Why Get Blood Cultures Before Antibiotics for Endocarditis?

If your doctor suspects you might have infective endocarditis (an infection of the heart valves or inner lining of the heart), they will likely want to draw blood for testing before starting you on antibiotic treatment ^[1]. It is critical that you do not self-start any leftover or unprescribed antibiotics while waiting for medical care. Taking even a single dose of an antibiotic without a doctor’s guidance can suppress bacteria in your bloodstream, making it much harder for tests to accurately identify the cause of your illness ^[2].

Important: Never stop taking a medication that was currently prescribed to you unless your treating clinician specifically tells you to do so. This guide only applies to self-starting unprescribed or leftover antibiotics.

How Blood Cultures Work

The primary test used to identify a bloodstream infection is called a blood culture. When you have your blood drawn for this test, the medical team will usually take multiple samples from different veins before your first dose of antibiotics ^[3]. These samples are placed in special bottles and kept in a warm incubator to see if any microorganisms (like bacteria or fungi) grow over several days ^[3].

If bacteria grow, the lab can help identify the exact organism causing your illness. The lab will also perform susceptibility testing, which exposes the bacteria to different medications to see which ones are most likely to destroy it. This process helps your medical team choose the most targeted, effective treatment and determine exactly how long you need to take it ^[4].

The Danger of a “False Negative”

If you take an antibiotic before your blood is drawn—even an old prescription you had in your medicine cabinet—the medication starts fighting the bacteria immediately. A short or incorrect course of antibiotics usually isn’t enough to cure a serious infection like endocarditis, but it can temporarily clear the bacteria from your bloodstream or stop them from growing in the lab ^[2].

When this happens, your blood culture results may show that no bacteria are present, resulting in a “false negative.” In some studies, prior antibiotic use is a leading cause of these false negatives, accounting for roughly a third of all endocarditis cases where blood cultures fail to grow an organism ^{[5] [6]}.

A negative blood culture does not mean you don’t have endocarditis, as other issues (like fungi or uncommon organisms) can also cause negative results ^[7]. However, a false negative caused by early antibiotics can complicate your care in several ways:

• It delays a clear diagnosis: Your care team may have a harder time confirming exactly what is making you sick ^[5].
• It makes targeted treatment difficult: Without knowing exactly which bacteria are causing the infection, doctors must rely on broad-spectrum antibiotics (medications designed to treat many different types of bacteria). While these are life-saving, they may have more side effects than a medication tailored specifically to your infection ^[4].
• It requires additional testing: To figure out the cause of a culture-negative infection (an infection where blood cultures don’t show growth), your doctor will likely need to order specialized blood work (like genetic or antibody tests) and detailed heart imaging, such as an echocardiogram, to look for signs of infection on your heart valves ^{[7] [8]}.

What To Do If You Already Took an Antibiotic

If you already took an antibiotic, do not panic and do not take extra doses. Simply tell your medical team exactly what medication you took, the dose, and when you last took it. This is vital information that helps them correctly interpret your tests and plan your care without judgement ^[9].

What If It’s an Emergency?

While it is standard practice to draw blood cultures before starting antibiotics in stable patients ^[10], your immediate safety is always the top priority ^[1].

If you are experiencing severe symptoms—such as confusion, fainting, severe shortness of breath, chest pain, or extreme weakness—do not wait at home. Seek emergency medical care immediately. In severe medical emergencies, your care team will draw your blood as quickly as possible and start intravenous (IV) antibiotics promptly to stabilize you ^[11]. Never delay an emergency evaluation just to get a blood culture.