Monitoring bacterial contamination of blood components
- Investigation of patient adverse reactions to transfusion
- Investigation of visually abnormal components
- Bacteriological screening of cord blood and autologous serum eyedrops
- Screening platelet concentrates to extend the shelf life to 7 days
- Monitoring donor arm disinfection
Bacterial contamination of blood components is still a major cause of patient morbidity and mortality. The source is usually the donor venepuncture site, via the blood stream or during blood processing.
Platelets are monitored retrospectively (i.e. once they have expired) to determine the bacterial contamination rate. This surveillance allows us to monitor the impact on the contamination rate of interventions such as diversion and new donor arm disinfection techniques.
The platelets are tested using an automated blood culture system and incubated in aerobic and anaerobic culture bottles for 7 days at 36°C.
Once contamination is confirmed, all associated units are recalled to prevent them from being transfused into a patient. The current contamination rate of time expired platelets are:
- Pooled Platelets: approx 1:300
- Apheresis Platelets: approx 1:900
- Bacteria that have been isolated from platelets:
- Staphylococcus epidermidis
- Other coagulase negative staphylococci
- Propionibacterium spp
- Micrococcus spp
- Group C Streptococcus
Platelets are stored at room temperature, which is ideal for growth of many species of bacteria. This is one of the reasons why platelets have such a short shelf life (5 days) compared with red cells which are stored at 4°C and have a shelf-life of 35 days.
All transfusion reactions suspected to be caused by bacterial contamination of the unit must be referred to the National Bacteriology Laboratory through an NBS consultant for investigation.
When a transfusion reaction occurs, the hospital must report the incident to an NBS consultant who decides on the course of action. If the reaction is suspected to be caused by bacterial contamination, the hospital must first return the unit to the Centre from which the unit was issued. The unit will then be sent to the NBL for investigation. All associated units will be put on hold until further results are known.
Testing of the implicated unit, investigation of the donor and molecular typing of isolates from the patient (if available), from the implicated pack and from the donor.
A report is compiled and sent to the Consultant Specialist for Transfusion Microbiology who will report back to the relevant hospital.
Investigations are reported into the UK Hemovigilance Scheme: Serious Adverse Blood Reactions and Events (SABRE) and Serious Hazard of Transfusion (ShoT)
Visually abnormal components suspected of being bacterially contaminated are also investigated. This may be at the request of the local Processing or Issue department or at the request of the hospital before the unit is transfused. Units are investigated for reasons such as unusual colour, leaks, clots or unidentified lumps in the pack.
There are guidelines on the NBS website outlining the correct procedure hospitals must take when a transfusion reaction occurs or a contaminated pack is noticed.
The blood from a baby’s umbilical cord is rich in stem cells and can be used to treat conditions such as leukaemia. The blood is taken from the cord just after the birth and is stored by the London Cord Blood Bank. The NBL screen each donation, using the automated blood culture system, before transplantation to ensure that it is not bacterially contaminated.
Some patients with certain conditions, for example those with keratoconjunctivitis, lose the ability to produce adequate tears, resulting in severe morbidity.
Autologous (i.e. patient’s own) blood is collected and the serum separated off and diluted with saline. This is then frozen for subsequent daily use by that patient as artificial tears. Prior to issue to that patient, each sample of artificial tears is screened for bacterial contamination.
In order to address supply problems caused by extended bank holiday periods, the NBS devised a strategy for the bacteriological screening of platelets to allow an increase in shelf life to 7 days. The screening enables the shelf life to be extended to 7 days, providing a microbiologically safer product.
Bacterial contamination of blood products by bacteria from the donor’s arm is associated with transfusion transmitted infections in patients. This is well documented in the literature.
The National Bacteriology Laboratory monitors the venepuncturist to ensure that the cleansing technique is being performed correctly and the current disinfection method is working effectively. This is done using agar contact plates on the donor venepuncturist site post disinfection.
Currently the National Blood Service uses Chloroprep Kit to disinfect the donor’s arm.