Proposed
approach to clearance of bacterial detection devices for screening of platelet
products prior to transfusion. J.
Vostal/LCH/DH
Issue:
FDA is
considering the appropriate approach to evaluate the efficacy of bacterial detection
devices in a clinical setting for screening of platelet products prior to
transfusion. Such methods are necessary
to decrease the incidence of platelet product transfusion-associated bacteremia
and sepsis.
Background:
Bacterial
contamination of platelet products at the time of collection is reported to
occur with a frequency of 1/2000 units.
The contamination of the product may occur as a result of improper skin
prep, sampling of a pocket of bacteria hidden in or under scar tissue as the
needle cuts through the skin or asymptomatic bacteremia in the donor at the
time of donation. The level of
contamination at the time of collection is low most likely on the order of 0-5
CFU/ml. However, since platelets are
stored at room temperature the bacteria can proliferate to levels of 100-1000s
of CFU/ml in a matter of days.
Transfusion of bacterially contaminated products can lead to serious
morbidity and mortality in approximately 1/100,000 transfusions.
FDA
recently cleared two devices for detection of bacterial contamination of
platelet products with the intended use for quality control of platelet
collection process in a blood center.
The clearance of the devices was based on in vitro data in which platelet
products were intentionally contaminated at the time of collection with
different species of bacteria. A sample
of the platelet product was taken under sterile conditions at 24-48 hours and
inoculated into the automatic bacterial culturing devices (1,2,). Sample collection 24 hours or longer after
blood draw is necessary to allow for the natural proliferation of the bacteria
to increase the chance obtaining enough bacteria in the sample to detect by the
device. The species of bacteria chosen
was based on the frequency of these organisms in platelet products reported in
literature (1). The species used to
clear these devices are listed below:
Bacillus
cereus
Bacillus
subtilis
Clostridium
perfringens
Corynebacterium
species
Echerichia
coli
Enterobacter
cloacae
Klebsiella
oxytoca
Propionibacterium
acnes
Pseudomonas
aeruginosa
Serratia
marcesens
Staphyloccocus aureus
Staphylococcus
epidermidis
Streptococcus
pyogenes
Streptococcus
viridans
Candida
albicans (fungus)
This
list includes both gram-positive and negative organisms and a fungal organism. Most of the organisms readily proliferate in
the automatic culture devices at 37 C and can be detected within 24 hours of
culture. However, there are several
organisms that grow slowly even under ideal conditions and may not be detected
until 48 hours in culture. These
include Staph epidermidis and Proprionibacterium acnes. Staph Epi. is a skin contaminant and has
been reported to account and gram- for approximately 50% of the bacteria
identified in contaminated platelet products.
To detect these organisms a sampling taken at 48 hours and cultured for
48 hours may be the most sensitive approach.
The
sensitivity of the devices range from 1-10 CFU/ml as defined by in vitro
testing. The in vitro tests do not
fully represent actual clinical use conditions and thus the FDA has decided not
to accept in vitro data alone as proof that the devices will be able to
identify platelet products contaminated during collection or processing. The FDA will propose that each device be
tested in a clinical setting. The test
design will have an initial sampling of the platelet product at a time that was
determined to be optimal by in vitro studies of the particular device tested. The product will then be stored at room
temperature and sampled a second time either at the point of transfusion or at
outdate if the product is not used. The
second sampling and culture is more likely to detect a contaminated unit since
the bacteria have had a chance to proliferate in the platelet product. The second culture thus serves as a
confirmation of the results obtained by the first sampling and culture. Since the contamination rate is on the order
of 1/2000 units, the whole study will need to sample approximately 10,000-12,000
units. The sampled platelet products
will be useable for transfusion if the sampling is done with a sterile
connecting device. Ideally the
confirmatory culture should be done at out date (after day 5 of storage) to
make sure that any slow growing organisms would have the possibility to
proliferate. However, that would
preclude the use of these products for transfusion and thus, taking the second
sample at the time of release will be acceptable. To assure that an adequate number of confirmatory samples are
taken on day 5 old platelets these should account for at least 3000 units (25%)
of the total study. Similarly, 4 day
old, 3 day old and 2 day old platelets should each account for 25% of the
second samples.
In
the early 1980s platelet products had 7 day shelf life but the long storage
time at room temperature allowed for bacterial proliferation that led to
transfusion associated sepsis. To
reduce this problem the shelf life of platelets was limited to 5 days in the
mid 1980s. Adequate bacterial screening
will make it possible to extend the shelf life of platelet products back to 7
days as long as platelet storage bags have been validated to maintain platelet
quality for this period. A clinical
study aimed at demonstrating that a bacteria detection device is capable of detecting
contaminated platelet units stored out to 7 days, will have a design similar to
that described above. The confirmatory
culture will need to be done at outdate of the product (day 7) and these
samples should account for at least 25% of the total number tested. The rest of the samples should be
distributed between day 6, day 5 and day 4 samples at approximately 25% each.
In
addition to a clinical trial, a post-marketing survey will be needed to assure
that actual clinical use of the device to screen platelets does not lead to an
increase in contamination rates or transfusion associated bacteremic
reactions. This study would be done
after a limited roll out of the cleared device to a small number of transfusion
centers. The results of all platelet units tested would be compiled and sent to
the FDA at prescribed intervals.
Discussion:
A
device that can be cleared as a method for screening platelets prior to
transfusion and to extend the shelf life of platelets should have 1) a high
detection accuracy rate (95% or better), 2) not cause additional contamination
of the units with the screening process, and 3) not have a high false-positive
rate that could lead to discarding platelet products unnecessarily. Initial evaluation of the device would be
done with in vitro studies to define the sensitivity and optimal sampling
strategy that will be used to design an appropriate clinical trial. The study should be of sufficient size so
that it could detect 5-6 naturally occurring contaminated platelet units. The readout of the initial screening sample
has to be compared to a recognized confirmatory detection method that could be
either a thioglycolate culture manual method, a cleared automated detection
system with known sensitivity or the device itself used later in the storage
time when bacteria are more easily detected.
If only a small volume of the platelet product is used for testing and
the sample was collected using a sterile connecting device, the product may be
transfused if it is less than 5 days old.
Platelets that are not transfused by day 5 can be sampled at day 5 and
then at day 7 to obtain data on the feasibility of the device for screening
platelets for storage out to 7 days.
The design of studies for 7 day storage may require collection of the
first sample later in storage to allow for proliferation of the slow growing
organisms.
References:
1)
Brecher, M. E. et al. Evaluation of an
automated culture system for detecting bacterial contamination of
platelets: an analysis with 15
contaminating organisms. Transfusion
41:477-482, 2001
2)
AuBuchon, JP et al. Experience with
universal bacterial culturing to detect contamination of apheresis platelet
units in a hospital transfusion service.
Transfusion 42:855-861, 2002