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Blood cultures

16 min

Skills you'll build3 skills
  • Check each blood-culture set's fill volume and collection site
  • Choose the first laboratory actions after a blood-culture bottle signals positive
  • Tell a blood-culture contaminant from a pathogen by organism and number of positive sets

At the bench: Read an organism stain and communicate a significant preliminary finding

Try first

Try first

Two adult blood-culture sets are ordered for suspected sepsis. Which collection practice does the most to help the bottles recover the organism?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Volume and site

An adult set contains 20 to 30 mL of blood, commonly about 10 mL in an aerobic bottle and 10 mL in an anaerobic bottle. At least two sets, 40 to 60 mL in all, are collected for one septic episode.1,2 Volume matters most, because many bloodstream infections have few organisms per milliliter. Two underfilled sets can look complete and still miss the organism.1,2

Each set comes from a separately prepared venipuncture, documented with its site, time and fill volume. The laboratory tracks the measured fill of every bottle as a quality indicator.3 When a catheter infection is in question, a peripheral set and a catheter-drawn set are collected at the same time, each labeled with its source.2,3

When a bottle signals

The Gram stain is the first result from a positive bottle.2,3

  1. Sample the bottle with an aerosol-controlled procedure.
  2. Gram stain it, and call the result under the urgent-result policy as soon as it is read.
  3. Subculture to media that fit the stain and the bottle.
  4. Run a validated rapid identification method when available.

A flagged bottle with no organisms on the smear is restained and subcultured.3

Contaminant or pathogen

Coagulase-negative staphylococci, Corynebacterium, Bacillus, Micrococcus and Cutibacterium acnes are skin commensals. Viridans streptococci belong to the oral flora. They are also frequent contaminants, and their significance varies. Each can also cause true infection, especially with a catheter or prosthetic device.4,5

PatternInterpretation
Same commensal species in independently collected setsTrue bacteremia
Commensal in one set among two or moreContamination, with low-grade bacteremia or a device still possible
S. aureus, beta-hemolytic streptococci, S. pneumoniae or Listeria in any setA clinically important pathogen

Independently collected sets are stronger evidence than bottles within one set. The report gives the exact set and bottle pattern, and a commensal in one set gets a qualified comment naming a possible skin contaminant.1,4

Follow the bench demonstration

Sources for this step
References
  1. Centers for Disease Control and Prevention. Collect adult blood culture sets. Updated March 31, 2026. Accessed September 27, 2026. https://www.cdc.gov/lab-quality/php/preventing-adult-blood-culture-contamination/collect.html
  2. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America and the American Society for Microbiology. Clin Infect Dis. Published online March 5, 2024. doi:10.1093/cid/ciae104
  3. Clinical and Laboratory Standards Institute. Principles and Procedures for Blood Cultures. 2nd ed. CLSI guideline M47. Clinical and Laboratory Standards Institute; 2022. Accessed September 27, 2026. https://clsi.org/shop/standards/m47/
  4. Sautter RL, Parrott JS, Nachamkin I, et al. American Society for Microbiology evidence-based laboratory medicine practice guidelines to reduce blood culture contamination rates: a systematic review and meta-analysis. Clin Microbiol Rev. 2024;37:e00087-24. doi:10.1128/cmr.00087-24
  5. Doern GV, Carroll KC, Diekema DJ, et al. Practical guidance for clinical microbiology laboratories: a comprehensive update on the problem of blood culture contamination and a discussion of methods for addressing the problem. Clin Microbiol Rev. 2019;33(1):e00009-19. doi:10.1128/cmr.00009-19

Watch one

Read each decision and its explanation.

Carol Draper, 74, has a prosthetic aortic valve. Two sets were drawn by separate venipunctures, one from each arm, with 9 to 10 mL in every bottle.

  • Set 1: aerobic bottle positive at 19 hours, anaerobic bottle positive at 23 hours.
  • Set 2: aerobic bottle positive at 21 hours, anaerobic bottle no growth.
  • Identification: Staphylococcus epidermidis from all three positive bottles, with matching susceptibility patterns.

How do you read this pattern?

  1. Name the organism group: S. epidermidis is a coagulase-negative staphylococcus, a common skin commensal.

    A skin commensal raises the question of contamination, and the culture pattern helps resolve it.

  2. Count the sets: both independently collected sets are positive.

    Independent sets are the strongest evidence, stronger than bottles within one set.

  3. Check the collection: separate sites, and every bottle contained 9 to 10 mL.

    A well-filled set makes a match between sets meaningful.

  4. Compare the isolates: species and susceptibility patterns match.

    The same species with the same susceptibility pattern in both sets indicates one organism in the bloodstream.

  5. Add the patient's device: a prosthetic valve.

    Coagulase-negative staphylococci infect prosthetic valves, so the device increases the significance of the finding.

The pattern supports true bacteremia. Report S. epidermidis in both sets with the bottle pattern, and leave off the possible-contaminant comment.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Two adult blood-culture sets are drawn before antimicrobials by separate venipunctures 5 minutes apart. Each set holds about 3 mL of blood split between an aerobic and an anaerobic bottle. What most limits recovery?

Incorrect. Routine adult sets can be drawn sequentially, and volume and aseptic collection matter more than the interval between sets.

Correct. An adult set holds 20 to 30 mL, and blood volume has the largest controllable effect on recovery because many bloodstream infections contain few organisms per milliliter. About 3 mL per set is severe underfilling, and the measured fill volume is tracked as a collection quality indicator.

Incorrect. A two-bottle adult set routinely places about 10 mL in each of an aerobic and an anaerobic bottle. The split is standard, so the problem here is the 3-mL total.

Hint
  1. Compare the blood in each set with what an adult set should contain.
  2. Many bloodstream infections have very few organisms in each milliliter of blood.

Review Collection for bloodstream infection

Problem 2 of 3 · Hints available

A coagulase-negative staphylococcus grows in one bottle of one set, and a second, independently collected set stays negative. What does the report convey?

Incorrect. The number of independently collected positive sets is more significant than the number of positive bottles within one set, and a commensal in one of two sets suggests contamination.

Correct. Recovery from one set among two makes collection contamination more likely. Low-grade bacteremia, antimicrobial exposure, and an intravascular device remain possible explanations. The report gives the exact set and bottle pattern and a comment naming a possible skin contaminant.

Incorrect. One positive set among two makes contamination more likely but does not confirm it. Coagulase-negative staphylococci can cause true infection, especially with a catheter or prosthetic device, so the comment stays qualified.

Hint
  1. Count the independently collected sets that grew the organism.
  2. Ask what one set among two can establish, and what it leaves open.

Review Contamination and clinical significance

Problem 3 of 3 · On your own

At 02:10 an aerobic bottle from a patient on the medical floor signals positive. The Gram stain shows gram-negative rods. The rapid identification panel will finish in about an hour. What do you do with the Gram stain?

The Gram stain is the first actionable result, and it goes out as soon as it is read. The subculture and panel continue after the call.

An hour's wait delays information the care team can act on now. The stain is called when it is read.

Waited for the identification to call the Gram stain

The Gram stain of a positive bottle is available first and guides the next steps while identification proceeds. Waiting for a final identification before calling the Gram stain delays urgent information by hours or days. The laboratory communicates the stain under its urgent-result policy as soon as it is read.

A result left in the chart is not seen until someone looks. The urgent-result policy calls for a call to a licensed caregiver.

Waited for the identification to call the Gram stain

The Gram stain of a positive bottle is available first and guides the next steps while identification proceeds. Waiting for a final identification before calling the Gram stain delays urgent information by hours or days. The laboratory communicates the stain under its urgent-result policy as soon as it is read.

Review Bottle monitoring and positive-culture workup

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Walter Tubbs, 68, spikes a fever during dialysis. He has a tunneled dialysis catheter.
  • Two sets are drawn at the same time. Set A comes through the catheter with 10 mL in each bottle. Set B comes from an arm vein with 2 mL in each bottle.
  • At 03:00 the aerobic bottle of set A signals positive at 11 hours. The Gram stain shows gram-positive cocci in clusters.
Decision 1 of 3

What do you do with the Gram stain?

Set B may take hours to signal, or never signal. The stain result is urgent now.

Waited for the identification to call the Gram stain

The Gram stain of a positive bottle is available first and guides the next steps while identification proceeds. Waiting for a final identification before calling the Gram stain delays urgent information by hours or days. The laboratory communicates the stain under its urgent-result policy as soon as it is read.

A positive bottle's Gram stain is called as soon as it is read. The call includes the source: set A, catheter-drawn.

Clusters include S. aureus, which is significant in any set. The organism is still unidentified.

Review Bottle monitoring and positive-culture workup

Decision 2 of 3

Both bottles of set A grow S. epidermidis. Set B has no growth at 5 days. How much significance does set B's negative result have?

An adult set needs 20 to 30 mL. With a fifth of that volume, set B could miss a low-grade bacteremia.

Two sets were collected, and set B contained a fifth of the blood it needed. Underfilled bottles lower sensitivity.

Counted bottles without checking the volume collected

An adult set needs 20 to 30 mL of blood, and two sets can arrive with only a few milliliters each. Blood volume has the largest controllable effect on recovery, because many bloodstream infections have few organisms per milliliter. Underfilled bottles lower sensitivity even when the set count looks complete.

A peripheral set is the preferred source. Its negative result is only as reliable as the volume of blood it contained.

Review Collection for bloodstream infection

Decision 3 of 3

How do you report the culture?

That is a clinical diagnosis. The laboratory reports what grew and where.

Two bottles of one set come from one draw. They are weaker evidence than two independent sets.

The report names the organism in both bottles of the catheter-drawn set, the negative underfilled peripheral set, and a comment naming a possible skin contaminant for clinical correlation.

Review Contamination and clinical significance

The finding that matters is set B's fill volume. With 4 mL in the set, its negative result cannot rule out bacteremia, and the laboratory cannot distinguish contamination from true infection. The report states the culture pattern, and the fill record goes to the collection quality review.

For reference

Sources checked