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Transmission of HIV through blood donations has been almost eliminated in the U.S. -- but the key word is "almost."

In 2008, the CDC reported, a Missouri man donated blood that escaped HIV surveillance and was later used to treat a patient getting a kidney transplant.

The kidney recipient's and blood donor's HIV were later shown to be 99% identical by genetic testing, the agency reported in the October issue of Morbidity and Mortality Weekly Report.

The agency noted that although such events are rare, they are possible because the tests in place don't pick up HIV infection during the so-called "eclipse" period -- the few days after infection and before detectable HIV RNA can be found in the blood.

Indeed, three previous cases were reported to the CDC in 2000 and 2002, in which blood donated during the eclipse period was negative on testing, both by enzyme immunoassay and nucleic acid amplification, but nonetheless caused HIV when transfused.

In this case, the CDC reported, the Missouri man reported no HIV risk factors when he gave blood in June 2008, and the blood was negative on both tests. One unit of packed red blood cells from the donation was used in July of that year to treat a patient undergoing cardiac surgery, who died two days later of heart disease.

Another unit of red blood cells was transfused in August into a Colorado patient undergoing kidney transplant. Both the kidney donor and the recipient were negative for HIV at the time of the transplant.

In November of that year, the Missouri man again donated blood, reporting no risk factors on his questionnaire. This time, however, testing showed HIV infection.

When the man was interviewed, he said that he was married but sometimes had sex outside the marriage with both men and women, usually when he was intoxicated. He had not reported those episodes on his donation questionnaire.

A rapid test during the interview confirmed the HIV infection, the CDC reported.

On testing, the Colorado kidney recipient was found to be HIV-positive, albeit without symptoms, because mycophenolic acid (CellCept, Myfortic) to prevent rejection was being given. The drug is a potent inhibitor of lymphocyte proliferation and HIV replication in CD4-positive T cells and macrophages.

Nonetheless, the CDC reported, the patient -- who was started on antiretroviral therapy -- had an HIV RNA viral load of 7,240 copies per milliliter of serum and a CD4 cell count of just 48 cells per microliter.

The CDC concluded that although rare, such episodes are likely also under-reported. A conservative estimate, the agency argued, would be that "11 infectious donations and 20 HIV-positive blood components released each year could potentially infect recipients."

Clinicians with HIV-positive patients who have no other risk factors should suspect the transfusion route, the CDC argued.

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