A 26-year-old woman becomes confused and then drowsy 5 days after giving birth, after relatives brought her large meat-rich meals. She says she has always disliked meat and eggs. She takes no regular drugs, and ALT, ALP, bilirubin, albumin and INR are normal. Plasma ammonia, collected on ice from a free-flowing vein and assayed within 15 minutes, is 210 µmol/L (reference 11-35 µmol/L), and a repeat sample confirms it. Venous blood gas: pH 7.49, pCO2 3.9 kPa (29 mm Hg). Plasma amino acids: glutamine high, citrulline low, arginine low. Urine orotic acid is markedly raised. The most likely cause is:

Ornithine transcarbamylase deficiency

Argininosuccinate synthetase deficiency

Hepatic encephalopathy from liver failure

Carbamoyl phosphate synthetase 1 deficiency

Which set of results is typical of laboratory tumor lysis syndrome?

Uric acid, potassium and phosphate high; calcium low

Uric acid low; potassium, phosphate and calcium high

Uric acid and calcium high; potassium and phosphate low

Uric acid, calcium and phosphate high; potassium low

A 28-year-old woman taking valproate for epilepsy becomes drowsy. ALT, ALP, bilirubin and INR are normal. Plasma ammonia, collected on ice and tested within 20 minutes, is 95 µmol/L (reference 11-35 µmol/L). What is the most likely cause?

Valproate-induced hyperammonemia

Uric acid and ammonia

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Uric acid and ammonia, part of Clinical Chemistry. The first 10 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 26-year-old woman becomes confused and then drowsy 5 days after giving birth, after relatives brought her large meat-rich meals. She says she has always disliked meat and eggs. She takes no regular drugs, and ALT, ALP, bilirubin, albumin and INR are normal. Plasma ammonia, collected on ice from a free-flowing vein and assayed within 15 minutes, is 210 µmol/L (reference 11-35 µmol/L), and a repeat sample confirms it. Venous blood gas: pH 7.49, pCO2 3.9 kPa (29 mm Hg). Plasma amino acids: glutamine high, citrulline low, arginine low. Urine orotic acid is markedly raised. The most likely cause is:
1. AArgininosuccinate synthetase deficiency
2. BOrnithine transcarbamylase deficiency
3. CHepatic encephalopathy from liver failure
4. DCarbamoyl phosphate synthetase 1 deficiency
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Answer: B · Ornithine transcarbamylase deficiency
The sample was handled correctly and the result was confirmed, so the hyperammonemia is real, and normal liver tests and INR rule out liver failure as its cause. The respiratory alkalosis is typical, because ammonia stimulates breathing. Low citrulline places the block before citrulline is made, at carbamoyl phosphate synthetase 1 (CPS1) or ornithine transcarbamylase (OTC); the high orotic acid separates them, because in OTC deficiency unused carbamoyl phosphate spills into pyrimidine synthesis. OTC deficiency is X-linked, and carrier women can present for the first time under a protein load or the catabolic stress of childbirth; a lifelong dislike of protein-rich food is a common clue. Argininosuccinate synthetase deficiency (citrullinemia type 1) would give very high citrulline.
- A: Argininosuccinate synthetase deficiency gives markedly raised citrulline.
- C: Her liver tests and INR are normal, so liver failure does not explain the ammonia.
- D: CPS1 deficiency also lowers citrulline but gives low or normal orotic acid.
Question 2 · Medium
Which set of results is typical of laboratory tumor lysis syndrome?
1. AUric acid low; potassium, phosphate and calcium high
2. BUric acid and calcium high; potassium and phosphate low
3. CUric acid, calcium and phosphate high; potassium low
4. DUric acid, potassium and phosphate high; calcium low
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Answer: D · Uric acid, potassium and phosphate high; calcium low
When large numbers of tumor cells die, they release nucleic acids (broken down to uric acid), potassium and phosphate. Phosphate binds calcium and precipitates as calcium phosphate, so serum calcium falls. Uric acid and calcium phosphate crystals can cause acute kidney injury, which worsens the high potassium and phosphate. Tumor lysis is most common after starting treatment for high-grade lymphomas and acute leukemias.
- A: Uric acid rises in tumor lysis, and calcium falls.
- B: In tumor lysis, calcium falls because phosphate binds it, and potassium and phosphate rise.
- C: Potassium rises in tumor lysis, and calcium falls.
Question 3 · Hard
A 6-year-old boy in an Abu Dhabi hospital is starting treatment for acute lymphoblastic leukemia with a very high white cell count. Uric acid is 690 µmol/L (11.6 mg/dL) (reference 120-330 µmol/L), and the team plans to give rasburicase. Which test should be checked before the first dose?
1. ADirect antiglobulin test
2. BSerum ferritin
3. CHemoglobin electrophoresis
4. DG6PD activity
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Answer: D · G6PD activity
Rasburicase converts uric acid to allantoin and produces hydrogen peroxide. In G6PD deficiency, red cells cannot make enough NADPH to neutralize this oxidant, so severe hemolysis and methemoglobinemia can follow. The product information advises screening patients at risk, and G6PD deficiency is common in the UAE. If the boy is deficient, hydration and allopurinol are used instead.
- A: The DAT detects antibody-coated red cells; rasburicase hemolysis is oxidative, not immune.
- B: Ferritin reflects iron stores and has no bearing on rasburicase safety.
- C: Hemoglobin electrophoresis detects hemoglobin variants, not the enzyme defect that rasburicase exposes.
Question 4 · Medium
A 28-year-old woman taking valproate for epilepsy becomes drowsy. ALT, ALP, bilirubin and INR are normal. Plasma ammonia, collected on ice and tested within 20 minutes, is 95 µmol/L (reference 11-35 µmol/L). What is the most likely cause?
1. AAcute liver failure
2. BDelayed sample processing
3. CValproate-induced hyperammonemia
4. DRecent heavy exercise
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Answer: C · Valproate-induced hyperammonemia
Valproate can raise ammonia even when liver tests are normal, mainly by reducing activity at the first step of the urea cycle (carbamoyl phosphate synthetase I, through lower N-acetylglutamate). Drowsiness or confusion in a patient on valproate should prompt an ammonia level. Here the sample was handled correctly, so an in vitro rise is unlikely. A normal INR and normal liver enzymes argue against liver failure.
- A: Acute liver failure would prolong the INR and usually raise the aminotransferases.
- B: The sample was iced and tested within 20 minutes, so an in vitro rise is unlikely.
- D: Exercise raises ammonia only briefly and would not explain drowsiness.
Question 5 · Easy
Which collection tube gives a falsely high plasma ammonia, because its anticoagulant contains ammonium ions?
1. ASodium fluoride-oxalate
2. BLithium heparin
3. CAmmonium heparin
4. DPotassium EDTA
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Answer: C · Ammonium heparin
Ammonium heparin adds ammonium to the plasma, so the measured ammonia is falsely high; the same tube also interferes with urease methods for urea. Lithium heparin or EDTA plasma is used instead. The tube should be filled without a prolonged tourniquet, placed on ice, and separated and tested promptly. Serum is unsuitable because ammonia rises while the sample clots.
- A: Fluoride-oxalate contains no ammonium salt, so it cannot add ammonia to the sample.
- B: Lithium heparin is an accepted ammonia tube if the sample is iced and tested promptly.
- D: EDTA plasma is an accepted ammonia specimen and adds no ammonium.
Question 6 · Easy
Ammonia absorbed from the gut reaches the liver in portal blood. A healthy liver removes most of it by converting it to:
1. ACreatinine
2. BCreatine
3. CUric acid
4. DUrea
Answer and explanation in LearnFree account
Question 7 · Medium
Which cause of hyperuricemia acts mainly by reducing renal excretion of uric acid rather than by increasing its production?
1. AExtensive psoriasis
2. BTumor lysis after chemotherapy
3. CThiazide diuretic therapy
4. DPolycythemia vera
Answer and explanation in LearnFree account
Question 8 · Easy
Allopurinol lowers serum uric acid in gout by inhibiting which enzyme?
1. AHGPRT
2. BXanthine oxidase
3. CUrate oxidase
4. DAdenosine deaminase
Answer and explanation in LearnFree account
Question 9 · Hard
A 50-year-old man with recurrent gout and a uric acid kidney stone has serum uric acid 540 µmol/L (9.1 mg/dL) (reference 200-420 µmol/L). On his usual diet, 24-hour urine uric acid is 6.5 mmol/day (1,090 mg/day); the laboratory flags values above 4.8 mmol/day (800 mg/day) as overproduction. These results best support:
1. ANormal excretion; dietary purine restriction alone is enough
2. BOverproducer; a uricosuric such as probenecid is preferred
3. COverproducer; a xanthine oxidase inhibitor is preferred
4. DUnderexcreter; a uricosuric such as probenecid is preferred
Answer and explanation in LearnFree account
Question 10 · Easy
Rasburicase, given to prevent tumor lysis syndrome, lowers plasma uric acid by converting it to:
1. AAllantoin
2. BHypoxanthine
3. CUrea
4. DXanthine
Answer and explanation in LearnFree account

Uric acid and ammonia

Which set of results is typical of laboratory tumor lysis syndrome?

A 6-year-old boy in an Abu Dhabi hospital is starting treatment for acute lymphoblastic leukemia with a very high white cell count. Uric acid is 690 µmol/L (11.6 mg/dL) (reference 120-330 µmol/L), and the team plans to give rasburicase. Which test should be checked before the first dose?

A 28-year-old woman taking valproate for epilepsy becomes drowsy. ALT, ALP, bilirubin and INR are normal. Plasma ammonia, collected on ice and tested within 20 minutes, is 95 µmol/L (reference 11-35 µmol/L). What is the most likely cause?

Which collection tube gives a falsely high plasma ammonia, because its anticoagulant contains ammonium ions?

Ammonia absorbed from the gut reaches the liver in portal blood. A healthy liver removes most of it by converting it to:

Which cause of hyperuricemia acts mainly by reducing renal excretion of uric acid rather than by increasing its production?

Allopurinol lowers serum uric acid in gout by inhibiting which enzyme?

Rasburicase, given to prevent tumor lysis syndrome, lowers plasma uric acid by converting it to: