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General information for laboratory and quality staff. This page is not medical advice and does not describe how to perform a blood draw.
Short answer
The blood culture contamination rate is the share of blood cultures that grow skin or environmental organisms picked up during collection rather than from the patient’s bloodstream. A 2026 laboratory-management article gives a national benchmark of 3% or lower, with 1% or lower considered optimal, and attributes both figures to a CDC quality tool. The number you report depends on which definition your hospital uses.
What counts as a contaminated blood culture
An article in Clinical Laboratory News (ADLM, 1 July 2026) describes blood culture contamination as skin flora or environmental organisms entering the culture bottle during collection, producing a false-positive result that can be mistaken for a true infection. Which cultures are classed as contaminated depends on the written definition a hospital applies.
That is why the same lab can report different rates depending on the rule it applies. The definitions in use are covered below.
Benchmark and who sets it
The ADLM article states that national benchmarks recommend a contamination rate of 3% or lower, and that 1% or lower is considered optimal for high-performing laboratories. It attributes these figures to the CDC’s “Prevent adult blood culture contamination” quality tool. It cites the CLSI guideline M47 (Principles and procedures for blood cultures, 2023) for general blood culture practice, not for the 3% figure, so the 3% number should be cited to the CDC tool and not to CLSI.
In practice, targets vary. In a CDC-funded survey of 52 US acute care hospitals, about half the hospitals targeted a contamination rate below 3%.
Why CAP, CLSI and NHSN definitions give different rates
The survey, led from Johns Hopkins and published in the Journal of Clinical Microbiology (August 2025), covered more than 360,000 blood cultures from 1 September 2019 to 31 August 2021 across 52 hospitals in 19 states and Washington, DC. It found three definitional approaches:
- 65.4% of hospitals used CLSI or College of American Pathologists (CAP) criteria.
- 17.3% used locally defined criteria.
- 17.3% used CAP or CLSI criteria plus the NHSN skin commensal list.
The two published rate pairs show how much the rule matters:
| Definition | ICU rate | Ward rate |
|---|---|---|
| CAP criteria | 1.38% | 0.96% |
| CAP criteria with the NHSN commensal list | 1.49% | 1.09% |
Adding the NHSN list moved both the ICU and ward rate upward. The lead author, Valeria Fabre, noted that hospitals vary in how they define contamination. The release warned that a lack of standardization could “dangerously stack the odds” toward cultures appearing to meet targets. For the NHSN side of this, see healthcare-associated infection definitions.
Questions to ask before comparing your rate with a benchmark
| Question | Why it matters (source) |
|---|---|
| Which definition does the rate use: CAP or CLSI, with or without the NHSN commensal list, or a local rule? | ICU rates were 1.38% under one definition and 1.49% under another (Johns Hopkins survey). |
| Is the target you are comparing with the CDC tool’s 3% or lower, or a local target? | The ADLM article attributes 3% and 1% to the CDC tool; about half of surveyed hospitals targeted below 3%. |
| Is the rate reported separately for ICUs and wards? | The survey reported ICU and ward rates separately and they differed. |
| Is anyone also tracking bottles collected per patient and culture positivity? | Only a minority of surveyed hospitals monitored these companion metrics. |
What else to monitor
The survey authors reported that few hospitals tracked the number of blood culture bottles collected or blood culture positivity alongside the contamination rate. The study also found an association between contamination and central line infections: a 1% rise in the contamination rate was associated with about a 9% increase in CLABSI. It is an association, not proof of cause. Background on the infection side is in the CLABSI guide.
Related guides
This page covers the metric, not the collection technique. For antiseptic categories see skin antisepsis before injection and what is an iodophor. Tube ordering is covered in blood draw tube colors, and the supplies a draw station stocks in the phlebotomy lab supply checklist.
Common misunderstandings
- “3% is a CLSI M47 requirement.” The source article attributes the 3% and 1% figures to a CDC quality tool and cites M47 for general principles.
- “A hospital’s rate is directly comparable with another’s.” Not unless both use the same definition; the survey found three approaches in use.
- “Every positive from skin flora is a contaminant.” Which cultures count as contaminated depends on the definition applied, which is why adding the NHSN commensal list changes the rate.
- “A low rate alone shows good quality.” The survey authors pointed to companion metrics such as bottles collected and positivity that few hospitals watch.
FAQ
What is an acceptable blood culture contamination rate?
The ADLM article gives 3% or lower as the national benchmark and 1% or lower as optimal for high-performing laboratories, attributing both to a CDC quality tool.
How do labs define a contaminated blood culture?
In the survey, most hospitals (65.4%) used CLSI or CAP criteria, some added the NHSN skin commensal list (17.3%), and others used local definitions (17.3%).
Does the definition change the rate?
Yes. In the survey, ICU rates were 1.38% under CAP criteria and 1.49% with the NHSN list added; ward rates were 0.96% and 1.09%.
Why does contamination matter beyond the lab?
Contaminated cultures can look like true infection. The survey associated each 1% rise in the rate with about a 9% increase in CLABSI.
Sources
- ADLM, Clinical Laboratory News, “Reducing blood culture contamination”, 1 July 2026: myadlm.org (checked 1 November 2026).
- Johns Hopkins Medicine news release, 3 September 2025, on a CDC-funded study in the Journal of Clinical Microbiology (August 2025): EurekAlert (checked 1 November 2026).
- CDC, Prevent adult blood culture contamination: A quality tool for clinical laboratory professionals, as cited in the ADLM article (benchmark attribution; not opened directly).
- CLSI guideline M47, Principles and procedures for blood cultures (2023), as cited in the ADLM article.
