Part 2 – Screening for infections
Screening blood for infections sounds very simple but the more you know about it the less simple it becomes…..
So, to start with, on every donation we test for some ‘mandatory’ markers. The fact that they are mandatory mean that we will not routinely issue the blood before the results are available and if they are positive then the blood cannot be issued through our computer system. Currently we test for Hepatitis B and C, HIV, HTLV (human lymphotrophic leukaemia virus), and syphilis. We can do tests that either look for evidence of the virus itself or we can look for antibodies in response to a previous infection or both. The techniques we use determine the ‘specificity’ and ‘sensitivity’ of the tests which means how often a positive result will be missed and how often a false positive result will be given. Also it is possible for a donor to be infectious but the infection is below the level of detection of the test even when the tests are done correctly. Even with both of these, the chance that an infectious donation enters the supply chain is really extremely small (less than 1 in several million depending on the virus) and even then this doesn’t necessarily mean that it will transmit an infection. But all of this is why we ask donors the questions about lifestyle behaviours and travel eg to minimise the risks to patients.
Even though people receiving blood usually worry most about transmission of a viral infection it is transmission of a bacterial infection that worries me more. Bacteria are most likely to grow in platelet transfusions as they are kept at a nice warm room temperature. The worry is particularly with bacteria that come from the skin and those from the gut because if they were to be transfused to someone with a reduced immune system, such as someone having chemotherapy, as they could cause a serious infection.
As you would expect we also have a number of processes in place to minimise the chances of bacteria being transmitted. This starts with the questions we ask donors about infections, through cleaning of the arm with antiseptic solution, a pouch for sample collection which takes the first draw of blood (containing the skin plug from the middle of the needle which could contain bacteria), the temperatures we transport our donations at, the sterile closed systems we use for processing, the testing that we do through to the inspections hospitals do on packs prior to transfusion. In addition we screen all our platelets for bacteria so the chances of any packs of platelets containing bacteria is also extremely low. With all of these in place, a pack of platelets that could potentially cause an infection getting to the hospitals occurs less than once a year on average – which is much lower that it was ten years ago.
So, as if all of this is not enough, we also do a number of ‘discretionary’ or optional tests on donations. These are tests done either because of the information a donor gives us or because we need products with additional tests done on them for certain patients. An example for patients is CMV (cytomegalovirus) tests where products given to new born babies and pregnant mothers are tested to ensure that they are CMV negative. Transmission of this virus, whilst very common and relatively harmless in the general population, can cause real problems in developing babies. An example of extra tests that are done because of donor information is West Nile Virus on people who have been on holiday in areas where the infection occurs (including common European destinations and the USA). Similarly malaria tests are done after travel to malarial risk areas and extra hepatitis B test are done for anyone who has had a tattoo or other piercings from 4 months after it was done.
Finally we have a duty of care to all of our donors which means that whenever we find a donor with a positive test result we will let you know and ensure that you are sent for the right NHS care. A few times every year testing positive for a test can potentially save a life – we hear of donors who have gone to their GP and been diagnosed with a serious illness such as bowel cancer they had no symptoms of after we have told them of their test results.
I hope this tells you two things – firstly a little bit about why we ask some of the questions that we do and secondly how hard we try to make our blood safe – using this work going on behind the scenes to ensure that transfusions in this country are among the safest, if not the safest, in the world.
Next time – tests in the future….