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Syphilis, Human T-Cell Lymphotropic Virus, and Chagas Screening
Testing blood donors for syphilis, human T-cell lymphotropic virus (HTLV), and Trypanosoma cruzi infection is mandated by the FDA and AABB Standards. Syphilis was the first transfusion-transmitted disease for which the blood supply was screened in the United States. HTLV is a retrovirus that can be transmitted by transfusion, and donor testing has mitigated the risk since put into place in 1988. Transfusion-transmitted Chagas disease, which is caused by T. cruzi , has rarely been reported.
Syphilis
Syphilis was the most commonly recognized transfusion-transmitted disease pre–World War II. For more than 50 years, donors have been screened for evidence of infection by Treponema pallidum , the organism responsible for causing syphilis. The last reported case of transfusion-associated syphilis was in 1966. Disappearance of transfusion-associated syphilis resulted from decline in incidence, antibiotic use, increased control of the disease spread, cessation of direct donor-to-patient transfusion, storage of blood components ( T. pallidum , which is anaerobic, is not infectious after ∼72 hours of refrigeration), and donation testing.
Background
Syphilis is transmitted primarily through sexual or vertical transmission; the spirochete passes through intact mucous membranes or compromised skin. The primary stage of syphilis is a chancre, which appears 10–90 days (average 21 days) after infection. The organism then disseminates to the blood. Four to ten weeks after the primary stage of infection, untreated individuals enter the secondary stage, developing a rash, fever, sore throat, malaise, weight loss, hair loss, and headache. At this stage they will become seropositive. Then, untreated individuals enter the latent stage, lasting years. The late stage (neurosyphilis and other damage to internal organs) presents in ∼15% of untreated individuals and can appear 10–20 years after infection. Both treponemal and nontreponemal antibodies are present during this stage.
Determination of Need and Requirement for Testing
Serologic Screening Tests
Antibody detection is either via nontreponemal or treponemal tests. Nontreponemal tests identify active or recent infections and become negative after disease treatment; treponemal tests identify current and distant infection. Until 2018, most blood centers used treponemal tests because they were automated and had better performance characteristics. The frequency of reactive treponemal screening tests of US donors is ∼0.05%. In January 2018, an automated nontreponemal test received approval for donor screening. Given that the vast majority of donors with reactive treponemal tests are nonreactive on nontreponemal testing, this should save many donors from deferral.
Nontreponemal Antibody Tests (Rapid Plasma Reagin Test or Venereal Disease Research Laboratory Test)
Nontreponemal tests detect antibodies that react against cardiolipin phospholipid produced in response to infected host tissue. Nontreponemal tests have a false-positive rate of 1%–2% owing to pregnancy, other infections (such as HIV, mononucleosis, tuberculosis, rickettsial infection, and other spirochetal and bacterial endocarditis), and disorders of immunoglobulin production (such as rheumatoid arthritis and ulcerative colitis cirrhosis). Rapid plasma reagin (RPR) test uses cardiolipin-coated carbon particles, which agglutinate with antibodies.
Treponemal Antibody Tests
Treponemal tests detect antibodies that specifically target T. pallidiun . Microhemagglutination for T. pallidum (MHA-TP) system has been widely used for donor screening because of its automation. MHA-TP uses sensitized sheep erythrocytes coated with T. pallidum , which agglutinate with anti–treponemal IgM and IgG antibodies.
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