Blood Culture: Understanding Your Report

Section Editor: Rodney E. Rohde PhD
August 18, 2026

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 organisms 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 blood sample, and then, if a microorganism that may be causing infection grows, further testing may be done to determine which medicines 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, treats 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 “no growth” result is reassuring. When a microorganism does grow, the report names what was found in the culture, and your healthcare team then decides whether it represents a true bloodstream infection or contamination from the skin, and which medicines are likely to 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. Bacteria in the bloodstream are called bacteremia, and fungi in the bloodstream are called fungemia.

Your healthcare team may order a blood culture when they suspect this is 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 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, where the blood is exposed to oxygen, and the other is an anaerobic bottle, where it is not. These two bottle types provide different growing conditions: some microorganisms grow best with oxygen, others grow better without it, and many can grow either way.

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

The amount of blood collected matters — Each bottle commonly holds about 8 to 10 milliliters of blood in adults, though the recommended amount depends on the culture system the laboratory uses. Collecting enough blood in total, often about 40 to 60 milliliters across two or more sets in adults, is one of the most important factors in whether a bloodstream infection is detected. Too little blood can cause a true infection to be missed.
More than one set is usually collected — Blood is usually drawn as two or more sets, commonly from separate needle sites. The sets do not need to be spaced widely apart in time. Comparing the sets helps the laboratory tell a true bloodstream infection from harmless skin bacteria that can enter the sample during collection.

When possible, blood cultures are collected before starting antibiotics because antibiotics given first can prevent a microorganism from growing in the laboratory.

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 monitor the bottles continuously and signal as soon as growth is detected, 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 may become available over the following days and, when appropriate, includes microorganism identification and antibiotic susceptibility results. Turnaround time varies by microorganism and laboratory method, and some microorganisms grow slowly and take longer.

The Gram stain: an early clue

When a bottle indicates growth, 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. The laboratory may also describe how the bacteria are arranged, for example, cocci in clusters or cocci in pairs and chains, which gives further clues about which microorganism is likely present. Your healthcare team uses this early information to estimate 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 some 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 bottles grew the microorganism, which is important for understanding the result, as explained in the next section.

Telling a true infection from contamination

The skin normally hosts 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 why blood is usually collected in more than one set. Several factors are weighed together: which microorganism was found, how many sets grew it, your symptoms and risk factors, and how the sample was collected. A common skin bacterium that grows in only one of several sets often suggests contamination. Other microorganisms, including Staphylococcus aureus, many gram-negative bacilli, and Candida, are treated as clinically important even when they grow in only one culture. The number of positive bottles alone does not settle the question, and your healthcare team makes this judgment by considering the result alongside your symptoms and overall condition, not 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.

Antimicrobial susceptibility (sensitivity) testing

If a microorganism grows, the laboratory tests how well different medicines stop it. This is called antimicrobial susceptibility testing, or sensitivity testing. Bacteria are tested against antibiotics, and fungi are tested against antifungal medicines when needed. 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. Depending on the method used, the laboratory may measure the size of that clear zone, or determine a specific concentration called the minimum inhibitory concentration.

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 medicines are most likely to treat the infection. A susceptible result does not automatically make an antibiotic the best choice for every patient, because the infection site, dose, allergies, kidney and liver function, and other medicines also matter.
Intermediate (I) — The microorganism’s response falls between susceptible and resistant. The medicine may still work in certain circumstances, such as when a higher dose can be given safely or when the medicine concentrates at the site of infection. Your healthcare team interprets this result based on the specific antibiotic, the dose, the infection site, and the laboratory’s standards.
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, you cannot directly compare MIC values 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.
Not enough blood was collected — Microorganisms may be present in the blood in very small numbers. If too little blood goes into the culture bottles, an infection can be missed.
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 — Routine blood cultures are designed mainly to find common bacteria and certain fungi, especially yeasts. They do not detect viruses, so a viral infection will show no growth. Some unusual or slow-growing microorganisms need specialized cultures or molecular tests instead.

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 doctors suspect a serious bloodstream infection, they often start antibiotics right away, before 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 resistance development. The team may also look for and treat the source of the infection. For certain bloodstream infections, doctors collect repeat blood cultures to confirm 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?
Was enough blood collected for the test to be reliable?
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 treatment?
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?