A blood culture is a laboratory test where a sample of blood is placed in a special medium to see if infection-causing microorganisms, like bacteria or fungi, grow. Its purpose is to detect bloodstream infections. Coagulase-negative staphylococci (CoNS) are a group of bacteria frequently found in these cultures. The presence of CoNS can be difficult for clinicians to interpret, as it may signify either a genuine infection or simple contamination of the sample.
What Are Coagulase-Negative Staphylococci?
Coagulase-negative staphylococci are a diverse group of bacteria, with over 30 different species. These microorganisms are normal inhabitants of the human body, particularly the skin and mucous membranes. They form a part of the natural microbiome, living on the skin surface without causing harm in most healthy individuals. In this environment, they exist as commensal organisms, meaning they benefit from living on a host without affecting it.
One of the most abundant species is Staphylococcus epidermidis, which is found all over the epidermis, or the outer layer of skin. While these bacteria are harmless on the skin’s surface, their ubiquitous nature is a primary reason they become a focal point in blood culture analysis. Their constant presence means they can easily be introduced into medical samples if procedures are not perfectly sterile.
The Challenge of CoNS in Blood Cultures
The primary issue with finding CoNS in a blood culture is determining its clinical relevance. Because these bacteria are the most common type found on skin, they are also the most frequent contaminants of blood cultures. Contamination can happen during the blood draw when bacteria from the patient’s skin or the healthcare worker’s hands are unintentionally introduced into the blood sample, even with adherence to aseptic techniques.
This presents a dilemma for physicians. A positive blood culture for CoNS might represent a true bloodstream infection, also known as bacteremia, or it could be a false positive from contamination. Making the wrong call has consequences; treating a contaminant unnecessarily exposes a patient to antibiotics and contributes to antibiotic resistance, while failing to treat a true infection can lead to severe health outcomes.
The high rate of CoNS contamination complicates clinical decision-making. Studies show that a large percentage of CoNS-positive blood cultures are ultimately deemed contaminants rather than true infections. This uncertainty requires careful evaluation to ensure patients receive appropriate care without the overuse of antibiotics.
Determining True Infection from Contamination
Clinicians rely on a combination of laboratory data and clinical observations to distinguish a true CoNS infection from a contaminant. Several clues guide the assessment:
- The number of positive blood cultures. If multiple blood samples, drawn from different sites on the body or at different times, all grow the same species of CoNS, it is more likely to be a true infection. A single positive culture is often presumed to be contamination.
- The time to positivity. This is the time it takes for the bacteria to grow in the culture. A shorter time to positivity, often less than 16 hours, suggests a higher density of bacteria in the bloodstream, which is more consistent with a genuine infection.
- The patient’s overall health status and symptoms. Signs of a systemic infection, like fever, chills, low blood pressure, or an elevated white blood cell count, support the diagnosis of a true bacteremia.
- The presence of indwelling medical devices. Central venous catheters, prosthetic heart valves, or joint implants are a predisposing factor, as these devices can become colonized by CoNS and serve as a source for bloodstream infections.
When CoNS in Blood Cultures Indicate a Real Problem
When evidence points to a genuine CoNS bloodstream infection, it is treated as a serious medical condition. These infections are particularly concerning in specific patient populations, including premature infants, individuals with weakened immune systems, and patients with implanted medical devices. For these groups, CoNS can transition from a harmless skin resident to a significant pathogen.
A confirmed CoNS bacteremia can lead to severe complications and is a primary cause of infections related to catheters and other medical implants. In more serious cases, the bacteria can cause endocarditis, an infection of the heart’s inner lining, or infect prosthetic joints, requiring complex medical and surgical intervention. Many hospital-acquired strains of CoNS are resistant to common antibiotics, making treatment challenging.
Treatment for a true CoNS infection involves a course of antibiotics. The choice of antibiotic is guided by laboratory testing to determine which drugs the specific bacterial strain is susceptible to. In cases involving infected medical devices like a central line catheter, removal of the device is frequently necessary to clear the infection from the bloodstream.