Page 66 - AIDSBK23C

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adults, while at age 3 months about half are detectable, but only a minority of HIV-1 infections
are diagnosable by HIV IgA assay under 1 month of age.[357,361]

IMMUNOBINDING ANALYSIS.-- Rapid serologic methods for cost-effective diagnosis
of HIV-1 and HIV-2 infection have been developed for use in places where the high cost or
longer turnaround time of the EIA assay with smaller numbers of samples makes application of
routine HIV testing more difficult. Both the enzyme immunobinding (dot-blot) and particle
agglutination assays do not require instrumentation or trained technicians and provide a rapid
turnaround time (hours). The tests utilize recombinant-expressed peptides, derived from the
protein envelope of HIV. The sensitivity and specificity of these assays are comparable to EIA
and Western blot when the test is performed properly. There may be fewer indeterminate results
than with conventional Western blot testing.[329,362] This form of rapid testing has been
applied to forensic science, providing a rapid means for postmortem diagnosis of HIV
infection.[363]

HIV-1 RNA ASSAY.-- The polymerase chain reaction (PCR) method can be applied to
both tissues and plasma for detection of HIV. In tissues, a DNA probe is used to detect HIV-1
proviral DNA, but is much more sensitive than in situ hybridization because the target DNA is
amplified many times to enhance sensitivity tremendously. Quantitation of the amount of HIV
present is also possible. PCR can detect one copy of viral DNA in one cell out of 100 000 to 1
000 000 cells present. The disadvantage of PCR is that the tissue, either fresh or formalin-fixed
paraffin-embedded, must be digested so that the exact localization of the HIV-1 within tissues
cannot be determined.[364]
The PCR method has also been employed for early viral detection in serum of perinatally
acquired HIV-1 infection. The sensitivity of this assay is sufficient to detect about half of
infections in the first month of life. Between 30 and 60 days following birth, PCR will detect
virtually all HIV infections of infants, and there should be no false negatives after 6 months, a
sensitivity equivalent to HIV culture. Sensitivity is 29% in the first week, 79% at 8 to 28 days of
age, and >90% at 29 days of age and thereafter. Therefore, HIV infection can be presumptively
excluded with 2 negative virological tests, with one at 2 or more weeks of age and the second at
1 or more months of age. HIV infection can be definitively excluded with 2 negative virological
tests, with one at 1 or more months of age and the second at 4 or more months of age.
[357,365,366]
Quantitation of HIV-1 RNA in plasma or peripheral blood mononuclear cells can be
performed by three methods: reverse transcriptase-polymerase chain reaction (RT-PCR),
branched DNA (bDNA) testing, and nucleic acid sequence-based amplification (NASBA).[332]
These assays provide a reliable means for monitoring progression of HIV infection
independently of CD4 lymphocyte counts. Levels of HIV-1 are reported in viral copies per
milliliter on patient plasma, and results may vary up to two-fold among these assays, so one
assay should be utilized consistently for a given patient.[367] The level of HIV-1 RNA may
vary up to three-fold in a single patient. However, there appears to be no diurnal variation. The
HIV-1 RNA level tends to increase as the CD4 lymphocyte count declines and HIV infection
progresses.[368] However, the commercial viral load assay kits vary in their ability to quantify
different HIV-1 subtypes.[369]
The levels of plasma HIV RNA detected correlate with the stages of HIV infection: a
viremic "spike" following initial infection, then suppressed levels of HIV during the long "latent"