Laboratory testing

In the past 15 years there has been a proliferation of new and better laboratory tests for infectious diseases that can be transmitted by transfusion. Before 1985, blood products were tested only for antibodies to Treponema pallidum (the bacterium that causes syphilis), and the Australian Antigen for hepatitis B surface antigen (HBsAg). Additional tests were developed for hepatitis B and C, HIV 1 and 2, and HTLV-I. Testing for parasites has been discussed for many years, but the FDA has yet to license a test to screen for malaria or Chagas' disease. With the risk of contamination from viral agents diminished, there is now a greater emphasis on developing assays for parasites.

In the United States, the policy is to screen each unit of blood as soon as a new assay is licensed; this currently includes the following laboratory tests (see Table 7.4):

• human immunodeficiency virus (HIV-1 and HIV-2)

• human T-lymphotropic virus (HTLV-I and -II)

Table 7.4 How blood is tested

Disease

Test

Implemented

Discontinued

HIV/AIDS

HIV-I Antibody test HIV-1/2 Antibody test HIV-I p24 Antigen test

1985 1992 1996

2003

Hepatitis B

Hepatitis B surface antigen Hepatitis B core antibody

1971 1987

Hepatitis C

Anti-HCV

1990

Hepatitis

ALT

1986

2003

Syphilis

Serologic test for syphilis - TP or RPR

1948

Human T-cell Lymphotropic Virus (HTLV)

HTLV-I Antibody test HTLV-I/II Antibody test

1989 1998

Hepatitis C and HIV/AIDS

Nucleic acid testing (NAT)

1999

West Nile Virus

Nucleic acid testing (WNV-NAT)

2003

Note: CMV testing is performed on some units of blood for patients who require CMV-negative blood - for example, neonates weighing less than 1500 g, and immuno-compromised or immune-suppressed patients.

Source: American Red Cross, 2006b (http://www.pleasegiveblood.org/education/blood_tested.php)

Note: CMV testing is performed on some units of blood for patients who require CMV-negative blood - for example, neonates weighing less than 1500 g, and immuno-compromised or immune-suppressed patients.

Source: American Red Cross, 2006b (http://www.pleasegiveblood.org/education/blood_tested.php)

• Testing of platelet donations for bacterial contamination.

Once approved, a laboratory test will be conducted on all donations until either a new and better test is licensed, or population-based data indicate that it is no longer productive. For example, ALT testing for hepatitis continued until 2003, and was discontinued only when a more specific and sensitive test became available to detect HCV Similarly, HIV1 p24 antigen testing was discontinued in 2003, but only when a specific and sensitive nucleic acid amplification test (NAT) was licensed for HIV1 and added to routine donor screening. From 15 March 1996 to 27 March 1999, HIV-1 p24 antigen testing was performed on approximately 45,000,000 units. Even with additional neutralization testing, there were many false-positive test results. Still, testing continued, and during this period only five donors were found to be HIV-antigen positive and HIV-antibody negative - which was about 1 in 9 million units tested. Currently, the United States is the only country to test for HIV-RNA (the United Kingdom and others test for HCV-RNA, but not for HIV-RNA). It is estimated that RNA testing reduces the risk of HIV and HCV transmission to about 1 in 2 million blood units.

It should be noted that there can be a difference between a laboratory test available for blood testing, and that licensed for the clinical assessment of patients. For example, there are diagnostic clinical tests for Chagas' disease and malaria, but still none licensed for screening blood. In contrast, the laboratory test for anti-HIV1 was licensed for blood-donor screening before it was approved for patient use.

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