A girl born at 29 weeks' gestation has received parenteral nutrition with amino acids since the first day of life. Her newborn screen, collected at 36 hours and run by tandem mass spectrometry, shows (µmol/L, program cut-offs in brackets): phenylalanine 205 (below 130), tyrosine 160 (below 300), phenylalanine/tyrosine ratio 1.3 (below 2.0), leucine/isoleucine 410 (below 300), valine 350 (below 280), methionine 88 (below 70). Second-tier testing finds no alloisoleucine, and succinylacetone is normal. The most likely explanation is:

Amino acids from parenteral nutrition

Phenylalanine hydroxylase deficiency

Two samples are drawn together from a patient receiving rasburicase. One is kept on ice and tested at once: uric acid 3.8 mg/dL [226 µmol/L]. The other stands at room temperature for 3 hours: 0.6 mg/dL [36 µmol/L]. The best explanation is:

The drug kept breaking down urate in the warm tube

Bacteria in the warm tube consumed the urate

Red cell glycolysis used up urate in the warm tube

Cold storage released urate from red cells

In the coupled urease-glutamate dehydrogenase method for BUN, the first enzyme, urease, converts urea into:

Ammonia (as ammonium) and carbon dioxide

Allantoin and hydrogen peroxide

Most US newborn screening programs now detect phenylketonuria by measuring phenylalanine in dried blood spots with:

Tandem mass spectrometry (MS/MS)

Nitrogen compounds: urea, creatinine, uric acid, ammonia, amino acids

15 ASCP MLS practice questions on Nitrogen compounds: urea, creatinine, uric acid, ammonia, amino acids, part of Chemistry. The first 15 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 girl born at 29 weeks' gestation has received parenteral nutrition with amino acids since the first day of life. Her newborn screen, collected at 36 hours and run by tandem mass spectrometry, shows (µmol/L, program cut-offs in brackets): phenylalanine 205 (below 130), tyrosine 160 (below 300), phenylalanine/tyrosine ratio 1.3 (below 2.0), leucine/isoleucine 410 (below 300), valine 350 (below 280), methionine 88 (below 70). Second-tier testing finds no alloisoleucine, and succinylacetone is normal. The most likely explanation is:
1. AMaple syrup urine disease
2. BPhenylalanine hydroxylase deficiency
3. CAmino acids from parenteral nutrition
4. DClassic homocystinuria
Show answerHide answer
Answer: C · Amino acids from parenteral nutrition
Several unrelated amino acids are raised together while the phenylalanine/tyrosine ratio stays normal. That pattern points to an amino acid load from outside, here parenteral nutrition, rather than a single enzyme block. In PKU, phenylalanine rises with low or normal tyrosine, so the ratio is high; in maple syrup urine disease, leucine, isoleucine and valine rise and alloisoleucine is present; in homocystinuria, methionine rises on its own. A repeat specimen is collected once parenteral nutrition is stopped or reduced, and preterm infants usually have repeat screens anyway. The ratio is reported because it cuts false positives from such nonspecific rises.
- A: Maple syrup urine disease does not raise phenylalanine, and alloisoleucine is absent.
- B: PKU raises phenylalanine alone, with a high phenylalanine/tyrosine ratio.
- D: Homocystinuria raises methionine alone, not the branched-chain amino acids or phenylalanine.
Question 2 · Hard
Two samples are drawn together from a patient receiving rasburicase. One is kept on ice and tested at once: uric acid 3.8 mg/dL [226 µmol/L]. The other stands at room temperature for 3 hours: 0.6 mg/dL [36 µmol/L]. The best explanation is:
1. ABacteria in the warm tube consumed the urate
2. BRed cell glycolysis used up urate in the warm tube
3. CCold storage released urate from red cells
4. DThe drug kept breaking down urate in the warm tube
Show answerHide answer
Answer: D · The drug kept breaking down urate in the warm tube
Rasburicase is a recombinant urate oxidase. It stays active in the blood sample and keeps converting uric acid to allantoin at room temperature, giving falsely low results. Samples should be collected into pre-chilled heparin tubes, kept in an ice-water bath and tested within 4 hours. The iced sample reflects the patient's true level.
- A: Bacterial growth in 3 hours is negligible and could not remove most of the urate.
- B: Glycolysis uses glucose, not urate.
- C: Chilling does not make red cells release large amounts of urate.
Question 3 · Easy
In the coupled urease-glutamate dehydrogenase method for BUN, the first enzyme, urease, converts urea into:
1. AGlycine and formaldehyde
2. BGlutamate and NAD+
3. CAmmonia (as ammonium) and carbon dioxide
4. DAllantoin and hydrogen peroxide
Show answerHide answer
Answer: C · Ammonia (as ammonium) and carbon dioxide
Urease hydrolyzes urea to ammonia (as ammonium) and carbon dioxide. Glutamate dehydrogenase then joins the ammonium to 2-oxoglutarate to form glutamate, oxidizing NADH to NAD+. The fall in absorbance at 340 nm is proportional to the urea. Ammonium already in the sample, or added by an ammonium heparin tube, adds to the signal.
- A: Glycine and formaldehyde come from sarcosine oxidase in enzymatic creatinine methods.
- B: Glutamate and NAD+ are products of the second, glutamate dehydrogenase step.
- D: Allantoin and hydrogen peroxide come from uricase acting on uric acid.
Question 4 · Medium
Which blood collection tube can falsely raise a BUN measured by the urease-glutamate dehydrogenase method?
1. ASodium citrate
2. BAmmonium heparin
3. CLithium heparin
4. DPotassium EDTA
Show answerHide answer
Answer: B · Ammonium heparin
The urease method measures ammonium produced from urea, so any ammonium already in the sample adds to the result. Ammonium heparin tubes therefore give a falsely high BUN. Lithium heparin plasma or serum is suitable. The same tube must also be avoided for plasma ammonia.
- A: Sodium citrate dilutes the sample and would lower, not raise, the result.
- C: Lithium heparin is the usual chemistry plasma tube and adds no ammonium.
- D: EDTA is mainly a hematology tube and does not add ammonium.
Question 5 · Easy
Most US newborn screening programs now detect phenylketonuria by measuring phenylalanine in dried blood spots with:
1. ATandem mass spectrometry (MS/MS)
2. BThin-layer chromatography
3. CA bacterial inhibition assay
4. DThe urine ferric chloride test
Show answerHide answer
Answer: A · Tandem mass spectrometry (MS/MS)
Tandem mass spectrometry (MS/MS) measures phenylalanine and tyrosine, along with many other amino acids and acylcarnitines, from one dried blood spot. A raised phenylalanine with a high phenylalanine/tyrosine ratio suggests PKU. MS/MS has largely replaced the Guthrie bacterial inhibition assay. The spot is usually collected at 24 to 48 hours of age, after feeding has begun.
- B: Thin-layer chromatography is not used for population newborn screening.
- C: The Guthrie bacterial inhibition assay was the original screen but has been largely replaced.
- D: The ferric chloride urine test is an old, insensitive test for phenylketones.
Question 6 · Easy
Which enzyme is used in the common enzymatic method for plasma ammonia?
1. AAlanine aminotransferase
2. BUrease
3. CGlutamate dehydrogenase
4. DUricase
Answer and explanation in LearnFree account
Question 7 · Easy
In the United States, routine creatinine methods are calibrated so that results are traceable to which reference method?
1. ACompensated kinetic Jaffe method
2. BIsotope dilution mass spectrometry (IDMS)
3. CJaffe reaction with Lloyd's reagent
4. DEnzymatic creatininase method
Answer and explanation in LearnFree account
Question 8 · Medium
In the kinetic Jaffe method for creatinine, the rate is read between about 20 and 80 seconds after mixing. Why does this window improve specificity?
1. ANon-creatinine chromogens are adsorbed first
2. BCreatinine finishes reacting in the first 20 s
3. CFast and slow interferents react outside it
4. DPicrate is fully consumed by 80 seconds
Answer and explanation in LearnFree account
Question 9 · Hard
In an enzymatic creatinine method, the sample is first incubated with creatinase, sarcosine oxidase and catalase. Creatininase and the peroxidase color reagent are added only afterwards. What is the purpose of the first step?
1. ATo remove endogenous creatine
2. BTo destroy ascorbic acid
3. CTo remove acetoacetate
4. DTo convert creatinine to creatine
Answer and explanation in LearnFree account
Question 10 · Medium
A BUN result of 21 mg/dL [7.5 mmol/L] needs to be reported as urea in mg/dL. What is the urea value?
1. A9.8 mg/dL
2. B21 mg/dL
3. C7.5 mg/dL
4. D45 mg/dL
Answer and explanation in LearnFree account
Question 11 · Medium
A uricase method measures the hydrogen peroxide formed with a peroxidase-coupled color reaction. Which substance in the sample gives a falsely low uric acid?
1. AAscorbic acid
2. BCefoxitin
3. CAmmonium ions
4. DAcetoacetate
Answer and explanation in LearnFree account
Question 12 · Hard
An ICU patient on a dobutamine infusion has an enzymatic creatinine of 0.3 mg/dL [27 µmol/L]; the day before it was 1.9 mg/dL [168 µmol/L]. The same sample gives 1.8 mg/dL [159 µmol/L] by a Jaffe method. The most likely cause is:
1. AOvernight recovery of kidney function
2. BHemodilution by the intravenous infusion
3. CDobutamine interfering in the peroxidase reaction
4. DAcetoacetate falsely raising the Jaffe result
Answer and explanation in LearnFree account
Question 13 · Easy
In the direct ultraviolet uricase method, uric acid is measured as:
1. AA rise in absorbance at 293 nm
2. BA fall in absorbance at 340 nm
3. CA rise in absorbance at 500 nm
4. DA fall in absorbance at 293 nm
Answer and explanation in LearnFree account
Question 14 · Medium
Which substance can falsely increase creatinine measured by the Jaffe (alkaline picrate) reaction?
1. ALipemia removed by ultracentrifugation
2. BAcetoacetate in ketoacidosis
3. CA high bilirubin
4. DA low total protein
Answer and explanation in LearnFree account
Question 15 · Medium
How should a blood sample for ammonia be handled?
1. ALeave at room temperature until fully clotted
2. BCollect just after a meal
3. CCollect in a gel serum tube and store overnight
4. DCollect on ice and analyze promptly
Answer and explanation in LearnFree account

Nitrogen compounds: urea, creatinine, uric acid, ammonia, amino acids

Two samples are drawn together from a patient receiving rasburicase. One is kept on ice and tested at once: uric acid 3.8 mg/dL [226 µmol/L]. The other stands at room temperature for 3 hours: 0.6 mg/dL [36 µmol/L]. The best explanation is:

In the coupled urease-glutamate dehydrogenase method for BUN, the first enzyme, urease, converts urea into:

Which blood collection tube can falsely raise a BUN measured by the urease-glutamate dehydrogenase method?

Most US newborn screening programs now detect phenylketonuria by measuring phenylalanine in dried blood spots with:

Which enzyme is used in the common enzymatic method for plasma ammonia?

In the United States, routine creatinine methods are calibrated so that results are traceable to which reference method?

In the kinetic Jaffe method for creatinine, the rate is read between about 20 and 80 seconds after mixing. Why does this window improve specificity?

In an enzymatic creatinine method, the sample is first incubated with creatinase, sarcosine oxidase and catalase. Creatininase and the peroxidase color reagent are added only afterwards. What is the purpose of the first step?

A BUN result of 21 mg/dL [7.5 mmol/L] needs to be reported as urea in mg/dL. What is the urea value?

A uricase method measures the hydrogen peroxide formed with a peroxidase-coupled color reaction. Which substance in the sample gives a falsely low uric acid?

An ICU patient on a dobutamine infusion has an enzymatic creatinine of 0.3 mg/dL [27 µmol/L]; the day before it was 1.9 mg/dL [168 µmol/L]. The same sample gives 1.8 mg/dL [159 µmol/L] by a Jaffe method. The most likely cause is:

In the direct ultraviolet uricase method, uric acid is measured as:

Which substance can falsely increase creatinine measured by the Jaffe (alkaline picrate) reaction?

How should a blood sample for ammonia be handled?