Blood Testing

The American Red Cross performs laboratory tests for multiple infectious disease markers on every unit of donated blood. Tests are upgraded or replaced with more sensitive technologies as these become available. These tests include:

Blood Testing

Chagas disease (T. cruzi) Antibody testing (2007)

Chagas disease is a serious, often fatal disease caused by the parasite Trypanosoma cruzi (T. cruzi). The agent is endemic in the Americas but most commonly occurs in Latin America. Blood donation screening is done using a qualitative chemiluminescent immunoassay (ChLIA) to detect antibodies to T. cruzi in human serum and plasma specimens. All reactive donations are further tested by a second licensed screening test and a licensed supplemental assay.  We use two tests to ensure that only true positives are notified of a confirmed-positive result.  All tests used are licensed by the Food and Drug Administration (FDA).  Because the rate of new infection in donors is very low (less than 1 per million), the Red Cross qualifies each donor (rather than each donation) for negativity to antibodies to T. cruzi.

Chagas disease

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Hepatitis B virus (HBV)
B Surface Antigen (HBsAg) (1971), Hepatitis B Core Antibody (HBc) (1986), and Nucleic Acid Testing (NAT) (2009)

HBsAg and HBV DNA are the first viral markers to circulate in an individual infected with HBV. Anti-HBc appears in the serum of individuals infected with HBV one to four weeks after the appearance of HBsAg, and at the onset of symptoms for the minority of adults (5% or less) who develop symptoms. A chemiluminescent immunoassay (ChLIA) is used for the qualitative detection of HBsAg and anti-HBc in human serum and plasma specimens. Specific antigen neutralization is used for HBsAg reactive confirmation; anti-HBc confirmation is done by testing each individual reactive donation sample by ultrasensitive HBV DNA detection by polymerase chain reaction (PCR, a type of NAT). HBV DNA detection by NAT for routine blood donor screening is done using transcription mediated amplification (TMA) technology; a type of NAT similar to PCR. Screening is performed in small minipools of 16 donations using a combined test that detects HBV DNA as well as HIV and HCV RNA. All TMA-reactive donations that are serology negative (HBsAg and anti-HBc) are further tested by PCR. NAT using TMA in minipools of 16 reduces the window as compared to HBsAg detection by 12days. The risk of hepatitis B infection through blood transfusion is between 1 in 843,000 to 1:1,208,000.

Hepatitis B virus

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Hepatitis C virus (HCV 3.0)
Antibody testing (1990) and Nucleic Acid Testing (NAT) (1999)

HCV is known to be the causative agent for most, if not all, blood-borne non-A, non-B hepatitis (NANBH). Blood donation screening is done using a qualitative third-generation, chemiluminescent immunoassay (ChLIA) in human serum and plasma samples. All anti-HCV reactive donations that are RNA negative are further tested by a second licensed screening test HCV RNA detection, by NAT using TMA in minipools of 16 as described above, reduces the window period between infection and the detection of antibody by about 1-2 months. The current risk of transfusion-transmitted HCV is 1 in 1,149,0001

Hepatitis C virus

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Human Immunodeficiency viruses, Types 1 and 2 (HIV 1,2)
Antibody testing (1985) and NAT (1999)

Blood donation screening is done using a qualitative third-generation, chemiluminescent immunoassay (ChLIA) in human serum and plasma samples for the detection of antibodies to both HIV-1 and HIV-2, the causative agents of AIDS. HIV-1 and HIV- 2 confirmation is performed using one or a combination of tests including an HIV-1 indirect immunofluorescence assay (IFA) and an HIV-2 enzyme immunoassay (EIA); a rapid diagnostic test is used for HIV-1 and HIV-2 antibody differentiation. HIV-1 antibody detection includes the major HIV groups and variants including HIV-1 Groups M, N and O. HIV RNA detection by NAT, using TMA in minipools of 16 (as described for HBV and HCV testing), closes the window period between infection and the detection of antibody by about 4-7 days. The current risk of transfusion-transmission of HIV is approximately 1 in 1,467,0001.

Human Immunodeficiency viruses - HIV

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Human T-Lymphotropic virus
Antibody testing (HTLV-I/II) (1988)

HTLV-I is a human Type-C retrovirus that has been associated with neoplastic conditions and a variety of demyelinating disorders. HTLV-II is not yet proven unequivocally to be of significant clinical concern. Qualitative antibody detection for both HTLV-I and HTLV-II in a combined test is performed with a ChLIA. The current risk of transfusion-transmitted (HTLV-I) is less than 1 in 2,000,0003.

Human T-Lymphotropic virus

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Syphilis (Treponema pallidum)
Antibody testing (1940's)

The test used for syphilis is a qualitative screening test that detects the presence of antibodies to the spirochete bacterium, Treponema pallidum. The assay is based on the principle of agglutination and pattern recognition. Confirmation is performed using another serologic test for total antibodies, an EIA, as well as a test for reagin (a protein-like substance that is present during acute infection and for several months following resolution of infection).  No cases of transfusion-transmitted syphilis have been recorded for more than 30 years.


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West Nile virus (WNV)
Antibody testing (2003)


WNV is a flavivirus originally described in Africa, and subsequently in West Asia, and the Middle East that is most commonly transmitted to humans through mosquito bites. WNV was first introduced in the US in 1999 and reached epidemic proportions in 2002, the same year that WNV was documented to be transmitted by blood and organs. WNV RNA is detected by NAT using the same type of assay as used for HBV, HCV and HIV-1. Following the introduction of blood donor screening there have been 11 documented cases of transfusion transmission from low-level viremic units attributed to not converting from minipool testing of 16 donations to more sensitive single unit donation testing in areas of the US experiencing increased WNV activity; however, none of these 11 cases involved donations to the Red Cross  However, one untested granulocyte donation from the Red Cross, released on an emergency basis, was shown to have transmitted WNV. As mentioned, measures are in place to reduce the risk of transmissions from units with low concentrations of virus during epidemic time periods by converting from testing donations in minipools to testing donors individually.


West Nile virus

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In addition to these tests, the Red Cross tests every unit to identify the donor’s blood group (O, A, B or AB) and Rh type, and screens for atypical or unusual red cell antibodies. Screening for antibodies to cytomegalovirus (CMV) is also available.


1 Stramer S. Current risks of transfusion-transmitted agents- A Review Arch Pathol Lab Med. 2007; 131: 702-707.

2 Zou S. et al Current Incidence and residual risk of hepatitis B infection among blood donors in the United States Transfusion 2009; 49:1609-1620.
3 Dodd R.Y. et al Current prevalence and incidence of infectious disease markers and estimated window-period risk in the American Red Cross blood donor population. Transfusion 2002; 42: 975-979.