A 58-year-old sheep farmer with a bioprosthetic aortic valve has had low-grade fevers, night sweats and weight loss for 4 months. Echocardiography shows a small vegetation on the valve. He took no antibiotics before three blood culture sets were drawn from separate sites, and all three are negative after 5 days in a continuous-monitoring system. Further results: Bartonella IgG negative; Brucella antibody negative; Coxiella burnetii IgG by immunofluorescence, phase I 1:4,096 and phase II 1:1,024. Which diagnosis best fits these results?

Acute Q fever without endocarditis

Bartonella quintana endocarditis

HACEK endocarditis missed by short incubation

A 34-year-old woman with acute leukemia and a tunneled central venous catheter is febrile and neutropenic (neutrophils 0.2 x 10^3/µL [0.2 x 10^9/L]). Two blood culture sets are drawn, one through the catheter and one by peripheral venipuncture, and both flag positive within 14 hours. The Gram stain shows large gram-positive rods in chains, some with central spores. Colonies on sheep blood agar are large, flat and beta-hemolytic, and the isolate is motile. No other patient's blood cultures have grown Bacillus in the past month. The best interpretation is:

True bacteremia needing full workup

Possible B. anthracis; refer to the LRN

Skin contaminant from blood collection

Pseudobacteremia from contaminated supplies

Blood and bone marrow cultures

30 ASCP MLS practice questions on Blood and bone marrow cultures, part of Microbiology. The first 25 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 58-year-old sheep farmer with a bioprosthetic aortic valve has had low-grade fevers, night sweats and weight loss for 4 months. Echocardiography shows a small vegetation on the valve. He took no antibiotics before three blood culture sets were drawn from separate sites, and all three are negative after 5 days in a continuous-monitoring system. Further results: Bartonella IgG negative; Brucella antibody negative; Coxiella burnetii IgG by immunofluorescence, phase I 1:4,096 and phase II 1:1,024. Which diagnosis best fits these results?
1. AAcute Q fever without endocarditis
2. BBartonella quintana endocarditis
3. CChronic Q fever endocarditis
4. DHACEK endocarditis missed by short incubation
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Answer: C · Chronic Q fever endocarditis
Blood cultures stay negative in Q fever because Coxiella burnetii is an obligate intracellular bacterium that does not grow in blood culture broth. The diagnosis is serologic: a very high phase I IgG titer, here also above the phase II titer, marks persistent (chronic) infection such as endocarditis, while acute Q fever shows phase II antibody higher than phase I. A phase I IgG titer above 1:800 is a major criterion for endocarditis. Contact with sheep and a prosthetic valve are classic risks. HACEK organisms are recovered within the standard 5 days in modern systems, and the Bartonella and Brucella tests are negative.
- A: Acute Q fever shows phase II antibody higher than phase I, not the reverse.
- B: Bartonella IgG is negative, and the Coxiella phase I titer explains the picture.
- D: HACEK organisms grow within the standard 5-day protocol in continuous-monitoring systems.
Question 2 · Hard
A 34-year-old woman with acute leukemia and a tunneled central venous catheter is febrile and neutropenic (neutrophils 0.2 x 10^3/µL [0.2 x 10^9/L]). Two blood culture sets are drawn, one through the catheter and one by peripheral venipuncture, and both flag positive within 14 hours. The Gram stain shows large gram-positive rods in chains, some with central spores. Colonies on sheep blood agar are large, flat and beta-hemolytic, and the isolate is motile. No other patient's blood cultures have grown Bacillus in the past month. The best interpretation is:
1. ATrue bacteremia needing full workup
2. BPossible B. anthracis; refer to the LRN
3. CSkin contaminant from blood collection
4. DPseudobacteremia from contaminated supplies
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Answer: A · True bacteremia needing full workup
Bacillus species are common contaminants when they grow late in a single bottle, but this isolate grew in both sets, including the peripheral one, within 14 hours. In a neutropenic patient with a central line, Bacillus cereus causes true catheter-related bloodstream infection that can be life-threatening. Beta-hemolysis and motility rule out B. anthracis, which is nonhemolytic and nonmotile, so no referral is needed. With no cluster of Bacillus isolates in other patients, pseudobacteremia from contaminated supplies is unlikely. The isolate should be identified and the clinical team told that it is probably significant.
- B: B. anthracis is nonhemolytic and nonmotile; this isolate is hemolytic and motile.
- C: A contaminant usually grows late in one set, not in both sets within 14 hours.
- D: Pseudobacteremia shows as a cluster in many patients, and none was found.
Question 3 · Hard
A 66-year-old man with iron-deficiency anemia (hemoglobin 9.1 g/dL [91 g/L], MCV 71 fL) and recent weight loss develops fever, shock and a rapidly spreading, painful swelling of the thigh without any injury. Only the anaerobic bottles of two blood culture sets flag positive, both at 10 hours. The Gram stain shows large gram-positive and gram-variable rods, some with oval subterminal spores. Anaerobic blood agar at 24 hours is covered by a thin, beta-hemolytic film of swarming growth. The isolate is lecithinase negative on egg yolk agar and does not grow on aerobic subculture. The most likely organism is:
1. AClostridium tetani
2. BClostridium perfringens
3. CClostridium septicum
4. DClostridium tertium
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Answer: C · Clostridium septicum
Spontaneous gas gangrene without trauma, with bacteremia, is the hallmark of Clostridium septicum, which enters the blood through a lesion in the colon, most often a carcinoma; the iron-deficiency anemia and weight loss point there. The isolate swarms across anaerobic blood agar, is beta-hemolytic and lecithinase negative, and forms oval subterminal spores. C. perfringens is lecithinase positive with a double zone of hemolysis, is nonmotile and seldom forms spores in culture, and C. tertium grows aerobically. C. tetani also swarms but has round terminal spores. C. septicum bacteremia should prompt a search for colorectal or blood cancer.
- A: C. tetani has round terminal spores, not oval subterminal ones.
- B: C. perfringens is lecithinase positive, shows a double zone of hemolysis and does not swarm.
- D: C. tertium is aerotolerant and would grow on the aerobic subculture.
Question 4 · Medium
Blood cultures from a 45-year-old with febrile neutropenia were loaded into a continuous-monitoring system; the sensor curves for both bottles of one set are shown. The Gram stain of the positive bottle shows gram-negative rods. The organism is most likely:
Figure: MedExams
1. AEscherichia coli
2. BPseudomonas aeruginosa
3. CFusobacterium necrophorum
4. DBacteroides fragilis
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Answer: B · Pseudomonas aeruginosa
The aerobic bottle's signal rises steeply from about 12 hours as CO2 from growth builds up, while the anaerobic bottle stays flat for 5 days. Growth in the aerobic bottle alone points to an obligate aerobe. Pseudomonas aeruginosa, a common cause of bacteremia in neutropenic patients, needs oxygen to grow and is recovered mainly from aerobic bottles. Facultative organisms such as E. coli usually grow in both bottles, and strict anaerobes grow only in the anaerobic bottle.
- A: E. coli is facultative and usually grows in both bottles.
- C: Fusobacterium necrophorum is a strict anaerobe and would grow in the anaerobic bottle.
- D: Bacteroides fragilis is a strict anaerobe and would grow in the anaerobic bottle.
Question 5 · Hard
In one month a hospital collected 1,600 blood culture sets (two bottles each). Contaminants grew in 56 sets. What is the contamination rate, compared with the laboratory's 3% benchmark?
1. A0.35%, within the benchmark
2. B28.6%, above the benchmark
3. C3.5%, above the benchmark
4. D1.75%, within the benchmark
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Answer: C · 3.5%, above the benchmark
Contamination rate = contaminated sets / total sets x 100 = 56 / 1,600 x 100 = 3.5%. This is above the 3% benchmark, so collection practice should be reviewed. Common causes are drawing through existing lines, poor skin antisepsis and not letting the antiseptic dry. Many programs now aim for 1% or less, using dedicated phlebotomy teams and initial-specimen diversion devices.
- A: 0.35% is a decimal error: 56 / 1,600 is 0.035, which is 3.5%.
- B: 28.6 comes from dividing sets by contaminated sets, an inverted ratio.
- D: 1.75% comes from dividing by 3,200 bottles instead of 1,600 sets.
Question 6 · Hard
A patient with a central venous catheter has two blood culture sets drawn at the same time, one through the catheter and one by venipuncture. Both grow Staphylococcus epidermidis with the same susceptibility pattern, and the catheter set flags 3 hours before the peripheral set. This most strongly suggests:
1. AEndocarditis on a native heart valve
2. BContamination of the peripheral set
3. CCatheter-related bloodstream infection
4. DContamination of the catheter hub only
Answer and explanation in LearnFree account
Question 7 · Easy
Continuous-monitoring blood culture systems such as BACTEC and BacT/ALERT flag a bottle as positive by detecting:
1. AHemolysis seen by an optical camera
2. BTurbidity measured by a photometer
3. CRadiolabeled CO2 in the headspace gas
4. DRising CO2 through a sensor in the bottle base
Answer and explanation in LearnFree account
Question 8 · Medium
Which organism can be inhibited by the SPS in blood culture broth?
1. AStaphylococcus aureus
2. BKlebsiella pneumoniae
3. CEscherichia coli
4. DNeisseria meningitidis
Answer and explanation in LearnFree account
Question 9 · Medium
Bone marrow culture is more sensitive than blood culture, even after antibiotics have started, for:
1. AEscherichia coli
2. BStaphylococcus aureus
3. CSalmonella Typhi
4. DStreptococcus pneumoniae
Answer and explanation in LearnFree account
Question 10 · Medium
A premature infant receiving intravenous lipid through a central line has budding yeast in a positive blood culture. Subcultures grow only when the plate is overlaid with sterile olive oil. Which organism is most likely?
1. ACryptococcus neoformans
2. BCandida parapsilosis
3. CCandida albicans
4. DMalassezia furfur
Answer and explanation in LearnFree account
Question 11 · Easy
The anticoagulant in most commercial blood culture broths is:
1. ASodium citrate
2. BEthylenediaminetetraacetic acid (EDTA)
3. CLithium heparin
4. DSodium polyanethol sulfonate (SPS)
Answer and explanation in LearnFree account
Question 12 · Hard
A 68-year-old man with endocarditis has blood cultures growing a catalase-negative, gram-positive coccus in chains. It is bile esculin positive, PYR negative and does not grow in 6.5% NaCl. Which investigation is recommended specifically because of this organism?
1. AColonoscopy for colorectal neoplasia
2. BUrologic evaluation of the urinary tract
3. CDental review for periodontal disease
4. DSkin examination for injection sites
Answer and explanation in LearnFree account
Question 13 · Medium
Why is positive blood culture broth processed before MALDI-TOF identification directly from the bottle?
1. ATo neutralize the SPS anticoagulant
2. BTo grow the organisms to a larger mass
3. CTo remove blood cells and host proteins
4. DTo remove antibiotics bound to resin
Answer and explanation in LearnFree account
Question 14 · Hard
A positive blood culture is tested with a rapid PCR panel whose only resistance target is mecA. It detects Staphylococcus aureus, Staphylococcus epidermidis and mecA. The correct interpretation is:
1. AmecA cannot yet be assigned to either species
2. BThe S. aureus is methicillin resistant (MRSA)
3. CThe panel result is invalid and must be repeated
4. DThe S. aureus is methicillin susceptible (MSSA)
Answer and explanation in LearnFree account
Question 15 · Hard
Seven weeks after aortic valve replacement, a febrile patient has three of three blood culture sets, drawn from separate sites, growing coagulase-negative staphylococci with the same susceptibility pattern. This finding is best interpreted as:
1. AContamination of the culture medium
2. BTransient bacteremia of no significance
3. CSkin contamination during collection
4. DLikely prosthetic valve endocarditis
Answer and explanation in LearnFree account
Question 16 · Medium
A positive blood culture Gram stain shows gram-negative rods, but a multiplex PCR identification panel detects no targets. The most likely explanation is that the organism:
1. AIs not one of the panel's targets
2. BIs a skin contaminant from collection
3. CCarries an ESBL resistance gene
4. DWas killed by the patient's antibiotics
Answer and explanation in LearnFree account
Question 17 · Easy
Which organism, found in only one of several blood culture sets, is most likely a skin contaminant?
1. AStreptococcus pneumoniae
2. BStaphylococcus aureus
3. CCutibacterium acnes
4. DEscherichia coli
Answer and explanation in LearnFree account
Question 18 · Medium
Blood culture bottles containing resin beads are most useful for patients who:
1. AAre newborns with low blood volume
2. BHave suspected mold infection
3. CAre suspected of having anaerobic sepsis
4. DAre already receiving antibiotics
Answer and explanation in LearnFree account
Question 19 · Hard
A blood culture bottle flags positive at 30 hours, but its Gram stain shows no organisms. What is the best next step?
1. AReport no growth and return the bottle for 5 days
2. BRequest a new blood culture set from the patient
3. CReport the bottle as a false positive and discard it
4. DDo an acridine orange stain and subculture the broth
Answer and explanation in LearnFree account
Question 20 · Hard
A man without a spleen develops septic shock 3 days after a dog bite. Blood cultures grow thin, fusiform gram-negative rods that need 2 to 3 days on blood and chocolate agar in CO2. They are oxidase and catalase positive. Which organism is most likely?
1. ACapnocytophaga canimorsus
2. BEikenella corrodens
3. CHaemophilus influenzae
4. DPasteurella multocida
Answer and explanation in LearnFree account
Question 21 · Hard
During an instrument breakdown, blood culture bottles are held at 35 °C for 20 hours before loading. One bottle looks cloudy and hemolyzed, yet the instrument still reads it negative at 5 days. The best action is:
1. AReport it as no growth at 5 days
2. BDiscard it and ask for a new set
3. CReload it for another 5-day cycle
4. DGram stain and subculture the bottle
Answer and explanation in LearnFree account
Question 22 · Hard
A homeless man with body lice has fever and a new heart murmur. Echocardiography shows a valve vegetation, but blood cultures stay negative after 5 days. What is the most likely cause?
1. ABartonella quintana
2. BCoxiella burnetii
3. CTropheryma whipplei
4. DBrucella melitensis
Answer and explanation in LearnFree account
Question 23 · Hard
A physician asks the laboratory to hold blood cultures for 3 weeks because HACEK endocarditis is suspected. The laboratory uses a continuous-monitoring system. What does current guidance support?
1. ASend blood for fungal culture instead
2. BStandard 5-day incubation is usually enough
3. CUse only anaerobic bottles for HACEK organisms
4. DHold all bottles 21 days with blind subcultures
Answer and explanation in LearnFree account
Question 24 · Medium
A positive blood culture shows gram-positive cocci in chains. On sheep blood agar, subcultures grow only as tiny satellite colonies around a Staphylococcus aureus streak. Which organism is most likely?
1. AHaemophilus influenzae
2. BEnterococcus faecalis
3. CAbiotrophia defectiva
4. DStreptococcus pneumoniae
Answer and explanation in LearnFree account
Question 25 · Medium
Subacute endocarditis on a previously damaged native valve, following dental work, is most often caused by:
1. ACoagulase-negative staphylococci
2. BViridans group streptococci
3. CStaphylococcus aureus
4. DStreptococcus pyogenes
Answer and explanation in LearnFree account

5 more Blood and bone marrow cultures questions

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Study Microbiology

Blood and bone marrow cultures

Blood cultures from a 45-year-old with febrile neutropenia were loaded into a continuous-monitoring system; the sensor curves for both bottles of one set are shown. The Gram stain of the positive bottle shows gram-negative rods. The organism is most likely:

In one month a hospital collected 1,600 blood culture sets (two bottles each). Contaminants grew in 56 sets. What is the contamination rate, compared with the laboratory's 3% benchmark?

Continuous-monitoring blood culture systems such as BACTEC and BacT/ALERT flag a bottle as positive by detecting:

Which organism can be inhibited by the SPS in blood culture broth?

Bone marrow culture is more sensitive than blood culture, even after antibiotics have started, for:

A premature infant receiving intravenous lipid through a central line has budding yeast in a positive blood culture. Subcultures grow only when the plate is overlaid with sterile olive oil. Which organism is most likely?

The anticoagulant in most commercial blood culture broths is:

A 68-year-old man with endocarditis has blood cultures growing a catalase-negative, gram-positive coccus in chains. It is bile esculin positive, PYR negative and does not grow in 6.5% NaCl. Which investigation is recommended specifically because of this organism?

Why is positive blood culture broth processed before MALDI-TOF identification directly from the bottle?

A positive blood culture is tested with a rapid PCR panel whose only resistance target is mecA. It detects Staphylococcus aureus, Staphylococcus epidermidis and mecA. The correct interpretation is:

Seven weeks after aortic valve replacement, a febrile patient has three of three blood culture sets, drawn from separate sites, growing coagulase-negative staphylococci with the same susceptibility pattern. This finding is best interpreted as:

A positive blood culture Gram stain shows gram-negative rods, but a multiplex PCR identification panel detects no targets. The most likely explanation is that the organism:

Which organism, found in only one of several blood culture sets, is most likely a skin contaminant?

Blood culture bottles containing resin beads are most useful for patients who:

A blood culture bottle flags positive at 30 hours, but its Gram stain shows no organisms. What is the best next step?

A man without a spleen develops septic shock 3 days after a dog bite. Blood cultures grow thin, fusiform gram-negative rods that need 2 to 3 days on blood and chocolate agar in CO2. They are oxidase and catalase positive. Which organism is most likely?

During an instrument breakdown, blood culture bottles are held at 35 °C for 20 hours before loading. One bottle looks cloudy and hemolyzed, yet the instrument still reads it negative at 5 days. The best action is:

A homeless man with body lice has fever and a new heart murmur. Echocardiography shows a valve vegetation, but blood cultures stay negative after 5 days. What is the most likely cause?

A physician asks the laboratory to hold blood cultures for 3 weeks because HACEK endocarditis is suspected. The laboratory uses a continuous-monitoring system. What does current guidance support?

A positive blood culture shows gram-positive cocci in chains. On sheep blood agar, subcultures grow only as tiny satellite colonies around a Staphylococcus aureus streak. Which organism is most likely?

Subacute endocarditis on a previously damaged native valve, following dental work, is most often caused by: