Routine Culture of Removed Cardiac Valves: More Trouble Than It's Worth
ABSTRACT & COMMENTARY
Synopsis: In the absence of previously identified endocarditis, most positive cultures of removed cardiac valves represent contamination rather than infection.
Source: Chuard C, et al. Clinical utility of cardiac valve Gram stain and culture in patients undergoing native valve replacement. Arch Pathol Lab Med 1998;122:412-415.
Chuard and colleagues at duke university retrospectively examined the value of routine microbiologic evaluation of removed native cardiac valves. Gram stain and culture were requested on 203 valves removed from 170 patients, representing approximately 65% of patients undergoing valve replacement, over an 18-month period.
Microorganisms were recovered from 41 (20%) of the valves, which were ground prior to culture. Six (14.6%) of the valves with positive cultures had been obtained from patients with known endocarditis. Only one positive culture proved to represent a case of endocarditis not diagnosed prior to surgery. This valve was removed from an elderly man with a long history of mitral regurgitation that had abruptly deteriorated three weeks after a myocardial infarction. The patient had no constitutional symptoms, and endocarditis was not suspected preoperatively. Gram stain of the removed valve demonstrated 4+ gram-positive cocci, the presence of which was confirmed on histopathological examination.
In 34 (83%) of the 41 cases with a positive culture, there was no evidence of endocarditis at the time of surgery or within the next three months. The quantity of growth did not distinguish contaminated valve cultures from cases of endocarditis. The organisms recovered from contaminated cultures represented skin flora in 94% of cases.
While the number of WBCs seen on Gram stain of the removed valve was not helpful in distinguishing between endocarditis cases and those without this infection, the detection of organisms was highly predictive, with a sensitivity and specificity each of 100%. On the other hand, the lack of visible organisms did not eliminate the diagnosis of endocarditis. Histopathological examination of the removed valve revealed inflammatory cells, microorganisms, or both in all cases of endocarditis, but in none of the cases of culture contamination.
COMMENT BY STAN DERESINSKI, MD, FACP
It remains a common practice among many cardiovascular surgeons to routinely send a portion of a removed cardiac valve, whether infection is suspected or not, to the microbiology laboratory. After receiving repeated calls about positive valve cultures at one community hospital where this was done, I reviewed their microbiological records and found that 6% of removed valves were culture positive, a value considerably lower than that experienced at Duke, despite a similar method of handling in the laboratory. The cultures also predominantly yielded skin flora and, with a single exception, represented contamination, not disease. The single case of endocarditis uncovered, which was due to an Enterococcus, was quite similar to the one described above, with the presence of valve infection confirmed by histopathology.
Another recent publication reported that, of 206 native valves and 13 prosthetic valves routinely cultured, cultures of 11.9% were positive, but none had evidence of endocarditis (Giladi M, et al. Clin Infect Dis 1997;24:884-888). In that same study, 11.6% of 224 tags removed from valves prior to insertion were culture positive but none of the recipients developed postoperative endocarditis.
The results of the study reviewed here provide a strong argument against routine performance of microbiological studies on all removed valves, in the absence of clinical suspicion of infection. Thus, as with other situations in which the pre-test probability of a disease is very low, the frequency of false positive results far outweighs the frequency of true positives.
Chuard et al recommend that the optimal approach to the diagnosis of endocarditis is the maintenance of a high index of suspicion and performance of preoperative blood cultures. If unexpected findings at the time of surgery raise concern about the presence of untreated endocarditis, they suggest maintaining a portion of the valve at 4° C and only performing culture if the histological studies are indicative of an inflammatory process.
In the interest of cost savings, some hospitals do not routinely perform histological examination of removed valves. One wonders if gross examination of the valve is not sufficient to select those with a higher likelihood of infection. If it is, it may be warranted to perform both culture and histological examination simultaneously on suspect specimens. An alternative approach would be to perform a Gram stain on all or selected valves and to only culture those on which organisms are seen.
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