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The diagnostic application of sonication on heart valves with suspected infective endocarditis

(2016) Bijker, K.L.B. (Kasper)

Introduction: Infective endocarditis (IE) is a very serious disease. Although relatively uncommon (5-11 per 100,000 people per year), it is associated with a high mortality and morbidity. The infection is predominantly caused by bacteria. However, in 10-20% of cases no causative organism is identified. This is a serious limitation to the ability of clinicians to provide the afflicted patients with optimal treatment.
Infected heart valves may become damaged through the disease process, resulting in impaired functioning. Valves affected by IE may require surgical replacement. An explanted heart valve may provide important additional opportunity to obtain information pertaining to the causative microorganism, particularly in cases of blood culture-negative IE. However, standard microbiological tests on explanted heart valves frequently lead to negative results (up to 49%). An increase in sensitivity for the detection of microorganisms would be beneficial to the diagnosis and treatment of IE.
Sonication has demonstrated an increased microbiological yield when carried out on various prosthetic materials that were suspect for infection, mainly in prosthetic joint infections. The added value is particularly apparent in material cultures of patients that were receiving prior antimicrobial therapy. It is hypothesized that similar added value may be seen when applied to explanted (prosthetic or native) heart valves.
Methods: Through a prospective case-control study, the results obtained from the experimental sonication protocol were compared to the results of the standard diagnostic protocol for explanted cardiac material. Results were obtained through cultures on agar plates, inoculation of aerobic and anaerobic blood culture bottles, and 16S PCRs. The sonication tests were always done after the standard workup, avoiding interference with regular diagnostics necessary for the clinics. Explanted valves with no suspicion of IE were used as negative controls, in order to assess the reliability of the method.
Based on a power analysis for dependent samples with α=0.05, β=0.80, and d=0.3, the necessary number of valves was determined to be n=16. Based on the UMCG’s records, the expected time needed to collect 16 valves per study group would exceed 1 year. The project was set up as a pilot study in order to evaluate and optimise the proposed study protocol, and to start with the collection of patient data. The results presented in this report are therefore limited to the number of valves collected within a 3 month period, between 01-04-2016 and 01-07-2016.
Results: Fourteen valves were included, of which 7 were suspected of IE. Polymerase chain reaction (PCR) methods identified microbial deoxyribonucleic acid (DNA) from present microorganisms on 4 of these valves. Cultures identified the presence of viable microorganisms on 2 of these valves. All negative control valves showed no evidence of microbial presence according the standard diagnostic method and the sonication protocol.
Notably, there was one clinical case that illustrates the potential for sonication as an addition to the standard workup for explanted valves from patients with IE, by making a significant contribution to the diagnosis and resulting therapy. Indeed, only sonication was able to demonstrate the presence of viable Propionibacterium acnes (P. acnes) in this case of culture-negative IE. The presence of P. acnes was confirmed by 16S PCR on the heart valve.
Conclusion: Based on the currently available data there is insufficient evidence to draw definite conclusions, though initial results are promising. However, this project enabled me to carefully formulate and validate a standard operating procedure (SOP) for the sonication of heart valves in the microbiology laboratory. This method has demonstrated the ability to
detect the presence of microorganisms. In one case sonication demonstrated additive value by detecting the presence of a bacterium through cultures that was not cultured using the standard diagnostic protocol, allowing for antibiotic susceptibility testing, which is crucial for final patient treatment. This finding suggests that there seems to be benefit to the detection of microorganisms by using sonication. Continuation of data collection will be needed to provide conclusive evidence.

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