Primary Measure: Blood Culture Contamination Rate

Overview

The microbiology laboratory determines a probable contaminated blood culture by the identification of a skin commensal organism in one set out of multiple sets collected in a 24-hour period. Understanding how often this occurs at the institutional level is critically important to maintaining quality practices at your facility and improving patient care. The primary measure (i.e., blood culture contamination rate) is a way for you to calculate the relative incidence of contaminated blood cultures at your facility. This allows you to monitor compliance with best practices and determine if mitigation strategies are needed. CDC encourages laboratories to evaluate their blood culture contamination (BCC) rate at least monthly.

This section provides a method to help you determine which specimens are eligible for inclusion when calculating the BCC rate and understand how to classify and report potential contaminants.

Primary measure eligibility criteria

If your laboratory identifies the presence of a skin commensal organism in a blood culture set, you should only include that set in the primary measure calculation if all the following criteria are met:

• Patient is at least 18 years old.
• Patient may be present in any hospital department such as the intensive care unit (ICU), emergency department (ED), inpatient floors, and step-down units.
• Institutions may choose to include outpatients in their analysis.
• At least two blood culture sets are collected in a 24-hour period.

Classify organisms

Laboratories can classify microorganisms using CDC's National Healthcare Safety Network (NHSN). The NHSN provides a list of bacterial skin commensal organisms categorized by both genus and genus and species. You can use this list to identify skin commensal organisms by name and corresponding SNOMED code.

Reporting the presence of commensal organisms in eligible sets

If your laboratory identifies the presence of a skin commensal organism in a set that meets the primary measure eligibility criteria, you could report the test result to the clinician as follows:

"Single-set positive out of two sets [or more, if this is the laboratory policy] may indicate the presence of possible skin contaminant, clinical correlation required. Please call the laboratory if further work–up is needed."

This comment alerts the clinician of a probable contaminant event and allows them to evaluate the clinical situation and determine next steps, if needed.

Calculate the overall BCC rate in your clinical setting

To calculate the BCC rate, divide the total number of eligible blood culture sets with growth of a skin commensal organism in only one of the sets collected within a 24-hour period by the total number of eligible blood culture sets collected during the monthly evaluation period, multiplied by one hundred.

• Denominator: Use the Laboratory Information System (LIS) to find all blood culture order codes within a specified timeframe (usually monthly) to identify the total number of blood culture sets collected
• Numerator: Use the LIS to find all probable contaminants by identifying all probable skin commensal organism result codes within a specified timeframe (usually monthly)
• Divide the total number of blood culture sets with growth of skin commensals without the same organism in other sets collected within 24 hours by the total number of eligible blood culture sets collected.

Eligible Blood Culture Sets = At least two blood culture sets collected from an adult patient within a 24-hour period