What Are Blood Cultures? Why Doctors Test the Blood for Infection
A clear explanation of blood cultures, how doctors detect bloodstream infections, and why this test is critical in serious infections
A Fever That Raises Concern
Angela arrives in the emergency department shivering with a high fever.
She recently started dialysis and had a catheter placed in her chest. Over the past two days she has developed fatigue, body aches, and redness around the catheter site.
Her temperature is high and her heart rate is fast. When infections reach the bloodstream, doctors worry about a condition that can become dangerous quickly. The test that helps answer that question is called a blood culture.
What a Blood Culture Actually Is
A blood culture is a laboratory test used to detect bacteria in the bloodstream. A small sample of blood is drawn and placed into special bottles that allow microorganisms to grow if they are present. The laboratory then monitors these bottles over time to see if bacteria multiply.
If bacteria grow in the culture, doctors can identify the exact organism responsible for the infection.
This information also allows doctors to determine which antibiotics will work best to treat it.
Why Doctors Worry About Bloodstream Infections
Most infections begin in one location. A urinary tract infection starts in the bladder. Pneumonia begins in the lungs. Skin infections start in the tissue beneath the skin.
Sometimes, however, bacteria escape that original location and enter the bloodstream. When this happens, the infection can spread throughout the body.
This condition is called bacteremia, and if it progresses further it can lead to sepsis, a life threatening infection that affects multiple organs.
Who Is at Higher Risk
Anyone can develop a bloodstream infection, but certain patients have higher risk.
People with weakened immune systems, such as those with cancer or HIV, are more vulnerable. Patients with medical devices inside the body, including dialysis catheters or IV lines, also face increased risk because bacteria can enter through those access points.
Injection drug use is another well known risk factor. Injecting substances directly into the bloodstream can introduce bacteria that quickly spread through the body. Because these infections can progress rapidly, doctors take fevers in these patients very seriously.
Why Doctors Draw Blood from Two Sites
Blood cultures are usually drawn from two different locations. This helps doctors distinguish between a true bloodstream infection and contamination from normal skin bacteria.
When blood is drawn through the skin, small amounts of harmless bacteria that normally live on the skin can sometimes enter the sample. If only one culture bottle grows bacteria, doctors may suspect contamination.
But if bacteria grow from multiple blood samples taken from different sites, the likelihood of a real bloodstream infection becomes much higher.
THE BOTTOM LINE
• Blood cultures are tests used to detect bacteria in the bloodstream
• They help doctors diagnose serious infections such as bacteremia and sepsis
• Drawing blood from multiple sites helps confirm whether bacteria found in the test represent a true infection
By Dr. Karim Ali, Emergency Physician