A blood culture is a laboratory test that identifies microorganisms like bacteria or fungi in the bloodstream. Normally, blood does not contain these pathogens. A “positive” result indicates these microorganisms are present, suggesting an active infection. This routine diagnostic tool helps detect and identify bloodstream infections, which can be serious. Test findings guide treatment strategies.
Understanding a Positive Blood Culture
A positive blood culture signifies microorganisms in the bloodstream, known as a bloodstream infection (BSI). When bacteria are detected, the condition is called bacteremia, while fungi presence is fungemia. This indicates an active infection that can impact various body parts.
The presence of bacteria or fungi in the blood is abnormal and can lead to sepsis. Sepsis is a serious inflammatory response to infection, potentially progressing to septic shock, a life-threatening condition with low blood pressure and organ dysfunction. While a positive result usually indicates a true infection, contamination is possible. Microorganisms from skin or lab equipment can cause a false-positive. Collecting multiple samples from different sites helps distinguish this; if only one sample grows, contamination is suspected.
Common Microorganisms and Infection Sources
Various microorganisms can cause positive blood cultures, including bacteria and fungi. Among bacteria, Staphylococcus aureus, Escherichia coli (E. coli), Klebsiella pneumoniae, and Enterococcus faecalis are frequently identified. Coagulase-negative staphylococci are also common, though their presence sometimes indicates skin contamination rather than true infection.
Fungal infections (fungemia) are most often caused by Candida species, especially Candida albicans. These microorganisms usually enter the bloodstream from other infected body sites. Common origins include pneumonia (lung infections), urinary tract infections, skin and soft tissue infections, and infections originating from surgical sites. Medical devices, like intravenous catheters, are also frequent sources.
Signs and Symptoms That Prompt a Blood Culture
Healthcare providers order blood cultures when a patient shows signs of systemic infection. Common indicators include high fever, often with chills and shivering. Patients may also experience a rapid heart rate and rapid breathing.
Other symptoms include confusion or altered mental status, generalized weakness, and fatigue. Low blood pressure, especially a significant drop, can also signal a severe systemic infection. These symptoms suggest an infection may have entered the bloodstream, requiring prompt investigation.
Managing a Positive Blood Culture
Upon a positive blood culture, the immediate next step is identifying the specific microorganism. Lab technicians perform a Gram stain for preliminary bacterial information, guiding initial treatment. Further testing, like antibiotic sensitivity testing (antibiogram), determines which antimicrobial medications are most effective.
Before full sensitivity results, healthcare providers initiate empirical antibiotic or antifungal therapy. This involves broad-spectrum medications chosen based on suspected infection type, patient condition, and local resistance patterns. Once the microorganism and its sensitivities are known, treatment is refined to targeted therapy, using specifically effective medications. Along with medication, identifying and controlling the infection source is important, possibly involving removing an infected catheter or draining an abscess. Prompt and appropriate treatment is necessary, as bloodstream infections can escalate to severe conditions like sepsis and septic shock.