What causes false positive blood cultures?

October 3, 2024

Blood cultures are a routine — but very important — part of healthcare as clinicians rely on them for patient treatment decisions. A blood culture can show if there are harmful bacteria in a patient’s bloodstream. However, blood cultures can sometimes become contaminated with bacteria from outside of the bloodstream, even if a healthcare professional has followed best practice techniques to clean and prepare the patient’s skin. This can cause a false-positive blood culture, and lead to patient misdiagnosis.

Common Causes of False-Positive Blood Cultures

Bacteria are everywhere, on every surface, all the time, including on human skin. To draw blood for a blood culture, healthcare professionals clean and disinfect the patient’s skin, as well as the tools they use as part of the blood draw. They do this to ensure that bacteria won’t enter the syringe or blood culture bottle when drawing blood. However, there are some common places where these tools might pick up bacteria.

Common sources of contamination:

Human Touch:
- When blood is drawn, there are several points in which bacteria can be accidentally included within the blood sample because human skin has accidentally touched the surface. This includes during the handling of the blood culture bottle or syringe, or during skin preparation (usually with an alcohol swab). A healthcare professional will attempt to disinfect supplies before drawing blood, but contamination during this step is possible.
Skin Plugs & Fragments:
- As the hollow-bore needle enters the vein (venipuncture), it may pull in skin fragments containing microbes. It is possible to disinfect the skin, but not to sterilize it, and up to 20% of skin flora remains viable in the keratin layer of the skin even after careful skin prep. These fragments may contain bacteria that will grow in the blood culture. Likewise, stray microbes that get pulled into the blood culture bottle with the venipuncture will also grow and contaminate the culture.

Though there are best practices in place for healthcare professionals, they aren’t fool-proof. Here’s why:

Skin antisepsis can’t completely kill bacteria. As mentioned, healthcare professionals will disinfect the skin before drawing blood. This is also known as skin antisepsis. Skin antisepsis, unfortunately, cannot entirely prevent the contamination of blood cultures by skin flora. Some studies show that as much as 20% of skin-associated bacteria have been found to survive disinfection. Researchers have been able to discern this by culturing skin samples post skin antisepsis.³
It’s not just the skin’s surface healthcare professionals have to worry about. Bacteria can be located in deeper layers of the skin or in other structures that antiseptics cannot penetrate, which means a syringe or needle entering the skin may pick up possible contaminants on its way to the patient’s vein, and enter the blood culture bottle when the blood is drawn. Nonetheless, inadequate skin preparation is thought to be the most common cause of blood culture contamination.⁴

How Can False-Positives Be Prevented?

A false-positive blood culture result can be prevented by using tools, like Steripath®, that divert the initial aliquot of blood that may contain bacteria and other contaminants to a separate chamber before collecting clean blood through a secondary sterile pathway. Steripath® is the Initial Specimen Diversion Device® (ISDD) that has been clinically proven to reduce blood culture contamination for improved patient health outcomes.

Learn More About Steripath Read More: Case Studies

Key Takeaway

Blood cultures are the gold standard for medical professionals to diagnose bloodstream infections in patients, but only if a non-contaminated blood sample can be obtained. Because there are microorganisms on the skin, blood cultures can become contaminated even after thorough skin disinfection. The resulting blood culture can grow the bacteria, resulting in a false-positive result. The use of ISDDs®, like Steripath®, can prevent these contamination events from occurring at the source.

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