JPAC Joint United Kingdom (UK) Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee

9.2: Microbiology screening

Update notice:  Section 9.2- has been updated following the the issue of Change Notification 12 - 2016.

Note: The meanings of certain terms used in this section are defined in section 9.2.6.

9.2.1: Screening of donations/donors

Donation/donor screening can be broadly divided into two main categories:

  • Mandatory: Absolute requirement prior to the release of components. There are, however, different reasons for a test to be defined as ‘mandatory’. These include a European Union requirement, a specific instruction from the Department of Health, including its Advisory Committees, and an Act of Parliament.
  • Additional (also known as Discretionary): Performed because of specific additional and identifiable donor or recipient risk.

Importantly, the mandatory requirements for blood donation and for tissue and stem cell donations are different, with some tests that are defined as ‘Additional’ for blood donations being ‘Mandatory’ for non-blood donations (Tables 9.1 and 9.2). Although not required for all donations, where

Table 9.2 Screening required for tissue and stem cell donations*

Infectious agent Minimum requirement Comments

HIV 1+2

anti-HIV 1+2 or HIV 1+2 Ag/Ab (M)


Maximum pool size of 24 donations**


anti-HCV (M)

HCV Ag and/or HCV Ag/Ab (O)


Maximum pool size of 24 donations**


HBsAg (M)

anti-HBc [+ anti-HBs] (M)


Donations that are anti-HBc reactive and have anti-HBs >100 mIU/mL are considered suitable for release


anti-treponemal Ab (M)



anti-HTLV I/II(M)***

ID or maximum pool size of 24 donations**


anti-HCMV (A)


Plasmodium sp.

anti-P. falciparum/vivax (A)


Trypanosoma cruzi

anti-T. cruzi (A)


West Nile Virus (WNV)


Maximum pool size of 16 donations****

(M) – mandatory

(O) – optional, genomic screening for HIV, HCV and HBV is not mandated but can be performed on the original donation sample as an alternative to 180 days’ quarantine and follow-up serological testing

(A) – additional due to specifically identifiable risk

* UK screening requirements. Other testing, e.g. Epstein-Barr virus, toxoplasmosis, may be
required as additional tests depending upon specific additional risk and/or special requests for individual recipients. For certain product types that are exported there may be additional end
user testing requirements.

** All screening of deceased tissue donations is performed on individual samples. HCV and HIV RNA and anti-HTLV I/II screening of surgical tissues/stem cells can be performed using pools of a maximum of 24 samples. HBV DNA screening should be on individual samples.

*** Not mandatory for avascular tissue donations but may be considered good practice.

**** The maximum validated pool size for WNV RNA screening is 16 donations.


additional tests are required, the results are an integral part of the criteria for the release of that donation/component/product. In addition, for certain donation types, there is the option of quarantine and follow-up serological testing before issue or the inclusion of genomic screening at donation.

Donations and any associated components/products must not be released to stock unless they have been tested and found negative for the mandatory, and any additional, microbiological screening required. In certain circumstances, for certain donation/component types, a reactive screen result may not preclude release of the donations/component.

9.2.2: Deceased neonatal and infant tissue donors

  • Full microbiology screening of a maternal sample is always required.
  • For still births and neonates less than 48 hours after birth, no microbiology screening of the neonate is required.
  • For neonates between 48 hours and 28 days after birth, a neonatal sample is only required when there are identifiable risks of possible viral transmission. In this scenario only nucleic acid amplification techniques (NAT) testing of the sample is required.
  • For infants more than 28 days after birth, full microbiology screening of an infant’s sample is required.

9.2.3: Serology screening algorithms Blood donations

  • No sample which tests initially reactive for the first time in the routine screening assay can be released for clinical use unless subsequently shown to have a negative result on both tests in duplicate repeat testing using the same assay.
  • Blood donations that are reactive on one or both of the repeat tests are unsuitable for use and must be labelled as biological hazard/not for transfusion.
  • Samples which test initially reactive in the routine screening assay, but which originate from donors who have been previously investigated in a reference laboratory and have been shown to be demonstrating non-specific reactivity, may be tested on a second (alternative) screening assay of at least equal sensitivity to the primary screening assay, and can be considered suitable for clinical use if the reaction in the alternative screening assay is negative.

See flowchart for screening of blood donations provided in Figure 9.1.


Figure 9.1 Serology screening: blood donations




Figure 9.2 Serology screening for tissue and stem cell donors Tissue and stem cell donations

  • All initially reactive samples (see Figure 9.2) must be re-tested in duplicate using either the same assay or using an alternative assay that has been specifically evaluated to have at least equal sensitivity and ideally is based on different antigens and/or antibodies, and/or principles.
  • Donations that are non-reactive on both of the repeat tests can be considered suitable for clinical use.
  • Donations that are reactive on one or both of the repeat tests may in some clinical circumstances, and depending on the confirmatory results, be considered suitable for use (SaBTO Guidelines 20112).

9.2.4: Molecular screening algorithm

  • All initially reactive pool samples (see Figures 9.3 and 9.4) must be resolved to an individual (or more) reactive donation. All other non-reactive donations can be considered suitable for clinical use.
  • Individual reactive donations are unsuitable for clinical use and must be labelled as biological hazard/not for transfusion.
  • Stem cell donations from known infected individuals that are reactive on screening may in some clinical circumstances be considered suitable for use (SaBTO Guidelines 20112).

* Donors confirmed to be HEV or WNV RNA positive need only be deferred for 6 months from pick-up.

Figure 9.3 Molecular screening: blood donations



Figure 9.4 Molecular screening for tissue and cell donors

9.2.5: Confirmatory testing

When a donation is screen reactive for any of the serological or molecular mandatory or additional microbiology tests described above (except for anti-HCMV and anti-HBc, where anti-HBs is present at a level 100 mIU/mL), samples from the donor/donation must undergo confirmatory testing at a designated reference laboratory.

  • If HEV or WNV RNA is confirmed in a donor, the donor record must be flagged as ‘temporary exclusion’ for 6 months.  The donor can be reinstated automatically at least 6 months after the date of the index HEV or WNV RNA positive donation: see section 9.4.
  • In all other cases, the donor record must be flagged as ‘permanent exclusion risk – not to be used for clinical use’ or equivalent.
  • In all cases where a positive result is confirmed, arrangements should be made to inform the donor and to ensure that the donor is given appropriate advice.

Note: Autologous stem cell donations may be collected from individuals who are known to be infected with one or more of the infectious agents for which donations are routinely screened. Such individuals are not generally classified as donors for the purposes of these guidelines.

  • If a negative, inconclusive or indeterminate result is reported following confirmatory testing, and the initial reactivity is determined by the reference laboratory to be non-specific, use of further donations or the same donation (tissue and stem cell donors only) may be possible, as covered in section 9.4. Specific requirements for HBsAg confirmation

The designated reference laboratory should, where appropriate, perform specific neutralisation tests for HBsAg to ensure that donors with low-level HBsAg reactivity are not incorrectly described as non-specifically reactive.

9.2.6: Definitions



Non-reactive (NR)

A sample whose reactivity when first tested falls inside the assay cut-off as defined by the manufacturer’s instructions. May also be referred to as a ‘Negative’ test result

Initial reactive (IR)

Any sample whose reactivity when first tested falls outside the cut-off as defined by the manufacturer’s instructions

Repeat reactive (RR)

Any sample reactive on two or more occasions either in the same screening test (duplicate) or in two or more screening tests that are used in combination sequentially, to determine the suitability of a donation for release for clinical use

Alternative assay testing

When a test of similar modality and sensitivity is used sequentially to screen a sample which is either IR or RR in a first screening assay

Confirmatory testing

Further testing of a repeat reactive sample using a number of different assays in a reference laboratory to define whether the reactivity is specific to the microbe being screened for and indicative of potential infectivity


A sample whose reactivity in confirmatory testing meets pre-defined criteria. This may indicate current or past infection depending on the markers and microbe concerned


A sample whose reactivity in confirmatory testing is not sufficient and/or specific enough to determine whether it reflects infection with the microbe being screened for


A sample which is either non-reactive in confirmatory testing or whose reactivity in confirmatory testing is deemed not to reflect infection