A patient on lithium has serum lithium 3.9 mmol/L (therapeutic 0.6-1.2) but is alert with no tremor. The sample, drawn 12 hours after the dose, was in a green-top lithium heparin tube. What is the most likely explanation?

Contamination from the lithium heparin

Sample taken too soon after the dose

A positive urine opiate screen by immunoassay is best confirmed by:

Gas chromatography-mass spectrometry

Thin-layer chromatography alone

For digoxin monitoring, the blood sample should be collected:

At least 6 hours after the dose

At any time; timing does not matter

Toxicology and drug screening

14 Kerala PSC Lab Technician Gr II practice questions on Toxicology and drug screening, part of Biochemistry. The first 14 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 · Easy
A violet colour on adding Trinder's reagent (a ferric salt reagent) to urine indicates:
1. ABarbiturates
2. BOpiates
3. CSalicylates
4. DBenzodiazepines
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Answer: C · Salicylates
Trinder's reagent contains ferric nitrate and mercuric chloride in acid, and the ferric ion forms a violet complex with salicylate. It is a quick bench screen for aspirin poisoning in urine or serum. Acetoacetate also gives a reddish colour with ferric salts, so a positive screen is confirmed with a quantitative serum salicylate level. The serum level also guides treatment such as urine alkalinisation.
- A: Barbiturates give no ferric colour and are found by immunoassay or chromatography.
- B: Opiates give no ferric colour and are found by immunoassay screening.
- D: Benzodiazepines are found by immunoassay screening.
Question 2 · Hard
A patient on lithium has serum lithium 3.9 mmol/L (therapeutic 0.6-1.2) but is alert with no tremor. The sample, drawn 12 hours after the dose, was in a green-top lithium heparin tube. What is the most likely explanation?
1. ALithium released by hemolysis
2. BTrue toxicity needing dialysis
3. CSample taken too soon after the dose
4. DContamination from the lithium heparin
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Answer: D · Contamination from the lithium heparin
A level of 3.9 mmol/L would normally cause marked tremor, confusion or worse, so a well patient points to a sample problem. Lithium heparin adds lithium to the specimen, and a small or underfilled sample raises the measured value strongly. Twelve hours after the dose is the standard sampling time, so early sampling does not explain it. Lithium must be measured in a plain (serum) tube or a sodium heparin tube, and the test is repeated on a correct sample.
- A: Red cells hold less lithium than plasma, so hemolysis does not raise the level.
- B: The clinical picture does not fit a true level of 3.9 mmol/L; repeat before treating.
- C: Twelve hours after the dose is the standard sampling time.
Question 3 · Medium
A positive urine opiate screen by immunoassay is best confirmed by:
1. AA second rapid card test
2. BGas chromatography-mass spectrometry
3. CRepeating the same immunoassay
4. DThin-layer chromatography alone
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Answer: B · Gas chromatography-mass spectrometry
Immunoassay screens are fast, but other compounds can cross-react; for example, some quinolone antibiotics can give false-positive opiate screens. Confirmation needs a highly specific method based on a different principle, and GC-MS (or LC-MS/MS) identifies the drug by its retention time and mass spectrum. Repeating the same immunoassay only repeats the same cross-reaction. Thin-layer chromatography is useful for screening but is less sensitive and less specific.
- A: A second rapid card test uses the same antibody-based principle.
- C: Repeating the same immunoassay repeats any cross-reaction.
- D: Thin-layer chromatography is less sensitive and less specific than GC-MS.
Question 4 · Easy
Low serum cholinesterase activity supports poisoning with which agent?
1. ABarbiturate sedatives
2. BOrganophosphates
3. CMethyl alcohol
4. DParacetamol
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Answer: B · Organophosphates
Organophosphate and carbamate insecticides inhibit acetylcholinesterase, causing salivation, sweating, pinpoint pupils and bradycardia. Serum (pseudo)cholinesterase is inhibited at the same time and is easy to measure, so a low level supports the diagnosis; red cell acetylcholinesterase is the more specific test. Serial activity also helps to follow recovery. Low pseudocholinesterase also occurs in liver disease and in inherited variants.
- A: Barbiturates are found by drug screening; they do not inhibit cholinesterase.
- C: Methanol poisoning causes high anion and osmolal gaps, not low cholinesterase.
- D: Paracetamol poisoning is assessed with a timed serum paracetamol level and liver tests.
Question 5 · Medium
For digoxin monitoring, the blood sample should be collected:
1. AAt least 6 hours after the dose
2. BWithin 1 hour after the dose
3. CAt any time; timing does not matter
4. DImmediately after the dose
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Answer: A · At least 6 hours after the dose
After an oral dose, digoxin takes about 6-8 hours to distribute from the blood into tissues such as the heart. A sample taken earlier shows a falsely high level that does not reflect the concentration at the site of action. Samples are therefore taken at least 6 hours after the dose, usually just before the next one. The time of the last dose should be written on the request form.
- B: At 1 hour digoxin is still distributing, so the level is falsely high.
- C: Timing matters greatly for digoxin because of its slow distribution.
- D: Straight after a dose the level is still rising and distributing, so it is misleading.
Question 6 · Medium
In suspected chronic arsenic poisoning, which specimens show exposure over the past several months?
1. AHair and nails
2. BBlood
3. CCerebrospinal fluid
4. DSaliva
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Question 7 · Medium
An immunoassay urine drug screen reports a sample as 'negative' for opiates. This means the opiate concentration was:
1. AZero in the sample
2. BBelow the assay's cut-off
3. CAbove the cut-off but unconfirmed
4. DToo high for the assay to read
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Question 8 · Hard
A driver who denies drinking has blood ethanol 30 mg/dL by gas chromatography. The skin was cleaned with a rectified spirit swab, and blood was drawn before it dried. Repeat blood, taken after an aqueous povidone-iodine swab, has no ethanol. What explains the first result?
1. ALoss of ethanol from the second tube
2. BRecent alcohol drinking
3. CEthanol made by gut fermentation
4. DContamination from the spirit swab
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Question 9 · Easy
A family is found drowsy in a closed room with a charcoal stove burning. Which blood test best confirms the likely poisoning?
1. ABlood alcohol
2. BCarboxyhemoglobin
3. CMethemoglobin
4. DSerum cholinesterase
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Question 10 · Easy
A farmer has swallowed a weedkiller. His urine turns blue when sodium dithionite in alkali is added. Which poison does this indicate?
1. AAluminium phosphide
2. BCarbamate
3. CParaquat
4. DOrganophosphate
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Question 11 · Hard
A confused man has pH 7.12, bicarbonate 8 mmol/L, a high anion gap and a raised osmolar gap. His urine shows many envelope-shaped and needle-shaped crystals. Which poisoning is most likely?
1. ASalicylate
2. BParaquat
3. CMethanol
4. DEthylene glycol
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Question 12 · Easy
Blood for ethanol estimation is best collected in a tube containing:
1. ALithium heparin
2. BSodium fluoride
3. CTrisodium citrate
4. DNo additive
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Question 13 · Easy
After a paracetamol overdose, the serum level used with the treatment nomogram is drawn no earlier than:
1. A4 hours after ingestion
2. B24 hours after ingestion
3. C1 hour after ingestion
4. D30 minutes after ingestion
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Question 14 · Medium
In methanol poisoning, which metabolite causes the metabolic acidosis and damage to the optic nerve?
1. AFormic acid
2. BAcetic acid
3. COxalic acid
4. DLactic acid
Answer and explanation in LearnFree account

Toxicology and drug screening

A violet colour on adding Trinder's reagent (a ferric salt reagent) to urine indicates:

A patient on lithium has serum lithium 3.9 mmol/L (therapeutic 0.6-1.2) but is alert with no tremor. The sample, drawn 12 hours after the dose, was in a green-top lithium heparin tube. What is the most likely explanation?

A positive urine opiate screen by immunoassay is best confirmed by:

Low serum cholinesterase activity supports poisoning with which agent?

For digoxin monitoring, the blood sample should be collected:

In suspected chronic arsenic poisoning, which specimens show exposure over the past several months?

An immunoassay urine drug screen reports a sample as 'negative' for opiates. This means the opiate concentration was:

A driver who denies drinking has blood ethanol 30 mg/dL by gas chromatography. The skin was cleaned with a rectified spirit swab, and blood was drawn before it dried. Repeat blood, taken after an aqueous povidone-iodine swab, has no ethanol. What explains the first result?

A family is found drowsy in a closed room with a charcoal stove burning. Which blood test best confirms the likely poisoning?

A farmer has swallowed a weedkiller. His urine turns blue when sodium dithionite in alkali is added. Which poison does this indicate?

A confused man has pH 7.12, bicarbonate 8 mmol/L, a high anion gap and a raised osmolar gap. His urine shows many envelope-shaped and needle-shaped crystals. Which poisoning is most likely?

Blood for ethanol estimation is best collected in a tube containing:

After a paracetamol overdose, the serum level used with the treatment nomogram is drawn no earlier than:

In methanol poisoning, which metabolite causes the metabolic acidosis and damage to the optic nerve?