A 49-year-old woman with mitral valve prolapse had a dental extraction 2 months ago and now has low-grade fever and weight loss. Two of three blood culture sets flag positive at 30 to 40 hours. The Gram stain of the broth shows pleomorphic gram-positive cocci in short chains, some of them elongated. After 48 hours in 5% CO2, subculture on sheep blood agar shows no growth. A second sheep blood agar plate, cross-streaked with Staphylococcus aureus, shows tiny alpha-hemolytic colonies only along the streak. The isolate is catalase negative, PYR positive and LAP positive. The most likely organism is:

Streptococcus mitis/oralis group

Two blood culture sets were drawn from separate venipunctures in a 52-year-old man with fever. The bottle readings are shown, and the one positive bottle grew coagulase-negative staphylococci. What is the most likely interpretation?

False-positive instrument signal

Catheter-related bloodstream infection

Blood culture bottles are designed for a blood-to-broth ratio of about 1:5 to 1:10 mainly to:

Dilute antibacterial factors and antibiotics

Supply enough oxygen for strictly aerobic organisms

Keep the broth pH alkaline for growth

Stop the instrument signalling too early

Blood cultures

15 DOH Abu Dhabi (HAAD) lab licensing practice questions on Blood cultures, part of Microbiology. The first 15 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 49-year-old woman with mitral valve prolapse had a dental extraction 2 months ago and now has low-grade fever and weight loss. Two of three blood culture sets flag positive at 30 to 40 hours. The Gram stain of the broth shows pleomorphic gram-positive cocci in short chains, some of them elongated. After 48 hours in 5% CO2, subculture on sheep blood agar shows no growth. A second sheep blood agar plate, cross-streaked with Staphylococcus aureus, shows tiny alpha-hemolytic colonies only along the streak. The isolate is catalase negative, PYR positive and LAP positive. The most likely organism is:
1. AEnterococcus faecalis
2. BHaemophilus parainfluenzae
3. CAbiotrophia or Granulicatella
4. DStreptococcus mitis/oralis group
Show answerHide answer
Answer: C · Abiotrophia or Granulicatella
The organism grows in blood culture broth but not on plain sheep blood agar, except beside a S. aureus streak that releases pyridoxal. This satellitism marks the nutritionally variant streptococci, Abiotrophia and Granulicatella, which are catalase negative, PYR positive and LAP positive and often look pleomorphic in broth. They cause a small but important share of streptococcal endocarditis, often after dental procedures, which fits this patient. Subculture with a pyridoxal disk, a staphylococcal streak or a pyridoxal-supplemented medium recovers them for identification by MALDI-TOF. Growth in two sets from a patient with suspected endocarditis is significant and must not be dismissed as a contaminant.
- A: E. faecalis grows readily on sheep blood agar and needs no satellitism.
- B: H. parainfluenzae is a gram-negative coccobacillus, not a gram-positive coccus in chains.
- D: Viridans streptococci such as S. mitis grow on blood agar and are PYR negative.
Question 2 · Medium
Two blood culture sets were drawn from separate venipunctures in a 52-year-old man with fever. The bottle readings are shown, and the one positive bottle grew coagulase-negative staphylococci. What is the most likely interpretation?
Figure: MedExams
1. ATrue bacteremia
2. BFalse-positive instrument signal
3. CCatheter-related bloodstream infection
4. DProbable skin contaminant
Show answerHide answer
Answer: D · Probable skin contaminant
Only the set 2 aerobic bottle shows a growth curve, flagged at about 38 hours, while the other three bottles stay flat for 5 days. Coagulase-negative staphylococci are common skin flora, and growth in one of two sets with a long time to positivity points to contamination at collection. True bacteremia with these organisms usually grows in more than one set and flags sooner. The isolate is reported with a comment that it is a probable contaminant, and contamination rates are monitored to improve collection.
- A: One late-positive bottle out of four does not support bacteremia with skin flora.
- B: The rising signal reflects real growth, and organisms were recovered.
- C: This needs paired catheter and peripheral cultures, usually both positive.
Question 3 · Easy
For an adult, the recommended blood volume per blood culture set (aerobic plus anaerobic bottle) is:
1. A50 to 60 mL
2. B20 to 30 mL
3. C5 to 8 mL
4. D2 to 4 mL
Show answerHide answer
Answer: B · 20 to 30 mL
Most adult bacteremias have fewer than one organism per mL of blood, so the yield depends heavily on volume. CLSI M47 recommends 20 to 30 mL per adult set, split between the bottles as the manufacturer directs. Two to three sets, about 40 to 60 mL in total, are drawn per episode. Under-filled bottles are a major cause of missed bacteremia, while overfilling can cause false-positive signals.
- A: 50 to 60 mL is the total for 2 to 3 sets, not one set.
- C: 5 to 8 mL per set under-fills both bottles and misses low-level bacteremia.
- D: 2 to 4 mL is closer to a paediatric bottle volume.
Question 4 · Medium
Blood culture bottles are designed for a blood-to-broth ratio of about 1:5 to 1:10 mainly to:
1. ADilute antibacterial factors and antibiotics
2. BSupply enough oxygen for strictly aerobic organisms
3. CKeep the broth pH alkaline for growth
4. DStop the instrument signalling too early
Show answerHide answer
Answer: A · Dilute antibacterial factors and antibiotics
Blood contains complement, lysozyme, phagocytes and often antibiotics that inhibit bacteria. Diluting it 1:5 to 1:10 in broth, together with SPS, reduces these effects so small numbers of organisms can grow. This is one reason bottles should be filled to the volume the manufacturer specifies. Overfilling reduces the dilution and can cause false-positive signals from blood cell metabolism.
- B: Oxygen for aerobes comes from the bottle headspace, not from the dilution.
- C: The broth is buffered, and an alkaline pH is not the aim.
- D: The ratio is about supporting growth, not the timing of the signal.
Question 5 · Medium
Which organism should be treated as a likely true pathogen even when it grows in only one of two blood culture sets?
1. ACorynebacterium species
2. BStaphylococcus aureus
3. CCutibacterium acnes
4. DMicrococcus luteus
Show answerHide answer
Answer: B · Staphylococcus aureus
Staphylococcus aureus is rarely a contaminant, so even a single positive set is treated as true bacteremia and investigated, often with echocardiography. Other organisms usually significant in one set include Streptococcus pneumoniae, Enterobacterales, Pseudomonas aeruginosa and Candida. Skin organisms such as micrococci, Cutibacterium, corynebacteria and most coagulase-negative staphylococci are usually contaminants when found in only one set. Interpretation still takes account of the clinical picture and any prosthetic material.
- A: Corynebacteria in one set are usually skin contaminants.
- C: Cutibacterium acnes in one set is usually skin contamination.
- D: Micrococcus in one set is almost always a skin contaminant.
Question 6 · Hard
A stable patient with a new murmur and 3 weeks of low-grade fever is suspected of subacute endocarditis. No antibiotics have been given. Which blood culture plan is best?
1. ATwo sets now, then one after antibiotics
2. BSix bottles drawn from one venipuncture
3. CThree sets from separate sites, spaced over time
4. DOne large set through a new cannula
Answer and explanation in LearnFree account
Question 7 · Medium
Initial specimen diversion devices lower blood culture contamination by:
1. AAdding an antiseptic to the first portion of blood
2. BFiltering bacteria out of the blood before the bottle
3. CDrawing the blood from a catheter instead of a vein
4. DDiverting the first 1-2 mL of blood from the bottles
Answer and explanation in LearnFree account
Question 8 · Hard
Over 10 days, six patients on one ward have a single blood culture set positive for Burkholderia cepacia complex. None had signs of sepsis or received treatment, and all recovered. What is the most likely source?
1. AA contaminated antiseptic or collection supply
2. BContamination of the laboratory's subculture plates
3. CPerson-to-person spread of a true infection
4. DA defect in the blood culture instrument sensors
Answer and explanation in LearnFree account
Question 9 · Easy
When blood cultures are drawn with other tubes at the same venipuncture, the culture bottles are filled:
1. AFirst, before any other tube
2. BAfter the serum tubes
3. CLast, after the EDTA tube
4. DAfter the citrate tube
Answer and explanation in LearnFree account
Question 10 · Hard
A woman at 30 weeks of pregnancy has fever and myalgia. Both blood culture sets grow short gram-positive rods, first reported as 'diphtheroids, probable contaminant'. Colonies show a narrow zone of beta-hemolysis and are catalase positive. Which organism is most likely?
1. AListeria monocytogenes
2. BCorynebacterium striatum
3. CBacillus cereus
4. DCutibacterium acnes
Answer and explanation in LearnFree account
Question 11 · Medium
Why are two blood culture sets drawn from separate venipunctures rather than both from one site?
1. ASo a skin contaminant can be told from true bacteremia
2. BSo aerobic and anaerobic organisms are kept apart
3. CSo the second set can be held longer for slow growers
4. DSo that a smaller volume of blood is taken at each site
Answer and explanation in LearnFree account
Question 12 · Hard
Two days after bowel surgery, a febrile patient has both anaerobic bottles flag positive while the aerobic bottles stay negative. The Gram stain shows pale, irregularly stained gram-negative rods. Subcultures grow only on anaerobic blood agar and BBE agar. The most likely organism is:
1. AEscherichia coli
2. BHaemophilus influenzae
3. CPseudomonas aeruginosa
4. DBacteroides fragilis group
Answer and explanation in LearnFree account
Question 13 · Easy
When blood cultures are drawn with a winged (butterfly) collection set, which bottle is filled first?
1. AEither, since order does not matter
2. BThe bottle with resin beads
3. CThe aerobic bottle
4. DThe anaerobic bottle
Answer and explanation in LearnFree account
Question 14 · Hard
A 70-year-old man with pneumonia has an aerobic blood culture bottle that flags positive at 16 hours. The Gram stain shows gram-positive cocci in pairs, many of them pale and gram-variable. Subcultures on blood and chocolate agar show no growth at 48 hours. What is the most likely explanation?
1. ACrystal violet precipitate mistaken for cocci
2. BAutolysis of Streptococcus pneumoniae in the broth
3. CSPS inhibition of Neisseria meningitidis
4. DA false-positive signal from a high white cell count
Answer and explanation in LearnFree account
Question 15 · Medium
What is the most important factor affecting how often blood cultures detect bacteremia in adults?
1. AThe size of the collection needle used
2. BRefrigerating the bottles before they go into the incubator
3. CThe time of day the blood is collected
4. DThe volume of blood cultured (about 20 mL per set)
Answer and explanation in LearnFree account

Blood cultures

Two blood culture sets were drawn from separate venipunctures in a 52-year-old man with fever. The bottle readings are shown, and the one positive bottle grew coagulase-negative staphylococci. What is the most likely interpretation?

For an adult, the recommended blood volume per blood culture set (aerobic plus anaerobic bottle) is:

Blood culture bottles are designed for a blood-to-broth ratio of about 1:5 to 1:10 mainly to:

Which organism should be treated as a likely true pathogen even when it grows in only one of two blood culture sets?

A stable patient with a new murmur and 3 weeks of low-grade fever is suspected of subacute endocarditis. No antibiotics have been given. Which blood culture plan is best?

Initial specimen diversion devices lower blood culture contamination by:

Over 10 days, six patients on one ward have a single blood culture set positive for Burkholderia cepacia complex. None had signs of sepsis or received treatment, and all recovered. What is the most likely source?

When blood cultures are drawn with other tubes at the same venipuncture, the culture bottles are filled:

A woman at 30 weeks of pregnancy has fever and myalgia. Both blood culture sets grow short gram-positive rods, first reported as 'diphtheroids, probable contaminant'. Colonies show a narrow zone of beta-hemolysis and are catalase positive. Which organism is most likely?

Why are two blood culture sets drawn from separate venipunctures rather than both from one site?

Two days after bowel surgery, a febrile patient has both anaerobic bottles flag positive while the aerobic bottles stay negative. The Gram stain shows pale, irregularly stained gram-negative rods. Subcultures grow only on anaerobic blood agar and BBE agar. The most likely organism is:

When blood cultures are drawn with a winged (butterfly) collection set, which bottle is filled first?

A 70-year-old man with pneumonia has an aerobic blood culture bottle that flags positive at 16 hours. The Gram stain shows gram-positive cocci in pairs, many of them pale and gram-variable. Subcultures on blood and chocolate agar show no growth at 48 hours. What is the most likely explanation?

What is the most important factor affecting how often blood cultures detect bacteremia in adults?