A 55-year-old woman with a BMI of 49 kg/m2 has daytime sleepiness and morning headaches. She has no lung disease, vomiting or diuretic use. Her chemistry panel shows total CO2 32 mmol/L (reference 22-29) and chloride 96 mmol/L, and the ward asks whether she has a metabolic alkalosis. Arterial results: pH 7.36 (reference 7.35-7.45), pCO2 7.6 kPa (57 mm Hg), HCO3- 31 mmol/L. For every 10 mm Hg rise in pCO2, HCO3- rises by about 1 mmol/L in acute and about 3.5 mmol/L in chronic respiratory acidosis (±2 mmol/L). Which interpretation fits best?

Chronic respiratory acidosis with expected compensation

Acute respiratory acidosis without compensation

Metabolic alkalosis with respiratory compensation

Respiratory acidosis plus metabolic alkalosis

A 34-year-old construction worker collapses on a building site after a long shift outdoors in summer. His core temperature is 41.6 °C and he is confused and breathing fast. Arterial results: pH 7.36 (reference 7.35-7.45), pCO2 2.9 kPa (22 mm Hg), HCO3- 12 mmol/L. Chemistry: sodium 140 and chloride 104 mmol/L, lactate 9.0 mmol/L (reference 0.5-2.2), CK 4,800 U/L. Use an anion gap reference of 8-16 mmol/L and Winter's formula: expected pCO2 (mm Hg) = 1.5 × HCO3- + 8 ± 2. The best interpretation is:

High anion gap acidosis plus respiratory alkalosis

High anion gap acidosis plus respiratory acidosis

High anion gap acidosis, appropriately compensated

Respiratory alkalosis with renal compensation

Acid-base disorders and blood gas interpretation

32 DOH Abu Dhabi (HAAD) lab licensing practice questions on Acid-base disorders and blood gas interpretation, part of Clinical 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 55-year-old woman with a BMI of 49 kg/m2 has daytime sleepiness and morning headaches. She has no lung disease, vomiting or diuretic use. Her chemistry panel shows total CO2 32 mmol/L (reference 22-29) and chloride 96 mmol/L, and the ward asks whether she has a metabolic alkalosis. Arterial results: pH 7.36 (reference 7.35-7.45), pCO2 7.6 kPa (57 mm Hg), HCO3- 31 mmol/L. For every 10 mm Hg rise in pCO2, HCO3- rises by about 1 mmol/L in acute and about 3.5 mmol/L in chronic respiratory acidosis (±2 mmol/L). Which interpretation fits best?
1. AAcute respiratory acidosis without compensation
2. BChronic respiratory acidosis with expected compensation
3. CMetabolic alkalosis with respiratory compensation
4. DRespiratory acidosis plus metabolic alkalosis
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Answer: B · Chronic respiratory acidosis with expected compensation
The pCO2 is high and the pH sits at the acid end of the range, so the primary disorder is a respiratory acidosis; a primary metabolic alkalosis would leave the pH alkaline. The pCO2 is 17 mm Hg above 40, so a chronic disorder should raise HCO3- by about 3.5 × 1.7 = 6 mmol/L, to about 30 (28-32), and an acute one by only about 2, to about 26. The measured 31 mmol/L fits chronic renal compensation, and the low chloride reflects bicarbonate retained in its place. No extra metabolic alkalosis is needed to explain the results. The pattern fits obesity hypoventilation, and the raised total CO2 on the panel is compensation, not a separate disorder.
- A: Acute retention would raise HCO3- only to about 26 mmol/L.
- C: Compensation does not overshoot; a primary alkalosis would make the pH alkaline.
- D: HCO3- 31 mmol/L lies within the expected 28-32, so no extra alkalosis is present.
Question 2 · Hard
A 34-year-old construction worker collapses on a building site after a long shift outdoors in summer. His core temperature is 41.6 °C and he is confused and breathing fast. Arterial results: pH 7.36 (reference 7.35-7.45), pCO2 2.9 kPa (22 mm Hg), HCO3- 12 mmol/L. Chemistry: sodium 140 and chloride 104 mmol/L, lactate 9.0 mmol/L (reference 0.5-2.2), CK 4,800 U/L. Use an anion gap reference of 8-16 mmol/L and Winter's formula: expected pCO2 (mm Hg) = 1.5 × HCO3- + 8 ± 2. The best interpretation is:
1. AHigh anion gap acidosis plus respiratory acidosis
2. BHigh anion gap acidosis plus respiratory alkalosis
3. CHigh anion gap acidosis, appropriately compensated
4. DRespiratory alkalosis with renal compensation
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Answer: B · High anion gap acidosis plus respiratory alkalosis
The anion gap is 140 − (104 + 12) = 24 mmol/L, a high anion gap acidosis explained largely by the lactate. Winter's formula predicts a pCO2 of 1.5 × 12 + 8 = 26 ± 2 mm Hg (24-28), but the measured pCO2 is 22 mm Hg, lower than compensation alone produces. A second, primary respiratory alkalosis is therefore present, driven here by the hyperthermia, which is why the pH is near normal despite a bicarbonate of 12 mmol/L. Renal compensation for a respiratory alkalosis takes days and would not raise the anion gap or the lactate. The rise in anion gap (12) matches the fall in bicarbonate (12), so no third process is hidden.
- A: A respiratory acidosis would leave the pCO2 above the predicted range, not below it.
- C: Winter's formula predicts 24-28 mm Hg; 22 mm Hg is lower than compensation explains.
- D: Renal compensation takes days and cannot explain a raised anion gap and lactate.
Question 3 · Hard
A 5-month-old boy is admitted during a hot summer with poor feeding, lethargy and poor weight gain. He has had two chest infections but no vomiting or diarrhea, and he takes no medicines. Arterial results: pH 7.55 (reference 7.35-7.45), pCO2 6.4 kPa (48 mm Hg), HCO3- 41 mmol/L (reference 22-26). Chemistry: sodium 127, potassium 2.8 and chloride 76 mmol/L. A spot urine chloride is below 10 mmol/L. For a metabolic alkalosis, expected pCO2 (mm Hg) = 40 + 0.7 × (HCO3- − 24) ± 5. Which test is most likely to identify the cause?
1. APlasma renin and aldosterone
2. BUrine diuretic screen
3. CSerum 17-hydroxyprogesterone
4. DSweat chloride test
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Answer: D · Sweat chloride test
This is a primary metabolic alkalosis: pH and HCO3- are high, and the expected pCO2 of 40 + 0.7 × 17 = 52 ± 5 mm Hg (47-57) shows that 48 mm Hg is appropriate compensation. A urine chloride below 10 mmol/L means the kidneys are conserving chloride, so it is being lost outside the kidney. With no vomiting or diarrhea, salt lost in sweat in hot weather is the likely route, and with chest infections and poor growth this points to cystic fibrosis. A sweat chloride of 60 mmol/L or more supports the diagnosis. Diuretics and Bartter-type disorders keep urine chloride high, and salt-wasting congenital adrenal hyperplasia causes hyperkalemia and acidosis, not alkalosis.
- A: Renin and aldosterone rise with any salt depletion and do not identify the cause.
- B: Diuretics keep urine chloride high while they act, and he takes no medicines.
- C: Salt-wasting CAH causes hyperkalemia and acidosis, not hypokalemic alkalosis.
Question 4 · Easy
Metabolic alkalosis is commonly accompanied by:
1. AHyperkalemia
2. BHypokalemia
3. CHyperchloremia
4. DHyperphosphatemia
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Answer: B · Hypokalemia
Many causes of metabolic alkalosis, such as vomiting, diuretics and mineralocorticoid excess, also cause renal potassium loss. Alkalosis itself shifts potassium into cells as hydrogen ions move out. Hypokalemia in turn increases renal H+ secretion and bicarbonate reabsorption, which helps maintain the alkalosis. Potassium should therefore be checked and replaced in these patients.
- A: Hyperkalemia is more typical of acidosis and kidney failure.
- C: Chloride is usually low, not high, in metabolic alkalosis.
- D: Phosphate is not characteristically raised in metabolic alkalosis.
Question 5 · Medium
A man taking large amounts of calcium carbonate antacid for dyspepsia has calcium 3.3 mmol/L (13.2 mg/dL; reference 2.15-2.55 mmol/L), HCO3- 35 mmol/L (reference 22-26), raised creatinine and a suppressed PTH. Which condition best explains the metabolic alkalosis?
1. ACushing syndrome
2. BMilk-alkali syndrome
3. CPrimary hyperparathyroidism
4. DPrimary aldosteronism
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Answer: B · Milk-alkali syndrome
Large intakes of calcium with absorbable alkali cause hypercalcemia, metabolic alkalosis and kidney injury. Hypercalcemia reduces GFR and causes volume loss, which helps the kidneys hold on to the extra bicarbonate. PTH is suppressed because the calcium comes from the diet. The disorder settles when the antacid is stopped and fluids are given.
- A: Cushing syndrome can cause alkalosis but not hypercalcemia with a suppressed PTH.
- C: Primary hyperparathyroidism has a raised PTH and tends to cause a mild hyperchloremic acidosis.
- D: Primary aldosteronism causes alkalosis with hypokalemia and hypertension, not hypercalcemia.
Question 6 · Hard
A patient with a metabolic alkalosis has HCO3- 36 mmol/L. Using expected pCO2 (mm Hg) = 40 + 0.7 × (HCO3- − 24), what pCO2 would show appropriate respiratory compensation?
1. A6.4 kPa (48 mm Hg)
2. B4.3 kPa (32 mm Hg)
3. C8.7 kPa (65 mm Hg)
4. D8.3 kPa (62 mm Hg)
Answer and explanation in LearnFree account
Question 7 · Hard
A report shows pH 7.40, pCO2 5.3 kPa (40 mm Hg) and HCO3- 15 mmol/L. Using H+ (nmol/L) = 24 × pCO2 (mm Hg) ÷ HCO3-, what is the best conclusion?
1. AMixed metabolic acidosis and respiratory alkalosis
2. BThe values do not agree, so one is likely wrong
3. CMetabolic acidosis with full compensation
4. DMetabolic acidosis with respiratory compensation
Answer and explanation in LearnFree account
Question 8 · Medium
A 64-year-old woman with type 2 diabetes takes metformin and has developed acute kidney injury. Blood pressure and oxygen saturation are normal. pH 7.18, lactate 11 mmol/L (reference 0.5-2.2), beta-hydroxybutyrate normal. Which type of acidosis is this?
1. ADiabetic ketoacidosis
2. BD-lactic acidosis
3. CType A lactic acidosis
4. DType B lactic acidosis
Answer and explanation in LearnFree account
Question 9 · Medium
A 16-year-old has had severe asthma for 6 hours and can speak only single words. Her arterial pCO2 was 3.9 kPa (29 mm Hg) on arrival and is now 5.6 kPa (42 mm Hg; reference 4.7-6.0 kPa, 35-45 mm Hg), with pH 7.37. How should the new result be interpreted?
1. ARecovery as the airways open up
2. BTiring with impending respiratory failure
3. CCompensation for a metabolic alkalosis
4. DNormal gas exchange with no concern
Answer and explanation in LearnFree account
Question 10 · Medium
In which primary disorder can compensation bring the pH back within the reference range?
1. AAcute respiratory acidosis
2. BSevere metabolic acidosis
3. CChronic respiratory alkalosis
4. DAcute respiratory alkalosis
Answer and explanation in LearnFree account
Question 11 · Medium
Advanced chronic kidney disease causes a metabolic acidosis mainly because the kidneys:
1. AProduce excess lactic acid
2. BCannot excrete enough ammonium (NH4+)
3. CRetain too much carbon dioxide
4. DLose bicarbonate as in proximal RTA
Answer and explanation in LearnFree account
Question 12 · Hard
Results: pH 7.50, pCO2 2.7 kPa (20 mm Hg), HCO3- 15, sodium 138 and chloride 90 mmol/L. Taking normal values as pH 7.40, pCO2 5.3 kPa (40 mm Hg), anion gap 12 and HCO3- 24 mmol/L, which disorders are present?
1. AHigh anion gap and normal anion gap metabolic acidosis
2. BRespiratory alkalosis with high anion gap acidosis only
3. CRespiratory alkalosis, high gap acidosis and metabolic alkalosis
4. DMetabolic acidosis with appropriate respiratory compensation
Answer and explanation in LearnFree account
Question 13 · Medium
An arterial sample shows a raised pCO2 together with a low bicarbonate. Whatever the pH, this combination indicates:
1. AMixed respiratory and metabolic acidosis
2. BMetabolic acidosis with respiratory compensation
3. CMixed respiratory and metabolic alkalosis
4. DRespiratory acidosis with renal compensation
Answer and explanation in LearnFree account
Question 14 · Easy
The kidneys compensate for a chronic respiratory acidosis by:
1. AExcreting more acid and retaining bicarbonate
2. BExcreting more bicarbonate in the urine
3. CIncreasing the rate and depth of breathing
4. DRetaining chloride in place of bicarbonate
Answer and explanation in LearnFree account
Question 15 · Medium
A 5-week-old infant with pyloric stenosis has a hypochloremic, hypokalemic metabolic alkalosis, yet the urine is acidic. The best explanation is:
1. ARespiratory compensation acidifies the urine
2. BLactic acid from dehydration is excreted
3. CSodium is reabsorbed in exchange for H+ and K+
4. DImmature kidneys cannot excrete bicarbonate
Answer and explanation in LearnFree account
Question 16 · Hard
A 70-year-old in septic shock is receiving morphine. Arterial pH 7.08, pCO2 5.6 kPa (42 mm Hg), HCO3- 12 mmol/L, lactate 8.0 mmol/L. Using Winter's formula, expected pCO2 (mm Hg) = 1.5 × HCO3- + 8 ± 2, the best interpretation is:
1. AMetabolic acidosis with appropriate compensation
2. BMetabolic acidosis with a respiratory acidosis
3. CRespiratory acidosis with renal compensation
4. DMetabolic acidosis with a respiratory alkalosis
Answer and explanation in LearnFree account
Question 17 · Hard
Reference: pH 7.35-7.45, pCO2 4.7-6.0 kPa (35-45 mm Hg), HCO3- 22-26 mmol/L, anion gap 8-16 mmol/L. Results: pH 7.40, pCO2 5.3 kPa (40 mm Hg), HCO3- 24, sodium 136 and chloride 86 mmol/L, creatinine 900 µmol/L (10.2 mg/dL). Which interpretation fits best?
1. AFully compensated respiratory acidosis
2. BHigh anion gap acidosis with metabolic alkalosis
3. CNo acid-base disorder is present
4. DNormal anion gap acidosis with respiratory alkalosis
Answer and explanation in LearnFree account
Question 18 · Medium
Blood gas analyzers report standard base excess (SBE). It is used to judge the metabolic part of an acid-base disorder because it:
1. ABecomes positive when lactic acid builds up
2. BIs measured directly with an electrode
3. CChanges little with acute changes in pCO2
4. DIncludes the respiratory part of the disorder
Answer and explanation in LearnFree account
Question 19 · Hard
A woman with short bowel syndrome has episodes of confusion and slurred speech after sugary meals. pH 7.29, HCO3- 12 mmol/L, anion gap 24 mmol/L (reference 8-16), lactate 1.2 mmol/L (reference 0.5-2.2) by the routine method, beta-hydroxybutyrate and creatinine normal. The most likely cause is:
1. ADiabetic ketoacidosis
2. BUremic acidosis
3. CD-lactic acidosis
4. DType A lactic acidosis
Answer and explanation in LearnFree account
Question 20 · Easy
Which condition typically causes a primary respiratory alkalosis?
1. AGuillain-Barré syndrome
2. BSevere COPD
3. CAcute pulmonary embolism
4. DOpioid overdose
Answer and explanation in LearnFree account
Question 21 · Hard
A patient's pCO2 has been about 4.0 kPa (30 mm Hg) for several weeks. Bicarbonate falls by 2 mmol/L per 10 mm Hg fall in pCO2 in acute respiratory alkalosis and by 5 mmol/L in chronic. What bicarbonate is expected if there is no other disorder?
1. A29 mmol/L
2. B19 mmol/L
3. C24 mmol/L
4. D22 mmol/L
Answer and explanation in LearnFree account
Question 22 · Hard
A teenager with diabetic ketoacidosis has received 0.9% saline for 10 hours. Now: pH 7.26, pCO2 3.1 kPa (23 mm Hg), sodium 140, chloride 108, HCO3- 10 mmol/L. Taking a normal anion gap as 12 and normal HCO3- as 24 mmol/L, what best describes the metabolic state?
1. AHigh anion gap acidosis with metabolic alkalosis
2. BNormal anion gap acidosis alone
3. CHigh and normal anion gap acidosis together
4. DHigh anion gap acidosis alone
Answer and explanation in LearnFree account
Question 23 · Medium
Some laboratories report hydrogen ion concentration with pH. At pH 7.40 it is 40 nmol/L. At pH 7.10 it is about:
1. A40 nmol/L
2. B80 nmol/L
3. C60 nmol/L
4. D100 nmol/L
Answer and explanation in LearnFree account
Question 24 · Hard
A malnourished 72-year-old woman recovering from sepsis has taken paracetamol 1 g four times daily for 3 weeks. She has a high anion gap acidosis (anion gap 25 mmol/L, reference 8-16) with normal lactate, beta-hydroxybutyrate, creatinine and osmolal gap; salicylate is not detected. Which test is most likely to identify the cause?
1. AUrine organic acids for 5-oxoproline
2. BSerum paracetamol concentration
3. CUrine ketones by dipstick
4. DPlasma D-lactate assay
Answer and explanation in LearnFree account
Question 25 · Easy
Renal compensation for a primary respiratory disorder is usually complete after about:
1. A6 to 12 hours
2. B3 to 5 days
3. C3 to 4 weeks
4. D10 to 30 minutes
Answer and explanation in LearnFree account

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Acid-base disorders and blood gas interpretation

Metabolic alkalosis is commonly accompanied by:

A man taking large amounts of calcium carbonate antacid for dyspepsia has calcium 3.3 mmol/L (13.2 mg/dL; reference 2.15-2.55 mmol/L), HCO3- 35 mmol/L (reference 22-26), raised creatinine and a suppressed PTH. Which condition best explains the metabolic alkalosis?

A patient with a metabolic alkalosis has HCO3- 36 mmol/L. Using expected pCO2 (mm Hg) = 40 + 0.7 × (HCO3- − 24), what pCO2 would show appropriate respiratory compensation?

A report shows pH 7.40, pCO2 5.3 kPa (40 mm Hg) and HCO3- 15 mmol/L. Using H+ (nmol/L) = 24 × pCO2 (mm Hg) ÷ HCO3-, what is the best conclusion?

A 64-year-old woman with type 2 diabetes takes metformin and has developed acute kidney injury. Blood pressure and oxygen saturation are normal. pH 7.18, lactate 11 mmol/L (reference 0.5-2.2), beta-hydroxybutyrate normal. Which type of acidosis is this?

A 16-year-old has had severe asthma for 6 hours and can speak only single words. Her arterial pCO2 was 3.9 kPa (29 mm Hg) on arrival and is now 5.6 kPa (42 mm Hg; reference 4.7-6.0 kPa, 35-45 mm Hg), with pH 7.37. How should the new result be interpreted?

In which primary disorder can compensation bring the pH back within the reference range?

Advanced chronic kidney disease causes a metabolic acidosis mainly because the kidneys:

Results: pH 7.50, pCO2 2.7 kPa (20 mm Hg), HCO3- 15, sodium 138 and chloride 90 mmol/L. Taking normal values as pH 7.40, pCO2 5.3 kPa (40 mm Hg), anion gap 12 and HCO3- 24 mmol/L, which disorders are present?

An arterial sample shows a raised pCO2 together with a low bicarbonate. Whatever the pH, this combination indicates:

The kidneys compensate for a chronic respiratory acidosis by:

A 5-week-old infant with pyloric stenosis has a hypochloremic, hypokalemic metabolic alkalosis, yet the urine is acidic. The best explanation is:

A 70-year-old in septic shock is receiving morphine. Arterial pH 7.08, pCO2 5.6 kPa (42 mm Hg), HCO3- 12 mmol/L, lactate 8.0 mmol/L. Using Winter's formula, expected pCO2 (mm Hg) = 1.5 × HCO3- + 8 ± 2, the best interpretation is:

Reference: pH 7.35-7.45, pCO2 4.7-6.0 kPa (35-45 mm Hg), HCO3- 22-26 mmol/L, anion gap 8-16 mmol/L. Results: pH 7.40, pCO2 5.3 kPa (40 mm Hg), HCO3- 24, sodium 136 and chloride 86 mmol/L, creatinine 900 µmol/L (10.2 mg/dL). Which interpretation fits best?

Blood gas analyzers report standard base excess (SBE). It is used to judge the metabolic part of an acid-base disorder because it:

A woman with short bowel syndrome has episodes of confusion and slurred speech after sugary meals. pH 7.29, HCO3- 12 mmol/L, anion gap 24 mmol/L (reference 8-16), lactate 1.2 mmol/L (reference 0.5-2.2) by the routine method, beta-hydroxybutyrate and creatinine normal. The most likely cause is:

Which condition typically causes a primary respiratory alkalosis?

A patient's pCO2 has been about 4.0 kPa (30 mm Hg) for several weeks. Bicarbonate falls by 2 mmol/L per 10 mm Hg fall in pCO2 in acute respiratory alkalosis and by 5 mmol/L in chronic. What bicarbonate is expected if there is no other disorder?

A teenager with diabetic ketoacidosis has received 0.9% saline for 10 hours. Now: pH 7.26, pCO2 3.1 kPa (23 mm Hg), sodium 140, chloride 108, HCO3- 10 mmol/L. Taking a normal anion gap as 12 and normal HCO3- as 24 mmol/L, what best describes the metabolic state?

Some laboratories report hydrogen ion concentration with pH. At pH 7.40 it is 40 nmol/L. At pH 7.10 it is about:

Renal compensation for a primary respiratory disorder is usually complete after about: