A 45-year-old woman had a ventriculoperitoneal shunt placed 4 months ago. She now has headache and signs of shunt malfunction, without fever. CSF from the shunt reservoir: WBC 60 × 10^6/L (reference under 5 × 10^6/L), 70% neutrophils; glucose 2.6 mmol/L (47 mg/dL) with plasma glucose 5.5 mmol/L (99 mg/dL); protein 0.9 g/L (90 mg/dL). The Gram stain shows a few pleomorphic gram-positive rods. Aerobic and anaerobic plates show no growth at 72 hours, when the laboratory usually finalises CSF cultures. The laboratory should:

Extend the anaerobic culture to at least 10 days

Report the rods as skin contaminants

Finalise the culture as no growth at 72 hours

Add a mycobacterial culture in MGIT broth

Why do many laboratories no longer perform routine bacterial antigen (latex agglutination) tests on CSF?

They seldom change treatment and are less sensitive than PCR

They need several days of incubation before reading

They detect only Cryptococcus and no bacterial antigens

They turn positive only after the culture is already positive

A 22-year-old has severe headache and confusion 5 days after swimming in a warm freshwater pond. CSF shows many neutrophils and red cells, low glucose and no organisms on Gram stain. To find the most likely cause, the laboratory should:

Examine a fresh, unrefrigerated wet mount for motile amebae

Refrigerate the CSF and plate it on chocolate agar the next day

Perform an India ink preparation on the CSF sediment

Request an acid-fast stain and mycobacterial culture

Compared with spontaneous bacterial peritonitis in cirrhosis, peritonitis after a perforated colon is typically:

Polymicrobial, including anaerobes such as Bacteroides fragilis

Sterile, with chemical inflammation only

Caused by a single gram-negative rod such as E. coli

Caused by coagulase-negative staphylococci from the skin

CSF and sterile body fluid cultures

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on CSF and sterile body fluid cultures, part of Microbiology. 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 45-year-old woman had a ventriculoperitoneal shunt placed 4 months ago. She now has headache and signs of shunt malfunction, without fever. CSF from the shunt reservoir: WBC 60 × 10^6/L (reference under 5 × 10^6/L), 70% neutrophils; glucose 2.6 mmol/L (47 mg/dL) with plasma glucose 5.5 mmol/L (99 mg/dL); protein 0.9 g/L (90 mg/dL). The Gram stain shows a few pleomorphic gram-positive rods. Aerobic and anaerobic plates show no growth at 72 hours, when the laboratory usually finalises CSF cultures. The laboratory should:
1. AReport the rods as skin contaminants
2. BFinalise the culture as no growth at 72 hours
3. CExtend the anaerobic culture to at least 10 days
4. DAdd a mycobacterial culture in MGIT broth
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Answer: C · Extend the anaerobic culture to at least 10 days
Shunt infections in the months after surgery are usually caused by skin organisms introduced at operation; with the staphylococci, Cutibacterium acnes is one of the commonest. The CSF shows a neutrophil pleocytosis with a low CSF-to-plasma glucose ratio (0.47), and the Gram stain shows pleomorphic gram-positive rods, which fits C. acnes. It is an aerotolerant anaerobe that often needs 5 to 10 days to grow, so a culture finalised at 72 hours can be falsely negative. Guidance on shunt and drain infections advises holding anaerobic cultures for 10 days when C. acnes is possible. Once it grows, an isolate from shunt CSF with pleocytosis is treated as significant, not as a contaminant.
- A: Organisms with neutrophils in shunt CSF need culture, not dismissal as contaminants.
- B: Finalising at 72 hours misses slow-growing Cutibacterium acnes.
- D: Nothing suggests tuberculosis; the likely organism is an anaerobic skin diphtheroid.
Question 2 · Medium
A CSF Gram stain from a 70-year-old woman on long-term prednisone shows a few short gram-positive rods among many neutrophils. Which organism should the technologist suspect first?
1. AListeria monocytogenes
2. BCutibacterium acnes
3. CCorynebacterium striatum
4. DLactobacillus species
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Answer: A · Listeria monocytogenes
Listeria monocytogenes causes meningitis mainly in newborns, pregnant women, adults over 50 and people with weakened cell-mediated immunity, such as those on corticosteroids. On Gram stain it appears as short gram-positive rods or coccobacilli, often few in number, and it can be mistaken for skin diphtheroids or even pneumococci. Gram-positive rods in CSF must never be dismissed as contaminants without discussion with the clinician. Listeria is intrinsically resistant to cephalosporins, so the finding changes empirical therapy to include ampicillin.
- B: C. acnes causes shunt and post-surgical infections, not community meningitis.
- C: C. striatum is a skin organism that rarely causes meningitis without a device.
- D: Lactobacillus is a mucosal commensal that almost never causes meningitis.
Question 3 · Medium
Why do many laboratories no longer perform routine bacterial antigen (latex agglutination) tests on CSF?
1. AThey need several days of incubation before reading
2. BThey seldom change treatment and are less sensitive than PCR
3. CThey detect only Cryptococcus and no bacterial antigens
4. DThey turn positive only after the culture is already positive
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Answer: B · They seldom change treatment and are less sensitive than PCR
Bacterial antigen panels for organisms such as S. pneumoniae, N. meningitidis, Hib and S. agalactiae add little when the Gram stain is positive and are often negative when it is negative. False positives also occur, so the results rarely change management. Specific PCR and multiplex PCR panels detect these pathogens more sensitively, even after antibiotics have been given. The cryptococcal antigen test is a separate test that remains highly useful.
- A: Latex tests are read within minutes, not after days of incubation.
- C: Bacterial panels detect bacterial capsular antigens; cryptococcal antigen is a separate test.
- D: Antigen tests give results in minutes, before culture, so timing is not the problem.
Question 4 · Hard
A 22-year-old has severe headache and confusion 5 days after swimming in a warm freshwater pond. CSF shows many neutrophils and red cells, low glucose and no organisms on Gram stain. To find the most likely cause, the laboratory should:
1. ARefrigerate the CSF and plate it on chocolate agar the next day
2. BPerform an India ink preparation on the CSF sediment
3. CExamine a fresh, unrefrigerated wet mount for motile amebae
4. DRequest an acid-fast stain and mycobacterial culture
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Answer: C · Examine a fresh, unrefrigerated wet mount for motile amebae
The history and a purulent, Gram-stain-negative CSF suggest primary amebic meningoencephalitis due to Naegleria fowleri. Trophozoites are best seen as actively motile amebae in a wet mount of fresh CSF examined at once at room temperature, and in a counting chamber they can be mistaken for white cells. The CSF must not be refrigerated, because cold kills the amebae; culture on non-nutrient agar with an E. coli overlay and PCR support the diagnosis. The disease progresses within days, so a positive finding is phoned immediately.
- A: Refrigeration kills the amebae and delays the diagnosis.
- B: India ink detects encapsulated Cryptococcus, which causes a slower, lymphocytic meningitis.
- D: Tuberculous meningitis is subacute and lymphocytic, which fits poorly with this rapid course.
Question 5 · Medium
Compared with spontaneous bacterial peritonitis in cirrhosis, peritonitis after a perforated colon is typically:
1. ASterile, with chemical inflammation only
2. BCaused by a single gram-negative rod such as E. coli
3. CPolymicrobial, including anaerobes such as Bacteroides fragilis
4. DCaused by coagulase-negative staphylococci from the skin
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Answer: C · Polymicrobial, including anaerobes such as Bacteroides fragilis
Perforation spills colonic flora into the peritoneum, so cultures grow mixed aerobes such as E. coli and enterococci together with anaerobes such as the Bacteroides fragilis group. Spontaneous bacterial peritonitis, by contrast, is usually caused by a single organism, most often E. coli, Klebsiella or a streptococcus. Several organisms in ascitic fluid from a cirrhotic patient should therefore raise concern for secondary peritonitis. Peritoneal fluid or pus for anaerobic culture must be sent in an anaerobic transport system.
- A: Colonic contents carry huge numbers of bacteria, so the peritonitis is infectious.
- B: A single gram-negative rod is typical of spontaneous peritonitis, not perforation.
- D: Coagulase-negative staphylococci are typical of peritoneal dialysis infection.
Question 6 · Medium
In suspected bacterial meningitis, lumbar puncture is delayed while a CT scan is done. Before antibiotics are started, the most important specimen to collect is:
1. AA urine culture
2. BBlood cultures
3. CA nasopharyngeal swab
4. DA throat swab
Answer and explanation in LearnFree account
Question 7 · Easy
A positive CSF Gram stain should be reported:
1. AImmediately by phone, as a critical result
2. BWith the final culture result in 2 days
3. CIn the next routine batch of reports
4. DOnly after the organism is identified
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Question 8 · Easy
Before routine conjugate vaccination, the leading cause of bacterial meningitis in children aged 3 months to 5 years was:
1. AHaemophilus influenzae type b
2. BStreptococcus agalactiae
3. CListeria monocytogenes
4. DEscherichia coli K1
Answer and explanation in LearnFree account
Question 9 · Hard
A 9-day-old preterm infant fed reconstituted powdered formula develops meningitis. CSF grows an oxidase-negative gram-negative rod that is indole negative and Voges-Proskauer positive, and forms yellow colonies on tryptic soy agar at 25 °C. Which organism is most likely?
1. ACronobacter sakazakii
2. BCitrobacter koseri
3. CSerratia marcescens
4. DEscherichia coli K1
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Question 10 · Hard
A patient has pain in a hip replacement 8 months after surgery. Five periprosthetic tissue samples are cultured, and Staphylococcus epidermidis with the same susceptibility pattern grows from three of them. The best interpretation is:
1. AProsthetic joint infection with S. epidermidis
2. BSkin contamination of the tissue samples
3. CColonization of the joint that needs no treatment
4. DA mixed infection that needs anaerobic culture
Answer and explanation in LearnFree account

CSF and sterile body fluid cultures

A CSF Gram stain from a 70-year-old woman on long-term prednisone shows a few short gram-positive rods among many neutrophils. Which organism should the technologist suspect first?

Why do many laboratories no longer perform routine bacterial antigen (latex agglutination) tests on CSF?

A 22-year-old has severe headache and confusion 5 days after swimming in a warm freshwater pond. CSF shows many neutrophils and red cells, low glucose and no organisms on Gram stain. To find the most likely cause, the laboratory should:

Compared with spontaneous bacterial peritonitis in cirrhosis, peritonitis after a perforated colon is typically:

In suspected bacterial meningitis, lumbar puncture is delayed while a CT scan is done. Before antibiotics are started, the most important specimen to collect is:

A positive CSF Gram stain should be reported:

Before routine conjugate vaccination, the leading cause of bacterial meningitis in children aged 3 months to 5 years was:

A 9-day-old preterm infant fed reconstituted powdered formula develops meningitis. CSF grows an oxidase-negative gram-negative rod that is indole negative and Voges-Proskauer positive, and forms yellow colonies on tryptic soy agar at 25 °C. Which organism is most likely?

A patient has pain in a hip replacement 8 months after surgery. Five periprosthetic tissue samples are cultured, and Staphylococcus epidermidis with the same susceptibility pattern grows from three of them. The best interpretation is: