Blood Culture: Understanding Your Report

A blood culture is a laboratory test that looks for microorganisms, such as bacteria or fungi, growing in your blood. Microorganisms (also called microbes) are living things too small to see without a microscope. They are not normally found in the blood, so their presence is an important finding. A blood culture is usually performed in two stages: first, the laboratory checks whether any microorganisms are growing in a sample of your blood, and then, if something grows, it tests which antibiotics are likely to stop it. An antibiotic is a medicine used to treat infections caused by bacteria, and a related group of medicines, called antifungals, is used to treat infections caused by fungi.

This article explains what a blood culture report shows, what the different results mean, and what to ask your doctor, so you can better understand a report you have received. A blood culture is ordered when there is concern that an infection may have reached the bloodstream, which can be serious. In most cases, a result of “no growth” is reassuring, while a result that names a microorganism tells your healthcare team what is causing the infection and which medicines should work against it.

Why is a blood culture done?

Microorganisms are not normally present in the blood. They can enter the bloodstream after an infection or injury involving the skin, lungs, urinary tract, or digestive tract. Once in the blood, they can spread throughout the body. An infection in the bloodstream caused by bacteria is called bacteremia, and one caused by fungi is called fungemia.

A blood culture is done when your healthcare team suspects this may be happening, because an untreated bloodstream infection can lead to serious conditions. These include endocarditis (an infection of the heart valves), meningitis (an infection of the lining around the brain and spinal cord), and sepsis (a serious, life-threatening response of the body to infection). Identifying the microorganism early and learning which antibiotics will work against it helps guide treatment decisions quickly.

How is a blood culture collected and tested?

A small amount of blood is drawn from a vein and added to special bottles. Each set of blood cultures usually includes two bottles. One is an aerobic bottle, in which the blood is exposed to oxygen, and the other is an anaerobic bottle, in which it is not. Both are used because some bacteria need oxygen to grow, while others cannot survive in it.

Two details about the collection often appear on the report or come up in conversation with your team:

The amount of blood collected matters — Filling each bottle with the recommended amount of blood, often about 8 to 10 milliliters in adults, improves the chance of detecting an infection. Too little blood can cause a true infection to be missed.
More than one set is usually collected — Blood is often drawn for two or more sets, sometimes from different veins or at slightly different times. Comparing the sets helps the laboratory tell a true bloodstream infection from harmless skin bacteria that can enter the sample during collection.

The bottles are placed in an incubator, a machine that maintains a temperature near body temperature (about 37 degrees Celsius, or 98.6 degrees Fahrenheit) so that any microorganisms present will grow. Modern laboratories use automated systems that watch the bottles continuously and signal as soon as growth is detected, which is often within one to two days. Because growth takes time, results usually arrive in stages. A preliminary report describes what is known so far, such as early growth or a Gram stain result. A final report, usually available after several days, gives the complete identification and the antibiotic susceptibility results. Some microorganisms grow slowly and take longer.

The Gram stain: an early clue

When a bottle indicates that something is growing, the laboratory usually performs a Gram stain immediately. A Gram stain is a quick test in which a drop of the sample is treated with a dye on a glass slide and examined under a microscope. It sorts bacteria into groups based on their color and shape and provides an early clue about the type of bacteria present before the full identification is complete.

Bacteria that hold the purple dye are called gram-positive, and bacteria that turn red or pink are called gram-negative. Round bacteria are called cocci, and rod-shaped bacteria are called bacilli. Your healthcare team uses this early information to make an initial estimate of where the infection may have started and which antibiotics are most likely to help, while waiting for the final results.

What your blood culture report shows

The first part of the report describes what grew, if anything. You may see any of the following.

No growth — No microorganisms grew in the sample. For most blood cultures, this is the expected, reassuring result. A preliminary report may state “no growth” after the first day or two, and the final report confirms it after the full incubation period.
Gram-positive cocci — Round bacteria that appear purple under the Gram stain. Examples include Staphylococcus aureus and Streptococcus pneumoniae.
Gram-negative cocci — Round bacteria that appear red or pink. An example is Neisseria meningitidis.
Gram-positive bacilli — Rod-shaped bacteria that appear purple. Examples include Listeria monocytogenes and Clostridium species.
Gram-negative bacilli — Rod-shaped bacteria that appear red. Examples include Escherichia coli (often written E. coli) and Klebsiella pneumoniae.
Yeast — A type of fungus. Examples include Candida species and Cryptococcus neoformans.

The report also states how many of the bottles grew the microorganism, which is an important part of understanding the result, as explained in the next section.

Telling a true infection from contamination

The skin is normally home to harmless bacteria. Sometimes these skin bacteria get into a blood sample as the needle passes through the skin during collection. When this happens, the microorganism that grows is called a contaminant, because it does not reflect a true infection in the blood.

This is the main reason blood is usually collected as more than one set. When the same microorganism grows in several bottles or several sets, it is more likely to represent a true bloodstream infection. When a common skin bacterium grows in only one of several bottles, it more often indicates contamination. Deciding between the two is not always straightforward, and your healthcare team makes this judgment by considering the result alongside your symptoms and overall condition rather than the report alone.

Identifying the exact microorganism

Once growth is detected, the next step is to identify exactly which microorganism is present. A small amount from the bottle is spread onto a culture plate, such as a blood agar plate, where the microorganisms grow into small visible clusters called colonies. The laboratory then identifies the microorganism using one or more methods. These may include automated instruments, mass spectrometry (a method that identifies a microorganism by its molecular fingerprint), or molecular tests that detect the microorganism’s genetic material. These newer methods can shorten the time to a name, sometimes to within hours of a bottle turning positive. The final report includes the name of the microorganism found.

Antibiotic susceptibility (sensitivity) testing

If a microorganism grows, the laboratory tests how well different antibiotics stop it. This is called antibiotic susceptibility testing, or sensitivity testing. In a common method, the microorganism is spread onto a plate containing small discs impregnated with different antibiotics; a clear zone around a disc indicates that the antibiotic inhibited the microorganism’s growth. Many laboratories now use automated systems that measure this more precisely and report a number for each antibiotic.

In the report, each antibiotic is usually given one of three results:

Susceptible (S) — The antibiotic is expected to work against this microorganism at the usual dose. These are the medicines most likely to treat the infection.
Intermediate (I) — The result is in between. The antibiotic may still work, but often only at a higher dose or in areas where it becomes concentrated. It may also mean the result is uncertain.
Resistant (R) — The microorganism can survive this antibiotic, so the medicine is not expected to work and is usually avoided.

Some reports also include a number called the minimum inhibitory concentration, or MIC. This is the smallest amount of an antibiotic required to prevent the microorganism from growing in the laboratory. The laboratory compares the MIC to a standard cutoff to decide whether the result is susceptible, intermediate, or resistant. A lower MIC for one antibiotic does not mean it is “stronger” or a better choice than another antibiotic with a higher number, because each antibiotic has its own separate cutoff. For this reason, MIC values cannot be directly compared between medicines. You may also notice that not every antibiotic is listed. Laboratories usually report a selected group that is appropriate for the microorganism and the site of infection.

What “no growth” means, and what it does not mean

A result of no growth usually means that no bacteria or fungi were found in your blood. In many situations, this is reassuring. However, no growth does not always mean there is no infection. A few situations can explain a negative result even when an infection is present:

Antibiotics were already started — If you began taking an antibiotic before the blood was collected, it may have prevented the microorganism from growing in the laboratory.
The microorganism is hard to grow — Some bacteria and fungi grow slowly or need special laboratory conditions, so they may not appear on a standard blood culture.
The infection is caused by something a blood culture does not detect — A blood culture looks for bacteria and fungi. It does not detect viruses, so a virus-caused infection will show no growth.

For these reasons, your healthcare team considers a no-growth result together with your symptoms and other test results, rather than relying on it alone.

What happens after a blood culture

A blood culture report describes what was found in your blood and which medicines are likely to work against it. It informs the decisions you and your healthcare team make together, but it does not, by itself, prescribe a treatment.

When a serious bloodstream infection is suspected, doctors often start antibiotics right away, before the results are available, choosing a broad-spectrum antibiotic that targets many common microorganisms. As the results come back in stages, the report helps the team refine that choice. The Gram stain may narrow the options within hours, the identification names the microorganism, and the susceptibility results show which antibiotics it responds to. Based on these findings, the team may switch to a narrower, more targeted antibiotic, which can reduce side effects and limit the development of resistance. The team may also look for and treat the source of the infection. For certain bloodstream infections, repeat blood cultures are collected to confirm that the blood has cleared.

Questions to ask your doctor

What were you concerned about when you ordered my blood cultures?
Did anything grow, and in how many of the bottles?
Does this result represent a true bloodstream infection, or could it be a contaminant?
What is the name of the microorganism found in my blood?
Why was my blood collected more than once or from different sites?
Which antibiotics is the microorganism susceptible to?
Why was the antibiotic I am receiving chosen, and will it change now that the results are back?
Could a recent antibiotic have affected whether anything grew?
Where do you think the infection started, and does that source need to be treated?
Will I need repeat blood cultures to confirm the infection has cleared?
How long is treatment likely to last?
Who should I contact if I feel worse while waiting for the final report?