likely diagnosis is:
1. APrimary hyperaldosteronism
2. BProximal (type 2) renal tubular acidosis
3. CDistal (type 1) renal tubular acidosis
4. DType 4 renal tubular acidosis
Answer and explanation in LearnFree account
Question 17 · Medium
Arterial pH 7.28, pCO2 30 mm Hg (4.0 kPa), HCO3- 14 mmol/L. What is the interpretation?
1. ARespiratory acidosis, uncompensated
2. BMetabolic acidosis, uncompensated
3. CMetabolic acidosis, partially compensated
4. DRespiratory alkalosis, compensated
Answer and explanation in LearnFree account
Question 18 · Hard
A man with cirrhosis has vomited for 3 days. Arterial pH 7.65, pCO2 30 mm Hg (4.0 kPa), HCO3- 33 mmol/L. Which interpretation fits best?
1. AMetabolic alkalosis with respiratory acidosis
2. BMetabolic alkalosis with expected compensation
3. CRespiratory alkalosis with renal compensation
4. DMetabolic alkalosis with respiratory alkalosis
Answer and explanation in LearnFree account
Question 19 · Medium
A normal anion gap metabolic acidosis is present. Urine sodium 30, potassium 20 and chloride 80 mmol/L give a urine anion gap of -30 mmol/L. The most likely cause is:
1. ADistal renal tubular acidosis
2. BChronic kidney disease
3. CSevere diarrhea
4. DType 4 renal tubular acidosis
Answer and explanation in LearnFree account
Question 20 · Hard
A 19-year-old with type 1 diabetes has glucose 540 mg/dL (30.0 mmol/L) and positive ketones. Arterial pH 7.21, pCO2 20 mm Hg (2.7 kPa), HCO3- 8 mmol/L. How should the respiratory response be judged?
1. AA coexisting primary respiratory alkalosis
2. BAppropriate compensation for the acidosis
3. CNo respiratory compensation has started
4. DA coexisting primary respiratory acidosis
Answer and explanation in LearnFree account
Question 21 · Hard
A 35-year-old arrives 4 hours after taking a large overdose of aspirin. Arterial pH 7.44, pCO2 25 mm Hg (3.3 kPa), HCO3- 17 mmol/L. These results are best explained by:
1. AFully compensated metabolic acidosis
2. BCompensated chronic respiratory alkalosis
3. CRespiratory alkalosis with metabolic acidosis
4. DNo acid-base disorder; the pH is normal
Answer and explanation in LearnFree account
Question 22 · Medium
A 24-year-old woman arrives during a panic attack with tingling fingers. Arterial pH 7.50, pCO2 30 mm Hg (4.0 kPa), HCO3- 23 mmol/L. The most likely disorder is:
1. AChronic respiratory alkalosis
2. BPrimary metabolic alkalosis
3. CCompensated metabolic acidosis
4. DAcute respiratory alkalosis
Answer and explanation in LearnFree account
Question 23 · Hard
A patient with a pure metabolic acidosis has HCO3- 12 mmol/L. By Winter's formula, the expected pCO2 is:
1. A38-42 mm Hg (5.1-5.6 kPa)
2. B27-31 mm Hg (3.6-4.1 kPa)
3. C16-20 mm Hg (2.1-2.7 kPa)
4. D24-28 mm Hg (3.2-3.7 kPa)
Answer and explanation in LearnFree account
Question 24 · Easy
A primary metabolic alkalosis begins with a primary rise in:
1. APlasma bicarbonate concentration
2. BHydrogen ion concentration
3. CArterial pCO2
4. DPlasma chloride
Answer and explanation in LearnFree account
Question 25 · Hard
A man with COPD usually has pCO2 65 mm Hg (8.7 kPa) and HCO3- 36 mmol/L. Six hours after intubation, his pH is 7.54, pCO2 40 mm Hg (5.3 kPa) and HCO3- 34 mmol/L. What is the most likely disorder?
1. ARespiratory alkalosis with renal compensation
2. BMetabolic acidosis with respiratory compensation
3. CPost-hypercapnic metabolic alkalosis
4. DChronic respiratory acidosis, fully compensated
Answer and explanation in LearnFree account

5 more Acid-base interpretation and disease correlation questions

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Study Chemistry

Acid-base interpretation and disease correlation

Acetazolamide causes a metabolic acidosis because it:

What do arterial blood gases usually show in the third trimester of a healthy pregnancy?

A 52-year-old with hypertension has K+ 2.9 mmol/L, HCO3- 34 mmol/L and urine chloride 60 mmol/L. Saline does not correct the alkalosis. The most likely cause is:

Poor tissue perfusion in shock most often causes which acid-base disorder?

A patient with COPD who takes a loop diuretic has arterial pH 7.41, pCO2 60 mm Hg (8.0 kPa) and HCO3- 38 mmol/L. The best interpretation is:

In acute respiratory acidosis, pCO2 rises from 40 to 70 mm Hg (5.3 to 9.3 kPa) within an hour. Starting from HCO3- 24 mmol/L, the expected bicarbonate is about:

A climber has spent 3 weeks at high altitude. pCO2 is in mm Hg and HCO3- in mmol/L. Which arterial result set fits his acid-base status?

What is the expected compensation for a primary metabolic acidosis?

pCO2 is in mm Hg and HCO3- in mmol/L. Which set shows a respiratory acidosis with partial renal compensation?

Why does bicarbonate rise more per 10 mm Hg of pCO2 in chronic than in acute respiratory acidosis?

An opioid overdose that slows breathing causes which primary acid-base disorder?

A patient has a metabolic alkalosis after five days of vomiting. Which urine result best predicts that saline will correct it?

A 30-year-old with kidney stones has a normal anion gap metabolic acidosis, K+ 2.9 mmol/L and no diarrhea. Urine pH stays above 6.0 on repeated tests, and imaging shows nephrocalcinosis. The most likely diagnosis is:

Arterial pH 7.28, pCO2 30 mm Hg (4.0 kPa), HCO3- 14 mmol/L. What is the interpretation?

A man with cirrhosis has vomited for 3 days. Arterial pH 7.65, pCO2 30 mm Hg (4.0 kPa), HCO3- 33 mmol/L. Which interpretation fits best?

A normal anion gap metabolic acidosis is present. Urine sodium 30, potassium 20 and chloride 80 mmol/L give a urine anion gap of -30 mmol/L. The most likely cause is:

A 19-year-old with type 1 diabetes has glucose 540 mg/dL (30.0 mmol/L) and positive ketones. Arterial pH 7.21, pCO2 20 mm Hg (2.7 kPa), HCO3- 8 mmol/L. How should the respiratory response be judged?

A 35-year-old arrives 4 hours after taking a large overdose of aspirin. Arterial pH 7.44, pCO2 25 mm Hg (3.3 kPa), HCO3- 17 mmol/L. These results are best explained by:

A 24-year-old woman arrives during a panic attack with tingling fingers. Arterial pH 7.50, pCO2 30 mm Hg (4.0 kPa), HCO3- 23 mmol/L. The most likely disorder is:

A patient with a pure metabolic acidosis has HCO3- 12 mmol/L. By Winter's formula, the expected pCO2 is:

A primary metabolic alkalosis begins with a primary rise in:

A man with COPD usually has pCO2 65 mm Hg (8.7 kPa) and HCO3- 36 mmol/L. Six hours after intubation, his pH is 7.54, pCO2 40 mm Hg (5.3 kPa) and HCO3- 34 mmol/L. What is the most likely disorder?