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Antibody response in six HACEK endocarditis cases under therapy

The antibody response to bacteria of the so‐called HACEK group, i.e. Haemophilus spp., Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens and Kingella kingae, was measured in sera of six patients with endocarditis. The corresponding isolates from their blood cultures were identified by conventional methods, including reactions for nitrate reduction and catalase as well as acid production from sugars. Crude antigens were prepared by glycine extraction and sonification of the blood culture isolates, and used to determine titers by complement fixation. A patient with Haemophilus parainfluenzae bacteremia received a short course of antibiotic therapy, and relapsed with spondylitis and endocarditis 5 months later. Titers of sera against his own isolate rose from 1:40 to 1:320 and fell to 1:40 after therapy within one year. A patient with C. hominis endocarditis had a similarly prolonged course. The complement fixation titer against his own isolate was already 1:240 before antibiotics were administered. Another patient with C. hominis endocarditis presented a titer of 1:320 2 weeks after the diagnosis. These three patients revealed C‐reactive protein values over 50 mg/l in the first serum sample. Decrease of both antibody titers and C‐reactive protein values correlated with clinical improvement. Two patients with prosthetic valve replacement 5 months earlier developed C. hominis and K. kingae endocarditis, respectively. At admission, C‐reactive protein values were 64 and 82, respectively, and therapy was instituted immediately. The first sera were received 3 and 6 weeks, respectively, after isolation of the corresponding blood culture isolates and revealed already low titers, i.e. 1:80 and 1:60, respectively. A woman with A. actinomycetemcomitans endocarditis received immediate therapy and did not develope titers against her own isolate. CRP was 100 at admission and remained over 50 5 weeks later. We conclude that the complement fixation assay with individual antigen preparations was easy to perform and allowed monitoring of the antibody response in 5 of 6 HACEK endocarditis cases under therapy, but the usefulness of this method to find culture‐negative HACEK endocarditis needs to be established.