Infection control suspects an outbreak after three ICU patients grow carbapenem-resistant Klebsiella pneumoniae in one week. What is the laboratory's most important immediate step?

Save all cluster isolates for molecular typing

Start nasal swab screening of all ICU staff

Discard isolates once final reports are issued

Hold carbapenem results until typing is finished

Over 3 weeks, bronchoalveolar lavage specimens from five unrelated patients grow Mycobacterium chelonae. None of the patients has signs of mycobacterial disease. All five were sampled with the same bronchoscope, and the cultures were set up on different days by different staff. The most likely explanation is:

Pseudo-outbreak from a contaminated bronchoscope

Community-acquired infection from tap water

Laboratory cross-contamination during processing

True outbreak spread from patient to patient

When a laboratory prepares its annual cumulative antibiogram by CLSI M39, which isolates are included?

The first isolate of each species per patient

Only isolates recovered from blood cultures

Every isolate tested during the year

Only isolates with multidrug resistance

The recommended specimen for Candida auris colonization screening is:

One swab of both axillae and the groin

Which method gives the highest resolution for deciding whether isolates from a suspected hospital outbreak are the same strain?

Pulsed-field gel electrophoresis

Infection control and surveillance

5 DOH Abu Dhabi (HAAD) lab licensing practice questions on Infection control and surveillance, part of Microbiology. The first 5 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 · Medium
Infection control suspects an outbreak after three ICU patients grow carbapenem-resistant Klebsiella pneumoniae in one week. What is the laboratory's most important immediate step?
1. AStart nasal swab screening of all ICU staff
2. BSave all cluster isolates for molecular typing
3. CDiscard isolates once final reports are issued
4. DHold carbapenem results until typing is finished
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Answer: B · Save all cluster isolates for molecular typing
Typing can show whether the cases share one strain only if the isolates are still available, so the laboratory keeps every isolate in the cluster and any new ones. It also searches its records for earlier cases and supports rectal swab screening of patient contacts. Results that guide treatment are never withheld during the investigation. CRE is carried in the gut, so staff screening is done only if the investigation points to a staff source.
- A: CRE is carried in the gut, and routine nasal screening of staff is not a first step.
- C: Discarded isolates cannot be typed, so the outbreak cannot be confirmed.
- D: Withholding results harms patients; reports continue as usual during the investigation.
Question 2 · Hard
Over 3 weeks, bronchoalveolar lavage specimens from five unrelated patients grow Mycobacterium chelonae. None of the patients has signs of mycobacterial disease. All five were sampled with the same bronchoscope, and the cultures were set up on different days by different staff. The most likely explanation is:
1. ACommunity-acquired infection from tap water
2. BLaboratory cross-contamination during processing
3. CTrue outbreak spread from patient to patient
4. DPseudo-outbreak from a contaminated bronchoscope
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Answer: D · Pseudo-outbreak from a contaminated bronchoscope
Positive cultures without disease, linked by one shared device, point to a pseudo-outbreak: the organism comes from the instrument, not the patients. Rapidly growing mycobacteria survive in water and can contaminate bronchoscopes and endoscope reprocessors when disinfection or drying fails. The laboratory saves the isolates for typing, and infection control takes the scope out of use and cultures it. Recognizing a pseudo-outbreak prevents needless treatment.
- A: Unrelated patients with no disease, linked by one instrument, do not suggest community infection.
- B: Cultures set up on different days by different staff make a laboratory source unlikely.
- C: The patients have no disease, and the shared bronchoscope, not contact, links them.
Question 3 · Medium
When a laboratory prepares its annual cumulative antibiogram by CLSI M39, which isolates are included?
1. AThe first isolate of each species per patient
2. BOnly isolates recovered from blood cultures
3. CEvery isolate tested during the year
4. DOnly isolates with multidrug resistance
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Answer: A · The first isolate of each species per patient
CLSI M39 counts only the first diagnostic isolate of each species from each patient in the period, whatever the body site or resistance pattern. Repeat isolates from long-stay patients would bias the percentages toward resistance. Screening cultures are left out, and species with fewer than 30 isolates are flagged or left out because their percentages are unreliable. The antibiogram guides empirical therapy and shows resistance trends over time.
- B: All diagnostic sites count; blood isolates alone would miss most urinary and wound pathogens.
- C: Repeat isolates from the same patient overcount resistant strains and bias the data.
- D: Selecting only resistant isolates would make the percentages meaningless.
Question 4 · Easy
The recommended specimen for Candida auris colonization screening is:
1. AOne swab of both axillae and the groin
2. BA rectal swab or stool sample
3. CA swab of both anterior nares
4. DA peripheral blood culture set
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Answer: A · One swab of both axillae and the groin
C. auris colonizes the skin, especially the axillae and groin, and a single composite swab of both sites detects most carriers. Screening is offered to contacts of a case and to patients transferred from facilities where C. auris has spread. The swab is tested by culture with salt enrichment or by C. auris PCR. Colonized patients stay on contact precautions, because carriage can last for months.
- B: Rectal swabs are the usual screen for CRE and VRE, not C. auris.
- C: The anterior nares are the main screening site for MRSA, not C. auris.
- D: Blood cultures diagnose bloodstream infection; they do not detect skin colonization.
Question 5 · Easy
Which method gives the highest resolution for deciding whether isolates from a suspected hospital outbreak are the same strain?
1. AMALDI-TOF identification
2. BWhole-genome sequencing
3. CComparison of antibiograms
4. DPulsed-field gel electrophoresis
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Answer: B · Whole-genome sequencing
Whole-genome sequencing compares the entire genome, so isolates can be judged related or unrelated by counting single-nucleotide differences. It has replaced pulsed-field gel electrophoresis as the reference typing method in many public health laboratories. Unrelated strains of one species can share an antibiogram and a MALDI-TOF identification, so these cannot prove transmission. Typing is only possible if the laboratory has kept the isolates.
- A: Routine MALDI-TOF identifies the species, not the strain.
- C: Unrelated strains can share a resistance pattern, and one strain can change it.
- D: PFGE was the former reference method, but band patterns resolve strains less finely than WGS.