No Value for Money in Using Broth Cultures
Abstract & Commentary
Synopsis: The additional time and expense of using fluid media for culturing clinical specimens is unwarranted.
Source: Derby P, et al. The value of including broth cultures as part of a routine culture. J Clin Microbiol 1997;35:1101-1102.
Three hundred seventeen clinical specimens of pus or body fluids from both superficially and deeply infected sites (thus excluding blood, cerebral spinal fluid, and urine) were prospectively examined to assess the value of using both solid and liquid media routinely. All specimens were inoculated onto a blood agar plate for anaerobic culture and both a McConkey agar and a heated blood agar plate for anaerobic culture, as well as into a cooked meat broth and a tryptic soy broth. All media were read after overnight incubation, and each fluid media was subcultured onto the same three-plate media. The isolates were then compared with those obtained on primary solid media. Of 273 specimens of pus and 44 body fluid specimens processed, additional isolates were recovered in fluid medium of 27 and three specimens, respectively. Twenty-two of the isolates recovered exclusively from the broth cultures were considered clinically relevant after reviewing the patients’ records, but their isolation led to a change in the management of only two patients. These results suggest that the additional expense and time involved in including fluid media for culturing clinical specimens is unwarranted.
COMMENT BY J. PETER DONNELLY, PhD
The use of fluid medium has long been part of the normal routine for handling fluid specimens from body sites, although the origins of the practice remain obscure. Morris et al (J Clin Microbiol 1995;33:161-165) detailed the four most frequent reasons given by microbiologists in defense of their using fluid media as a backup to solid media, namely, using broth cultures to allow recovery of so-called "fastidious" bacteria that fail to grow on solid media, permit the growth of anaerobes, encourage the growth of smaller numbers of bacteria, and dilute out any inhibitory substances. These beliefs had bolstered the practice for most of this century and have provided fertile ground for a steady stream of reports comparing a modern formulation over a more staid and traditional recipe. But, it seems, all to no avail. For whatever the merits or otherwise of a given broth medium, the clinicians employed by the University of Cape Town Medical School and Groote Schuur Hospital in Cape Town, South Africa, are ignoring them or, at least, not allowing this hard won and expensive information to alter their management. Morris et al found the same resounding apathy by their colleagues at Duke University Medical Center to the results of broth cultures from 356 tissue and body fluid specimens sent to them.
Simply put, events have overtaken traditional clinical microbiology. Clinicians have long since gotten into the habit of taking a sample and then treating empirically. Only if things are going badly wrong or their best guess was way off beam are they likely to consider altering their management of the patient. Thus, the odds are heavily stacked against microbiology ever showing its value in terms of bringing about an appropriate change in treatment. Moreover, we can no longer escape the fact that any laboratory finding not used to inform action either now or in the future remains essentially little more than an item of data left to gather dust in the archives and, hence, a luxury we can ill afford. I happen to agree that broth cultures in this day and age are redundant except for blood and, perhaps, cerebral spinal fluid. However, my concern is that empirical treatment may have become the norm at these institutions and so any attempt at microbiological diagnosis would be more readily seen as an additional expense that could be avoided. The authors did not subject the results of plate cultures to the same test for clinical relevance, namely, that they either brought or should have brought an appropriate change in the management of the patient. My feeling is that they might also have found the same lack of impact. Therefore, before attempting to assess the clinical relevance or otherwise of a particular microbiological investigation, investigators would be well advised to establish beforehand the likelihood of patient management being altered by the results.
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