Question:

Dear doctors,
I have a question that it is causing me a lot of concerns. 9 months ago I got injured while in Nigeria. At the hospital they gave me blood. Soon as I got back I got tested for HIV1 and 2 and I have had until now, 3 antibody tests, the last one at 6 months after the event.

Now my questions: I have heard that in Nigeria the subtype N and O are present and that they might not be detected by the standard Elisa test I did.
What do you suggest me to do?

Thanks for your attention.

Answer provided by:

Rodger MacArthur, M.D.
Wayne State University
Division of Infectious Diseases

In general, the currently-used ELISA assays are quite good at detecting antibodies to non-Clade B subtypes like Clade N and Clade O. However, there have been reports (in the past) of ELISA assays not reacting to some specimens infected with Clade N or Clade O strains of HIV. These subtypes are present in much of Africa, whereas Clade B predominates in Western Europe and North America.

Data from primarily Clade B subtypes suggest that over 99% of persons who ultimately seroconvert from HIV-negative to HIV-positive will do so within 6 months of the exposure. At the Detroit Medical Center, we recommend that one additional test be drawn at 1 year following an occupational exposure, although no one has ever been HIV-negative at 6 months and HIV-positive at one year.

Given the excellent performance of currently-used ELISA assays, the chance that someone exposed to HIV in Nigeria, who tested negative through 6 months post-exposure, will actually be positive (or turn positive) from that exposure is very low. There are, however, several other steps that could be taken to confirm that the ELISA results are truly negative:

1) Have the laboratory run a Western Blot on the specimen(s). The Western Blot test typically is not run if the ELISA is negative, but can be performed if requested. It will likely detect antibodies (if present) that may not have cross-reacted with the ELISA assay.

2) Have a specimen tested for HIV RNA by the bDNA assay. Note that neither the bDNA assay nor the RT PCR assay for detection of HIV RNA is licensed for the purpose of determining infection status, because an undetectable result does not exclude infection. Nevertheless, a positive result, especially at copy number greater than 1000/ml, would indicate infection with HIV RNA. The bDNA assay is much better at accurately quantifying copy number for all clades than is the RT PCR assay, which performs best for Clade B subtypes. Note, however, that the RT PCR assay is about twice as sensitive as the bDNA assay for detecting copy number of Clade B subtypes, especially at low copy number.

3) Have a thorough physical examination performed, looking for signs (e.g., enlarged lymph nodes) of HIV infection. Such signs do not indicate or confirm infection with HIV, but do increase the a priori likelihood of ongoing HIV infection. Conversely, a negative examination, when done by a person experienced in examining HIV-infected persons, decreases the a priori likelihood of infection, especially when paired with previously negative ELISA results.

4) Do not order a CD4 cell count, as a low result on this test does not necessarily suggest infection, nor does a high (normal) result exclude infection.

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