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Millar et al in their interesting review discussed culture negative endocarditis.1 The mainstay of diagnosis of infective endocarditis is still conventional blood culture; however, blood culture may be negative in 1%–79% of all cases. The incidence of culture negative endocarditis has been increasing. This could be for a number of reasons. Prosthetic heart valves are prone to infection and in many of these cases the culture is negative. Many aetiological agents causing infective endocarditis may be fastidious in nature, such as the HACEK group of organisms2 or unusual and require specialised microbiological techniques.
Within our national survey of 180 cases in Slovakia,3 culture negative endocarditis appeared in 35 cases (19.5%), which is higher than that reported in the Netherlands (1%), the USA (5%), Sweden (12%), the UK (15%), France (18%), but lower than in Russia (26%) and Spain (37%–43%) and much lower than in India (53%–79%).
In univariate analysis comparing all cases (180) to culture negative (35 cases), prior cardiosurgery within two weeks (p<0.045), probable endocarditis (p<0.04) according to Duke's criteria,4 and emboli (p<0.001) were more frequently observed among the group with culture negative endocarditis, and prior dental surgery (p<0.03) and a definitive diagnosis (p<0.045) among all cases of endocarditis (see table 1). In addition multivariate analysis (STAT ADV computerised package of the postgraduate medical school) was performed. The only significant risk factor for culture negative endocarditis in multivariate analysis was presence of complications. The odds ratio was 2.45 (confidence interval 0.95 to 2.35) in the group with culture negative endocarditis, which was 2.45 times higher than in culture positive endocarditis.
Interestingly mortality was lower in culture negative endocarditis than among all cases (24.5% v 44.4%,p<0.001). Millar et al in his excellent review analysed reasons for culture negative endocarditis. We found according to our experience one more risk factor—prior cardiac surgery. Probably, those undergoing cardiac surgery and receiving antibiotic prophylaxis (first generation cephalosporins/cefazolin in Slovakia) have lower death rates in endocarditis due to protective effect of antimicrobials for occurrence of infection.