98 What percentage of blood cultures will be
positive in endocarditis?
Peter Wilson
The great majority of patients with endocarditis have positive
blood cultures within a few days of incubation and only a few
cases will become positive on further incubation for 1–2 weeks.
The proportion of culture-negative cases depends on the volume
of blood and method of culture but a common estimate is 5% with
a range from 2.5% to 31%.^1 Most cases of culture-negative endo-
carditis are related to use of antibiotics within the preceding two
weeks and probably represent infections with staphylococci,
streptococci or enterococci. If antibiotics have been given, with-
drawal of treatment for four days and serial blood cultures will
usually demonstrate the pathogen.
A number of organisms may grow only if incubated under the
correct conditions. Nutritionally-deficient streptococci may fail to
grow in ordinary media and yet are part of the normal mouth flora
and can cause endocarditis.^2 The HACEK organisms are slow
growing and easily missed. Coxiella burnetti, Chlamydia spp. and
Mycoplasma spp. are rare causes of endocarditis and are difficult to
grow, diagnosis requiring biopsy or serology. Bartonella spp. are
now known to cause endocarditis in homeless patients and diag-
nosis is difficult by conventional methods.^3
Three sets of blood cultures will demonstrate at least 95% of
culturable organisms causing endocarditis. After four negative
cultures there is only a 1% chance of an organism being identified
by later culture.^4 Contamination as the result of poor collection
technique makes interpretation difficult and is a greater risk
when repeated sets of culture are collected.
RReeffeerreenncceess
1 Barnes PD, Crook DWM. Culture negative endocarditis. J Infect
1997; 3355 : 209–13.
2 Stein DS, Nelson KE. Endocarditis due to nutritionally deficient strepto-
cocci: therapeutic dilemma. Rev Infect Dis1987; 99 : 908–16.
3 Raoult D, Fournier PE, Drancourt M et al. Diagnosis of 22 new cases of
Bartonella endocarditis. Ann Intern Med1996; 112255 : 646–52
4 Aronson MD, Bor DH. Blood cultures. Ann Intern Med1987; 110066 : 246–53.