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Journal of Clinical Microbiology, July 2000, p. 2803-2804, Vol. 38, No. 7
0095-1137/00/$04.00+0
LETTERS TO THE EDITOR
Failure of an Automated Blood Culture System To Detect
Nonfermentative Gram-Negative Bacteria
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LETTER |
Dr. Klaerner and colleagues (1) reported interesting
data about the automated BacT/Alert blood culture system. A total of
605 out of 8,107 blood culture bottles yielded relevant microorganisms. The system failed to detect 15 nonfermentative species out of 605 bottles. The authors speculated that this was due to growth of
microorganisms during preincubation at 36°C, and they suggested keeping the bottles at room temperature until loading. Preincubation at
room temperature may reduce the failure to detect nonfermentative species; however, detection of other clinically relevant microorganisms may be delayed.
Automated blood culture systems (BACTEC 9240 and BacT/Alert) detect
growth of microorganisms by continuous monitoring of CO2 without manipulations of the bottles, which reduces the contamination risk and the workload (5). The microbiological yield is
determined by an adequate filling of the blood culture bottles (3,
4). Therefore, the remark of Dr. Klaerner et al. that a smaller
inoculum can reduce the number of false negatives appears not to be
clinically useful.
Rapid detection and identification of bacteremia by a clinical
laboratory add to prompt and adequate antibiotic therapy. Hence, the
reported failure in detection of bacteremia is of great concern. Fast
transportation of the bottles to the laboratory and immediate loading
are not always feasible, and the growth of microorganisms in the
bottles may already have reached steady state. The algorithm for
detection of growth estimates both the initial CO2 level
(reflection units) and changes in this level but proved to be not
sensitive enough. Adaptation of the software may be indicated; however, this should not cause an increase in the number of false-positive readings (2).
The results of Dr. Klaerner et al. (1) indicate the need for
continuous quality improvement to ensure optimal use of the benefits of
the automated blood culture systems, including short transportation
time, adequate filling of the bottles, and evaluation of the detection
algorithm of the blood culture system. The authors suggest subculturing
of all bottles with a transportation time longer than 4 h prior to
loading. This causes a substantial increase in the workload. In order
to cope with the advanced growth in blood culture bottles, in our
laboratory the technicians screen the color at the bottom of the
bottles prior to loading. From "suspect" bottles with yellow color
indicators a sample is drawn for Gram staining and subculture. In our
experience the BacT/Alert system (software version BacT/Link E.00)
detects growth of microorganisms in such bottles soon after loading,
though we did not record this routinely. Subculturing on indication
seems reliable and is less laborious than subculturing all bottles with
a transportation time longer than 4 h; moreover, often the
transportation time is unknown.
Measurement of the absolute number of reflection units by the system is
more accurate than evaluation of the color by the human eye; however, a
plot of reflection units can be generated only after six measurements
(60 min). If there is a correlation between the number of bacteria in
the bottles at arrival in the laboratory and the degree of yellowness
(or the absolute number of reflection units), this can be used to
detect preincubational growth and it would be worthwhile to investigate
whether the software of the blood culture system can select bottles
which should be subcultured.
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FOOTNOTES |
*
Phone: 31 50 3633510
Fax: 31 50 3633528
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REFERENCES |
| 1.
|
Klaerner, H.-G.,
U. Eschenbach,
K. Kamereck,
N. Lehn,
H. Wagner, and T. Miethke.
2000.
Failure of an automated blood culture system to detect nonfermentative gram-negative bacteria.
J. Clin. Microbiol.
38:1036-1041[Abstract/Free Full Text].
|
| 2.
|
Meessen, N. E. L.,
E. C. M. van Pampus, and J. A. Jacobs.
1999.
False-positive blood cultures in a patient with acute myeloid leukaemia.
Clin. Microbiol. Infect.
5:769-770.
|
| 3.
|
Mermel, L. A., and D. G. Maki.
1993.
Detection of bacteremia in adults: consequences of culturing an inadequate volume of blood.
Ann. Intern. Med.
119:270-272[Abstract/Free Full Text].
|
| 4.
|
Washington, J. A., II, and D. M. Ilstrup.
1986.
Blood cultures: issues and controversies.
Rev. Infect. Dis.
8:792-802[Medline].
|
| 5.
|
Weinstein, M. P.
1996.
Current blood culture methods and systems: clinical concepts, technology, and interpretation of results.
Clin. Infect. Dis.
23:40-46[Medline].
|
| | | | |
Nico E. L. Meessen*
Hilly G. de Vries-Hospers
Department of Medical Microbiology
University Hospital Groningen
P.O. Box 30001
9700 RB Groningen, The Netherlands
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AUTHOR'S REPLY |
We agree that blood culture bottles have to be filled adequately in
order to allow optimal conditions for the detection of microorganisms
that invade the bloodstream. Our remark that smaller inocula reduce the
number of false-negative blood cultures detected by the BacT/Alert
system was in conjunction with the experimental results where smaller
numbers of microorganisms but not smaller volumes were added to the
blood culture bottles; this procedure allowed the detection of
nonfermentative species even though the bottles were preincubated. This
experimental result should not be interpreted as a recommendation to
fill blood culture bottles with smaller volumes of blood.
We are not convinced that a change in the software of the BacT/Alert
system in order to minimize the problem with preincubated blood
cultures in detecting nonfermentative species will solve the problem.
An increased sensitivity of the software will probably also result in
more cases of false-positive readings. This issue, of course, can be
solved only by the manufacturer. The results of our study reemphasize
the need of rapid transportation to the laboratory. In our eyes, the
speed of transportation is increased by the presence of automatic
transportation systems and, more importantly, by the awareness of
physicians and nurses. We have to bear witness to the fact that blood
culture bottles were forgotten and left in the incubators in the ward
for several days, a situation which cannot be tolerated. To detect
growth of nonfermentative species in preincubated bottles, we decided
to subculture all blood culture bottles on their arrival in the
laboratory. The workload, of course, is increased; on the other hand,
the bottles have to be vented manually, and at this point of the
working process a subculture is performed quickly. We checked visually
whether a change in the color indicator of the blood culture bottles
from the clinic occurred upon their arrival in the laboratory, and "suspect" bottles, as discussed by Drs. Messen and de
Vries-Hospers, were subcultured immediately. However, this procedure
did not prevent our failure to detect nonfermentative species, because the color indicators of these bottles did not change. In addition, our
experimentally seeded blood culture bottles did not show changes in the
color indicator after the period of preincubation. Therefore, in our
experience changes in the color indicator do not predict reliably
whether subcultures need to be performed or not.
| | | | |
T. Miethke
Institute for Medical Microbiology, Immunology and Hygiene
Technical University of Munich
Trogerstrasse 9
81675 Munich, Germany
Phone: 49 89 4140 4187
Fax: 49 89 4140 4868
E-mail: Thomas.Miethke{at}lzz.tu-muenchen.de
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Journal of Clinical Microbiology, July 2000, p. 2803-2804, Vol. 38, No. 7
0095-1137/00/$04.00+0
