How endocarditis is diagnosed:
blood cultures, echocardiography and other tests

Article written and reviewed by Dr Luca Moretti

The diagnosis of infective endocarditis does not depend on a single test. The physician combines symptoms and risk factors, blood culture results and cardiac imaging to establish whether infection is present and whether it has already caused damage to the valves or other structures.
The two initial pillars are therefore microbiology and imaging: blood cultures look for the microorganism in the blood, while echocardiography looks for vegetations, abscesses, valvular abnormalities or other lesions compatible with endocarditis.

In practice, the diagnostic pathway may include:

• at least three sets of blood cultures before antibiotics, when conditions allow;
• transthoracic echocardiography as the first cardiac imaging test;
• transesophageal echocardiography when suspicion remains high or prosthetic valves and devices are present;
• repeat echocardiography if the first study is negative or uncertain but suspicion persists;
• cardiac CT, PET/CT or other imaging techniques in selected cases;
• serological or molecular tests when blood cultures remain negative;
• tests of other organs if emboli or other complications are suspected.

An accurate diagnosis is also important for choosing the antibiotic and establishing whether surgery is necessary. The monograph on infective endocarditis discusses diagnostic criteria, microbiology and classification in depth; this guide instead explains what the tests that are requested mean for the patient and why they sometimes need to be repeated or combined.

Blood cultures: why they are the first test to perform before antibiotics

Blood cultures are blood samples sent to the laboratory to determine whether bacteria or fungi are present and, when possible, which antimicrobials they are susceptible to. In endocarditis, the microorganism tends to circulate in the bloodstream relatively continuously, so it is not necessary to wait for the fever to rise before collecting samples.
ESC guidelines recommend obtaining at least three sets of blood cultures before antibiotic therapy is started when endocarditis is suspected, using peripheral venipuncture and sterile technique. Each set normally includes bottles designed to detect aerobic and anaerobic microorganisms.

The reason samples are collected before antibiotics is simple: even a few doses of an effective drug can reduce the number of microorganisms in the blood and make them more difficult to identify. This does not mean that treatment should be dangerously delayed in a severely unstable person. In sepsis or shock, samples are collected rapidly and antibiotics are started without waiting for laboratory results.
A single positive blood culture does not automatically mean endocarditis. Some bacteria may accidentally contaminate the sample from the skin, whereas others, especially when found in multiple samples, have much greater significance. Interpretation therefore depends on the type of microorganism, the number of positive samples and the overall clinical picture.

Alongside blood cultures, general blood tests such as complete blood count, creatinine, liver function, C-reactive protein and other markers of inflammation or organ damage are performed. They may show anemia, elevated inflammatory markers or renal abnormalities, but none of these values alone diagnoses endocarditis. Rather, they help assess severity, complications and treatment safety.

Transthoracic and transesophageal echocardiography: what is the difference

Transthoracic echocardiography, performed by placing the probe on the chest, is normally the first imaging examination. It is rapid, noninvasive and allows assessment of the valves, cardiac function and findings compatible with endocarditis. One of the best-known lesions is the vegetation, an infected mass that can form on a valve, but echocardiography also looks for abscesses, perforations, fistulas, new valvular regurgitation and, in prosthetic valves, signs of dehiscence or perivalvular infection.
A normal result does not always exclude the disease. Small vegetations, certain locations and especially the presence of prosthetic material can make transthoracic echocardiography less sensitive.

Transesophageal echocardiography uses a probe inserted into the esophagus, which lies immediately behind the heart. The closer proximity provides more detailed images and makes the examination particularly useful when transthoracic echocardiography is negative or equivocal but clinical suspicion remains high, when the first echocardiogram is already positive and complications must be sought, or when prosthetic valves or intracardiac devices are present.
Because it is semi-invasive, transesophageal echocardiography requires preparation and is performed in a healthcare setting with appropriate precautions. It is not automatically replaced by CT or PET: the different techniques provide complementary information.

If the first echocardiogram is normal or inconclusive but suspicion of endocarditis remains high, ESC guidelines indicate repeating TTE and/or transesophageal echocardiography after approximately 5-7 days, or according to the clinical situation. Endocarditis is a dynamic disease: a lesion that is not visible initially may become evident later, while a follow-up study may show progression of a vegetation or the development of a complication.

When cardiac CT, PET/CT and other imaging tests are needed

Echocardiography remains the starting point, but diagnosis of endocarditis today is often multimodal. Cardiac CT can be particularly useful for evaluating abscesses, pseudoaneurysms, fistulas and other complications around the valve or prosthesis. In some situations it better defines the anatomical extent of infection and can contribute to surgical planning.
18F-FDG PET/CT looks for areas of increased inflammatory metabolism. It has a role particularly when prosthetic material is present, for example a prosthetic valve, a transcatheter-implanted valve or certain cardiac devices, in which echocardiography may be less conclusive.

In specialized centers, radiolabeled leukocyte SPECT/CT may also be used. The choice depends on the clinical situation, local availability and the time elapsed since surgery: after a recent procedure, postoperative inflammation can complicate interpretation of some nuclear imaging studies.
Whole-body CT, brain magnetic resonance imaging or other investigations may be requested not to visualize the valve better, but to look for distant complications. Endocarditis can cause emboli or infectious foci in the brain, spleen, kidneys, vertebrae or other organs, sometimes even without obvious symptoms.

What happens if blood cultures are negative

A negative result does not automatically exclude endocarditis. The most common cause of blood culture-negative endocarditis is antibiotic use before blood sampling, but there are also microorganisms that grow poorly with conventional culture techniques. In these cases, the laboratory and infectious disease specialist direct more specific tests based on the patient's clinical history and exposures.
Serological tests and molecular techniques such as PCR and sequencing may be used to detect microorganisms such as Coxiella burnetii, Bartonella species, Tropheryma whipplei and other agents that are difficult to identify with a standard blood culture.

If the patient undergoes surgery, valve tissue or removed material can be analyzed with cultures, histology and molecular techniques. These tests can provide decisive information when blood has not identified the causative agent.
The monograph on blood culture-negative endocarditis discusses this subgroup in detail. For the patient, the practical point is that a negative culture does not end the diagnostic pathway if symptoms, risk factors and imaging continue to suggest endocarditis.

Diagnostic criteria: why “having a vegetation” or a positive culture is not enough

Physicians combine findings using structured diagnostic criteria, now updated both by ESC guidelines and the 2023 Duke-ISCVID criteria. Major elements include certain microbiological results and specific lesions demonstrated by echocardiography or other imaging techniques; minor elements may include predisposition, fever, vascular or immunological manifestations and microbiological results that do not meet a major criterion.
These systems classify the clinical picture as definite, possible or rejected, but they are not a test that patients can apply independently. Diagnosis requires clinical interpretation, because a mass on a valve may have noninfectious causes and bacteremia may occur even without endocarditis.

In complex cases, assessment is entrusted to an Endocarditis Team, integrating cardiology, infectious diseases, microbiology, imaging and cardiac surgery. The advantage of this approach is that diagnosis and treatment are assessed together: it is not enough to establish whether infection is present, but it is necessary to understand which microorganism is causing it, how extensive it is, whether emboli or heart failure are present and whether the valve can be treated with medication alone or requires surgery.

Frequently asked questions about the diagnosis of endocarditis

Which tests are most important?
Blood cultures and echocardiography are the initial cornerstones. Cardiac CT, PET/CT, SPECT/CT, MRI and molecular tests are added when they can clarify the clinical picture or identify complications.

Do blood cultures have to be taken while there is fever?
No. In endocarditis, bacteremia tends to be continuous, so it is not necessary to wait for the fever peak. It is instead important, when possible, to collect samples before antibiotics are started.

Is a negative echocardiogram enough to exclude endocarditis?
No. If suspicion remains high, transesophageal echocardiography may be necessary, the echocardiogram may need to be repeated after a few days or other imaging techniques may be required.

Why is PET/CT used especially with prosthetic valves?
Prosthetic material can create artifacts and make it more difficult to identify vegetations or perivalvular infection with echocardiography alone. PET/CT can show abnormal inflammatory activity around prosthetic material.

If cultures are negative, does that mean there is no infection?
No. Antibiotics taken before blood sampling or microorganisms that are difficult to culture can cause culture-negative endocarditis and require additional tests.