A Blood Culture and Sensitivity test is a critical microbiology lab test used to check whether there are germs (most often bacteria or fungi) in the bloodstream. Because blood is normally sterile, finding an organism in a blood culture may indicate a serious blood infection, such as bacteremia or septicemia (sepsis). This test is often simply called a blood culture or culture and sensitivity.

The test has two main parts:

1) Culture: Your blood sample is placed into special bottles that help germs grow if they are present. The lab monitors these bottles for signs of growth and then identifies the specific organism (for example, Staphylococcus aureus, E. coli, Streptococcus species, Candida, and others).

2) Sensitivity (antibiotic susceptibility): If an organism grows, the lab tests which antibiotics (or antifungals) are likely to work best. This helps your clinician choose targeted therapy—treatment aimed at the exact germ—rather than relying only on broad-spectrum antibiotics.

What results can mean:

Negative / No growth: No bacteria or fungi were found in the sample during the time the lab monitored it. This makes a bloodstream infection less likely, but it does not always rule it out—especially if antibiotics were started before the sample was drawn or if the infection is intermittent.

Positive / Growth detected: A germ was found. The report typically includes the organism name and, when available, the sensitivity results showing which medications are “susceptible” (likely effective), “intermediate,” or “resistant” (less likely to work). Because skin bacteria can sometimes contaminate the sample, clinicians interpret a positive culture along with symptoms, number of positive bottles, and timing to decide whether it represents a true infection.

Blood cultures are most often ordered when a clinician suspects a serious infection that may be spreading through the bloodstream, especially sepsis. Early detection and the right antibiotics can be lifesaving.

Common reasons your doctor may order a blood culture and sensitivity include:

Symptoms suggesting sepsis or a bloodstream infection: fever or chills/rigors, very fast heart rate, rapid breathing, low blood pressure, confusion or unusual sleepiness, severe weakness, dizziness, reduced urine output, or feeling suddenly very ill.

Possible infection source with systemic symptoms: pneumonia, urinary tract/kidney infection (pyelonephritis), abdominal infection, skin/soft tissue infection, infected wounds, bone/joint infection, or an infection after surgery.

Higher-risk situations: weakened immune system (chemotherapy, transplant medications, HIV, long-term steroids), uncontrolled diabetes, older age, recent hospitalization or ICU stay, or recent invasive procedures.

Concern for endocarditis: infection of the heart valves, especially if there is a new heart murmur, persistent fever, a history of valve disease, or a prosthetic valve.

Suspected catheter-related bloodstream infection: symptoms of infection in people with central lines, ports, PICC lines, or other indwelling devices.

Before or soon after starting antibiotics: Blood cultures are ideally drawn before antibiotics when possible, because antibiotics can reduce the chance of detecting the organism. In urgent cases, treatment may begin immediately, but cultures are still valuable.

How it’s usually collected: Blood is drawn from a vein using sterile technique. Often, two or more sets of cultures (from different sites and/or at different times) are collected to improve detection and help distinguish true infection from contamination.

• Sepsis (systemic inflammatory response to infection)
• Septicemia / bloodstream infection (bacteremia or fungemia)
• Infective endocarditis (heart valve infection)
• Pneumonia with suspected bloodstream spread
• Pyelonephritis (kidney infection) and complicated urinary tract infection
• Meningitis or other invasive bacterial infections
• Osteomyelitis (bone infection)
• Septic arthritis (joint infection)
• Intra-abdominal infections (e.g., perforation, abscess)
• Catheter-related bloodstream infection and other device-associated infections
• Candidemia (Candida bloodstream infection)

• Rapid diagnosis of a suspected blood infection to guide urgent care decisions
• Early detection and treatment of sepsis to reduce complications and improve survival
• Choosing the most effective antibiotic/antifungal based on culture and sensitivity results (targeted therapy)
• Reducing antibiotic resistance by avoiding unnecessary or ineffective antibiotics
• Monitoring response to treatment in persistent or recurrent bacteremia/fungemia (repeat cultures when indicated)
• Identifying the infection source (e.g., catheter, heart valve, urinary tract) to guide further testing and prevention
• Improving safety for high-risk patients (immunocompromised individuals, hospitalized/ICU patients, people with implanted devices)