5.1. BLOOD SCREENING PROCESS

The screening of donated blood and the quarantine of blood and blood components represent critical processes that should be followed to ensure that blood units are safe. Based on the screening results, they should either be released for clinical or manufacturing use or be discarded. Laboratory screening for TTIs should be performed on blood samples collected at the time of donation. All tests on blood samples should be performed and recorded in accordance with standardized procedures in laboratories that are properly equipped to undertake them.

All blood samples, donations and components should be correctly labelled to ensure correct identification throughout the screening process. The BTS should also have appropriate, validated systems for linking all test results to the correct donations and donors so that donors' records can be reviewed each time they come to donate. These systems will ensure that the correct results are allocated to each donation and prevent errors resulting in the transfusion of an unsafe unit.

Laboratory staff should always adhere to the national screening strategy, algorithm and standardized procedures when conducting the tests and analysing the results. The performance of laboratory tests in a quality environment with competent staff and a functional documentation system will minimize the risk of analytical and transcription errors, particularly false negative results.

The objective of blood screening is to detect markers of infection in order to prevent the release of infected blood and blood components for clinical use. Blood screening strategies are designed to assure the safety of blood units, but should not be used for notifying blood donors of reactive test results. The appropriate confirmatory testing strategy for blood donor management should be applied before notifying donors of their infectivity status (see Section 6). The results of all tests performed for infection markers for TTIs and blood group serology should be evaluated when making final decisions on the release of blood units for therapeutic use.

5.2. APPROACHES TO BLOOD SCREENING

Two approaches to blood screening are recommended for blood safety, depending on whether or not an effective quality system has been established in the laboratory in which the testing is carried out (see Section 7). These options represent the processes recommended for screening blood for each TTI in laboratories where:

1. Quality systems are weak or have not yet been established.
   OR
2. Effective quality systems are in place.

The assay selected for blood screening should be highly sensitive and specific. The aim is to detect all possibly infected donations while minimizing wastage due to false positive results. Donations that yield reactive or indeterminate test results should be discarded using methods in accordance with standard safety precautions (47).

Option 2. In laboratories with established quality systems

1. Use a single assay (A) and test each blood sample singly in accordance with standard operating procedures. The assay should have been validated for the specific TTI.
2. Collate and analyse the results of the assays. If a result is non-reactive (A−), the blood unit can be released for clinical use.
3. If a blood sample is initially reactive for a TTI (A+), immediately segregate the blood donation and all blood components derived from it.
4. Repeat test in duplicate, from the same sample and using the same assay.
5. Analyse the results of the repeat tests:
   • If both repeat tests are non-reactive (A+, A−, A−), the initial result could be due to false reactivity or technical error and the donation can be released for clinical use
   • If one or both of the repeat tests are reactive (A+, A+, A−)/(A+, A+, A+) immediately segregate and then discard the blood donation and all blood components derived from it. Send the sample for confirmatory testing.

The algorithm shown in Figure 1 shows the decision points on whether the blood and blood components should be released or discarded, based on the screening results, and whether confirmatory testing should be performed for blood donor management and epidemiological monitoring (see Section 6).

Figure 1

Algorithm for blood screening.

5.3. POOLING FOR SEROLOGICAL ASSAYS

The pooling of samples before testing has been the subject of debate for some years. It has been considered to be a cost-saving measure, but any cost savings have to be balanced against the risk of failing to detect a positive donation. This is likely in some assays where sensitivity is compromised in diluted sample. In a pooled sample each individual sample is diluted. There is also a high risk of errors being made as a result of poor quality procedures during the preparation of the pool and when recording individual samples in each pool. An additional complication is the resolution of pools that test positive and the subsequent delay in releasing the units that comprise the pool. The pooling of samples for serology testing is therefore not recommended for a blood screening programme.

5.4. SEQUENTIAL SCREENING

Blood transfusion services routinely screen for TTI markers (HIV antigen-antibody, HBsAg, anti-HCV and syphilis) at the same time. The main reason for this is to reduce the time needed for screening so that the blood or blood components, especially labile components such as platelets, can be released in a timely manner. Initially reactive donations are segregated and quarantined. Depending on the algorithm used by the laboratory, the donation is then either discarded or repeat testing is performed.

Some laboratories may use sequential screening by initially testing for one or two infection markers. If a reactive result is obtained, no further testing is performed on this donation. The screening strategy for determining the test or tests that are undertaken first will be influenced by the prevalence of infections in the blood donor population. Sequential screening is sometimes used in countries where the prevalence of one TTI is higher than others; for instance, HBsAg might be screened for first when the prevalence of hepatitis B is higher than the prevalence of HIV and HCV. In this situation, only HBsAg negative donations would then be tested for HIV antigen-antibody, anti-HCV and syphilis. No tests for these viral markers would be performed on the donations that test reactive on the HBsAg screening test. Thus there is potential for cost savings, especially if the more expensive assays do not need to be performed on donations that have already tested positive for HBsAg.

While sequential testing may be perceived to have economic benefits, the potential cost savings need to be balanced against factors such as the increased turnaround time for results and increased staff costs owing to longer shifts. It may result in delays in screening and releasing blood and blood components, leading to poor blood stocks, especially if there are chronic shortages. Another disadvantage of this strategy is that donors with co-infections (i.e. with more than one infection) will not be identified and cannot, therefore, be notified and counselled about these additional infections as part of the duty of care towards blood donors. Sequential testing could also increase the possibility of mix-ups and errors owing to frequent handling of blood samples, donations or the components derived from them. In centres with limited or no quality systems, this might lead to an increased risk of untested or unsafe units being transfused. The opportunity to study the epidemiological profile of infections in donors will also be lost. Sequential screening is therefore not recommended for a blood screening programme.

5.5. BLOOD SCREENING AND DIAGNOSTIC TESTING

In general, there is no difference between screening and diagnostic assays themselves; the differences lie in the reasons for the testing, the population being tested, the interpretation of the results and the subsequent actions. The screening algorithms used and the focus of quality systems may also differ as blood screening is product-related and diagnostic testing is not.

Microbiological screening of blood is performed on donations from apparently healthy, asymptomatic donors to rule out the presence of infections and assure safe blood for transfusion. Diagnostic testing is performed as part of a clinical investigation to pursue a diagnosis of infection either as a result of signs and symptoms in an individual or a specific or identifiable risk of infection.

Blood screening involves a single test with the resultant action, such as the release or discard of the donation arising from that single test alone, even though an initially reactive result may be followed by repeat testing. Diagnostic testing often involves additional testing over a period of time either to pursue the diagnosis in early infections or to follow-up or monitor infection. A single test result alone is not relied upon to determine infection or subsequent actions.

Diagnostic samples are high-risk samples as they are generally taken from symptomatic patients; they should not be mixed with blood samples from blood donors. In hospital-based blood services, diagnostic testing facilities should be separate from those used for blood screening.

5.6. EMERGENCY SCREENING

In emergency situations in which blood and blood components are needed urgently, but are not readily available from blood inventory, screening with rapid/simple single-use assays could be used to obtain results quickly and enable blood to be released for clinical use in consultation with the prescribing clinician.

Wherever possible, however, the blood sample should be retested as soon as possible using an EIA or another assay used routinely for blood screening in the laboratory in order to check the validity of the test results. Any discrepant results should immediately be investigated further and corrective action taken, including communication with the clinician who has prescribed the blood. Countries should work towards systems that avoid these situations.

5.7. SCREENING PLASMA FOR FRACTIONATION

In blood transfusion services where plasma is collected for fractionation, either as recovered plasma (whole blood) or as source plasma (apheresis), the screening requirements and algorithm adopted may be different from those needed for blood donations for clinical use. Plasma for fractionation may require additional screening tests, depending on its source and the regulatory requirements applied to the fractionation facility. Such requirements may be national or international, depending on the location, ownership, nature and scale of the facility.

5.8. PRE-DONATION TESTING

The testing of blood donors for TTIs before they donate blood (pre-donation testing) is the subject of debate. It is sometimes considered to be a cost-saving measure, particularly in high-prevalence situations. However, pre-donation testing of the donor does not ascertain the infectious status of the donation and will need to be followed by tests on the blood sample collected during the blood donation process. Pre-donation testing may lead to a wastage of resources and increased screening costs unless prevalence is extremely high. It increases the time taken for a donor to donate blood, causing undue inconvenience to donors, and also the risk of discrimination and stigmatization. The practice of pre-donation testing could undermine the long-term development of a sustainable blood donor programme based on well-selected voluntary non-remunerated blood donors who donate regularly.

All screening of blood donations for TTIs should be carried out only on samples taken during the donation process and in a quality controlled environment. In an effective national blood screening programme, pre-donation testing of blood donors has limited applicability. In settings where the prevalence of an infection is very high and donor selection would not be effective in reducing prevalence in first-time donors, pre-donation testing may be useful as an interim strategy while building a stable pool of regular voluntary non-remunerated donors.

5.9. QUARANTINE OF BLOOD AND BLOOD COMPONENTS PRIOR TO RELEASE OR DISCARD

A quarantine system should be in place for the physical segregation of all unscreened donations and their blood components until screening for infection markers has been completed and the suitability of donations for therapeutic use has been determined. A system should be in place to ensure that screened and unscreened units are stored in separate blood storage equipment to prevent the issue of unscreened units. All reactive or positive donations and all components derived from these donations should be labelled “Not for transfusion” and segregated for discard or non-clinical use.

The BTS should ensure that separate blood storage equipment is clearly designated for:

• Unscreened units
• Reactive/positive units
• Unresolved/indeterminate units
• Units suitable for clinical use: i.e. available blood stock.

There should be a fully documented system that identifies the current location and eventual fate of all blood and blood components, whether destined for clinical use or disposal. The BTS should also have documented policies and procedures to deal with the emergency release of blood components prior to all screening being completed.

Reactive or positive units of blood or plasma are valuable resources for quality control samples and panels, evaluations and validations, and for research purposes. Blood screening laboratories can provide blood or plasma to be used as reagents to institutions involved in research or to quality assessment schemes for the production of proficiency panels.

5.10. RELEASE OF BLOOD AND BLOOD COMPONENTS

Only blood and blood components from donations that are non-reactive for all markers screened for should be released for clinical or manufacturing use. When all the required blood screening tests have been performed, the results have been checked and any other required checks have been made, formal release procedures can be undertaken to release quarantined units and physically move the released blood stock from one location to another. The BTS should have appropriate systems for labelling the blood and blood components as ready for clinical use. The label on each blood unit should contain the relevant details of the donation and the tests carried out on the donation. When this has been carried out, the screening process is considered to be complete.

All reactive units should be removed from the quarantined stock and stored separately and securely until further handling.

5.11. LONG-TERM STORAGE OF DONATION SERUM/PLASMA SAMPLES

The long-term archiving of donation serum/plasma samples can be very useful for a BTS in facilitating the investigation of adverse transfusion events and transfusion-transmitted infections or the evaluation of new screening assays or reagents. However, archiving should be considered only if adequate and suitable resources are available, including sufficient space and efficient paper-based or software-based warehousing systems to manage sample retrieval.

A number of crucial issues should be considered before building a sample archive. These include:

• System for the identification and history of each sample in the archive related to its use and length of time of storage
• Type of storage containers required
• Specified temperature at which samples are to be stored
• Volume of samples to be archived
• Criteria and documentation of the reasons for the recovery of an archive sample.