Culture-negative endocarditis is an infection and inflammation of the lining of one or more heart valves, but no endocarditis-causing germs can be found in a blood culture. This is because certain germs do not grow well in a laboratory setting, or some people have received antibiotics in the past that keep such germs from growing outside of the body.
Endocarditis is usually a result of a bloodstream infection. Bacteria can enter the bloodstream during certain medical procedures, including dental procedures or through intravenous injection using non-sterile needles. Then bacteria can travel to the heart, where they can settle on damaged heart valves.
Baddour LM, Freeman WK, Suri RM, Wilson WR. Cardiovascular infections. In: Zipes DP, Libby P, Bonow RO, Mann DL, Tomaselli GF, Braunwald E, eds. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Philadelphia, PA: Elsevier; 2019:chap 73.