accessioned and placed in a 37°C incubator; aerobic bottles were
agitated at 200 rpm for 24.to 48 h (1, 4). Anaerobic bottles
were incubated but not agitated. BACTEC bottles were
tested according to the recommendations of the manufacturer,
i.e., daily readings of both bottles for 7 days and an
additional reading of the aerobic bottle on days 1 and 2.
BacT/Alert bottles were entered into the prototype system,
which provided incubation at 35°C with continuous agitation
and monitoring. Although the system reported positive bottles
as they occurred, to accommodate the daily workload
pattern technologists checked the system for positive cultures
four times a day during the week and three times each
day on weekends. Bottles in both systems were incubated
for 7 days or until they became positive. Samples from
suspected positive bottles were Gram stained and subcultured
onto the appropriate plate media according to routine
laboratory procedures. False-positive bottles were returned
to their respective systems for additional incubation and
testing. Samples from negative bottles from discrepant pairs
(a culture positive with one system but not the other) were
Gram stained and subcultured at the end of 7 days. Time to
positivity for both systems was defined as the difference
between the time the bottles began incubation in the laboratory
and when they became positive.
RESULTS
Media, sensor, and instrument evaluation. Figure 2 shows
the relationship between C02 introduction and the voltage
signal produced by the photodiode. It can be seen that
additions of CO2 from 0.1 to 3.0 mmol can be detected by the
system. The organisms listed above were successfully cultured
in the appropriate BacT/Alert aerobic or anaerobic
medium. Carbon dioxide production, as measured by the
instrument as a change in total voltage, ranged from 0.73 to
6.25 V. Growth of all organisms was detected by the
instrument. Figure 3 shows the stability of readings in
uninoculated bottles, the steady production of C02 by fresh
whole blood, and the rapid increase in C02 because of
growth of seeded microorganisms. These readings are expressed
as reflectance units rather than as a change in
voltage. Table 1 shows the relative speed of detection of
seeded blood cultures by each system.
Preliminary clinical trial. Because of the limited nature of
the study, all blood cultures submitted from the wards
involved in the study were evaluated without regard to
clinical importance. Table 2 lists the organisms detected by