Isolators versus broth for blood cultures

The first report describing a blood culture technique based on lysis and centrifugation appeared in 1976 (6). During the next few years improvements were made in the system, and in the early 1980s the Isolator system was introduced by DuPont Co. (Wilmington, Del.). There has since been a number of evaluations of the Isolator 7.5 ml, 10 ml, and 1.5 ml systems (2-5, 8, 13, 19). The evaluations have included comparisons of the Isolator to broth systems for the recovery and time to detection of bacteria and fungi, and the interpretation of colony counts from the Isolator system. The various Isolator systems have been compared to a variety of broth systems including conventional broth bottles with 50mL, 100 mL, and biphasic fills with Columbia, tryptic soy, and Brain Heart Infusion broth; the Roche Septi-Chek (Roche Diagnostics, Nutley, N.J.) bottles; and BACTEC (Johnston Laboratories, Towson, Md.) resin and nonresin broth media. I would like to summarize these findings and include my own opinion as to our observations and those of others. The evaluations represent a wide variety of study protocols, patient populations, media, etc., thus they must be interpreted with some caution. For example, at this institution, many of our patients are immunosuppressed, approximately 50% are receiving antibiotics at the time the blood culture is taken, and approximately 50% of blood cultures are taken through indwelling catheters. These circumstances may predispose our studies to different results from those of a large general hospital. A summary of the studies suggests, in general, that neither Isolator nor broth has consistently provided the greatest sensitivity for all organism groups. Thus for optimal recovery of a variety of aerobic and anaerobic bacteria, yeasts and molds, at least two blood culture systems should be used. In several studies, Isolator recovered more yeast and staphylococci, and the broth systems recovered more streptococci and anaerobes. We have compared Isolator to conventional Columbia broth, BACTEC 6B-7D, and BACTEC 16B-17D media. Of the three broth media combinations, the BACTEC resin media was the most effective and recovered aerobic and facultative anaerobic bacteria and yeasts as efficiently as did the Isolator system. Both Isolator and resin media remove antibiotics, thus in a hospital where many patients have received antibiotics before culture, these systems may be preferred. Recovery of yeasts has often been favored by the Isolator system, and Streptococcus pneumoniae has been recovered more often by the broth systems. Several large studies referenced earlier and individual case reports (18, 20) also showed that Candida spp., Histoplasma capsulatum, and Cryptococcus neoformans were detected more often and sooner on an agar surface from the Isolator system or broth subcultures than from broth media alone. Perhaps increased yeast recovery on agar is due to the aerobic metabolism of the yeast. Thus when fungemia is suspected, the Isolator system may be the blood culture system of choice. For those using only broth systems for detecting fungemia, terminal subcultures of the broth should be performed. Decreased yield of S. pneumoniae by the Isolator system may be caused by lysis of the bacteria by saponin in the Isolator tube. Saponin, used to lyse red blood cells in the Isolator system, was shown in 1933 to lyse pneumococci that had been sensitized with cholesterol and ergosterol (16). Organisms have been detected at approximately the same time with both the Isolator and broth systems. Colonies on Isolator plates may more readily provide inoculum for identification and antibiotic tests, and the presence of different colonial morphologies on Isolator plates may indicate polymicrobic infection earlier than would a broth system. These potential advantages, however, may not necessarily translate to earlier reports of important information. The availability of blood culture information depends, in addition to the system used, on the hours of operation of the laboratory, culture sampling times, the use of rapid procedures for organism identification, and the timely ability to communicate the results to the clinician. In our laboratory, initially more contaminants were recovered from the Isolator than by broth systems. These results suggested that cultures should be processed with minimum air flow and that hooded environments should be used. We now inoculate Isolator sediment to agar plates and examine those plates daily inside a SteriLGARD biosafety hood. We do not routinely report the presence of one or two staphylococci colonies on Isolator plates if these organisms are present only in that culture. The number of colony-forming units of organism per milliliter of blood (CFU/ml) can readily be determined by the Isolator system. How important is this information? Interest in quantifying organisms in the blood as an aid in the diagnosis and management of septicemia is not new. In 1985 I reviewed 52 years of blood culture quantitation (14) and found that many papers showed that colony counts have been used in studies of the diagnosis and treatment of endocarditis, the relationship between magnitude of bacteremia and mortality, and the identification of sites of infection in adults, children, and neonates. In most of these studies, labor-intensive pourplate techniques have been used for quantitation of organisms, and these cumbersome procedures have tended to discourage routine determinations of colony counts in septicemia. Introduction of the Isolator system has caused a renewed interest in the quantitation of bacteria and fungi in