A 31-year-old man from Bangladesh, who returned to Abu Dhabi 5 days ago after a family visit, has had fever, headache and muscle pain for 2 days. CBC: WBC 2.8 × 10^9/L (reference 4.0-11.0 × 10^9/L); platelets 118 × 10^9/L (reference 150-400 × 10^9/L). A combined rapid dengue cassette gives: NS1 antigen positive; IgM negative; IgG negative. The control line is valid, and a laboratory NS1 ELISA on the same serum is also positive. What is the best interpretation?

Past dengue infection, now immune

Acute secondary dengue infection

The amplification plot shown is from a herpes simplex virus PCR run that included a patient's CSF. What should be done with the patient's result?

Reject the run and repeat the test

Report the specimen as inhibited

A MALDI-TOF system uses score cut-offs of 2.00 or more for high confidence, 1.70-1.99 for low confidence and below 1.70 for no reliable identification. A colony smeared directly on the target scores 1.84 for Klebsiella pneumoniae. What is the best next step?

Re-spot it with an on-target formic acid extraction

Subculture it for 7 days before repeating the test

Report no identification and refer the isolate

Report Klebsiella pneumoniae to the clinician

Rapid tests and molecular microbiology

12 DOH Abu Dhabi (HAAD) lab licensing practice questions on Rapid tests and molecular microbiology, part of Microbiology. The first 12 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 31-year-old man from Bangladesh, who returned to Abu Dhabi 5 days ago after a family visit, has had fever, headache and muscle pain for 2 days. CBC: WBC 2.8 × 10^9/L (reference 4.0-11.0 × 10^9/L); platelets 118 × 10^9/L (reference 150-400 × 10^9/L). A combined rapid dengue cassette gives: NS1 antigen positive; IgM negative; IgG negative. The control line is valid, and a laboratory NS1 ELISA on the same serum is also positive. What is the best interpretation?
1. APast dengue infection, now immune
2. BEarly primary dengue infection
3. CAcute secondary dengue infection
4. DLate primary dengue infection
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Answer: B · Early primary dengue infection
NS1 is a viral protein secreted into the blood from the first day of fever and is usually detectable for about the first 5 to 7 days. IgM appears from about day 4 or 5 of illness, and in a primary infection IgG appears only after about 10 to 14 days. On day 2, a positive NS1 with negative IgM and IgG therefore fits an early primary infection. In a secondary infection, IgG rises quickly within the first days of fever, and NS1 is detected less often. The low WBC and mild thrombocytopenia fit dengue, and the platelet count should be followed through the critical phase.
- A: Past infection would give IgG without NS1 antigen.
- C: Secondary infection gives a rapid, strong IgG response in the first days of fever.
- D: Late primary infection shows IgM, and NS1 often becomes negative.
Question 2 · Easy
Three nasopharyngeal specimens were tested on the same lot of a rapid RSV antigen cassette, as shown. Which cassette's result is invalid?
Figure: MedExams
1. ACassettes 2 and 3
2. BCassette 1
3. CCassette 2
4. DCassette 3
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Answer: D · Cassette 3
Cassette 3 has a test line but no control line. The control line shows that the specimen flowed along the strip and that the labelled reagents worked, so without it no result can be reported, not even a positive one, and the test is repeated with a new cassette. Cassette 1, with both lines, is positive even though its test line is fainter than the control line. Cassette 2, with the control line only, is a valid negative.
- A: Cassette 2 is valid; only cassette 3 lacks the control line.
- B: Both lines are present, so it is a valid positive.
- C: A control line alone is a valid negative result.
Question 3 · Medium
The amplification plot shown is from a herpes simplex virus PCR run that included a patient's CSF. What should be done with the patient's result?
Figure: MedExams
1. AReport the patient as negative
2. BReject the run and repeat the test
3. CReport the specimen as inhibited
4. DReport the patient as positive
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Answer: B · Reject the run and repeat the test
The no-template control, which holds only water and reagents, crosses the threshold at about cycle 36, so target DNA has contaminated the reagents or the run. The patient's target signal (Ct about 31) could come from the same contamination, so it cannot be reported. The positive control and the patient's internal control both amplified, so the problem is contamination, not reagent failure or inhibition. The run is rejected, the source of contamination such as amplicon carry-over is found, and the specimen is retested with fresh reagents.
- A: The patient's target crossed the threshold, so a negative report is not supported.
- C: The internal control amplified normally, so the specimen is not inhibited.
- D: The patient's signal may be contamination, since the no-template control amplified.
Question 4 · Medium
A MALDI-TOF system uses score cut-offs of 2.00 or more for high confidence, 1.70-1.99 for low confidence and below 1.70 for no reliable identification. A colony smeared directly on the target scores 1.84 for Klebsiella pneumoniae. What is the best next step?
1. ASubculture it for 7 days before repeating the test
2. BReport no identification and refer the isolate
3. CReport Klebsiella pneumoniae to the clinician
4. DRe-spot it with an on-target formic acid extraction
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Answer: D · Re-spot it with an on-target formic acid extraction
A score of 1.84 is in the low-confidence band, so the species call should not be reported as it stands. The usual fix is to re-spot the colony with an on-target formic acid overlay, which breaks the cell wall and releases more ribosomal proteins, improving the spectrum. A thin, even smear from a fresh, pure colony also helps. Only if the score stays low is the isolate identified by another method or referred.
- A: Prolonged incubation gives old colonies, which give poorer spectra.
- B: Referral is premature; a simple extraction usually gives a reliable score.
- C: A low-confidence score does not support reporting the species.
Question 5 · Hard
A rapid influenza antigen test has a sensitivity of 70% and a specificity of 98%. In summer, local surveillance suggests 0.5% of the patients tested have influenza. About what proportion of positive results will be true positives?
1. AAbout 98%
2. BAbout 70%
3. CAbout 85%
4. DAbout 15%
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Answer: D · About 15%
Work per 10,000 patients: 0.5% prevalence gives 50 with influenza, and 70% sensitivity detects 35 of them. Of the 9,950 without influenza, 2% (199) test falsely positive. Positive predictive value = 35 / (35 + 199) = 35 / 234, about 15%. Out of season most positive rapid results are false, so they should be confirmed by RT-PCR.
- A: 98% is the specificity, which applies to people without influenza.
- B: 70% is the sensitivity, not the chance that a positive result is true.
- C: 85% is the share of positives that are false, the inverse of the answer.
Question 6 · Easy
In MALDI-TOF identification, the matrix solution placed over the smeared colony:
1. AFixes and stains the cells so they can be counted
2. BAbsorbs the laser energy and helps ionize the proteins
3. CSeparates the proteins by size before the laser fires
4. DBinds species-specific antibodies to the proteins
Answer and explanation in LearnFree account
Question 7 · Easy
In a lateral flow (immunochromatographic) antigen test, the appearance of the control line shows that:
1. AThe patient's sample has no interfering antibodies
2. BThe antigen level is above the clinical cut-off
3. CThe patient's sample contains the target antigen
4. DThe sample flowed and the reagents worked
Answer and explanation in LearnFree account
Question 8 · Hard
A SARS-CoV-2 RT-PCR on a nasopharyngeal swab gives: N gene target not detected; human RNase P internal control not detected. The run's positive and negative controls are acceptable. How should the result be handled?
1. AReport it as invalid and re-extract or recollect
2. BReport it as SARS-CoV-2 detected at a low level
3. CReport it as SARS-CoV-2 not detected
4. DRepeat the run controls, then report it as negative
Answer and explanation in LearnFree account
Question 9 · Medium
A multiplex gastrointestinal PCR panel detects Salmonella in a stool. Why does the laboratory still culture the specimen?
1. ABecause PCR cannot detect Salmonella reliably
2. BTo confirm that the patient has diarrhea
3. CTo get a live isolate for testing and typing
4. DBecause culture is faster than the panel
Answer and explanation in LearnFree account
Question 10 · Easy
Most rapid antigen tests for SARS-CoV-2 detect which viral component?
1. AThe envelope (E) gene
2. BPatient antibody to spike
3. CThe RNA genome
4. DNucleocapsid (N) protein
Answer and explanation in LearnFree account
Question 11 · Medium
A nurse with fever and a sore throat that began this morning has a negative SARS-CoV-2 rapid antigen test. Why is a NAAT, or a repeat antigen test in 48 hours, advised?
1. AA negative antigen test means the nurse is immune
2. BAntigen tests detect only the earliest virus variants
3. CAntigen tests cross-react with common cold coronaviruses
4. DAntigen tests miss many infections with a low viral load
Answer and explanation in LearnFree account
Question 12 · Easy
A rapid urease test on a gastric biopsy is used to detect which organism?
1. ACandida albicans
2. BProteus mirabilis
3. CHelicobacter pylori
4. DCampylobacter jejuni
Answer and explanation in LearnFree account

Rapid tests and molecular microbiology

Three nasopharyngeal specimens were tested on the same lot of a rapid RSV antigen cassette, as shown. Which cassette's result is invalid?

The amplification plot shown is from a herpes simplex virus PCR run that included a patient's CSF. What should be done with the patient's result?

A MALDI-TOF system uses score cut-offs of 2.00 or more for high confidence, 1.70-1.99 for low confidence and below 1.70 for no reliable identification. A colony smeared directly on the target scores 1.84 for Klebsiella pneumoniae. What is the best next step?

A rapid influenza antigen test has a sensitivity of 70% and a specificity of 98%. In summer, local surveillance suggests 0.5% of the patients tested have influenza. About what proportion of positive results will be true positives?

In MALDI-TOF identification, the matrix solution placed over the smeared colony:

In a lateral flow (immunochromatographic) antigen test, the appearance of the control line shows that:

A SARS-CoV-2 RT-PCR on a nasopharyngeal swab gives: N gene target not detected; human RNase P internal control not detected. The run's positive and negative controls are acceptable. How should the result be handled?

A multiplex gastrointestinal PCR panel detects Salmonella in a stool. Why does the laboratory still culture the specimen?

Most rapid antigen tests for SARS-CoV-2 detect which viral component?

A nurse with fever and a sore throat that began this morning has a negative SARS-CoV-2 rapid antigen test. Why is a NAAT, or a repeat antigen test in 48 hours, advised?

A rapid urease test on a gastric biopsy is used to detect which organism?