Page 60 - AIDSBK23C

Page 60

CHAPTER 2 - DIAGNOSIS OF AIDS

DIAGNOSTIC TESTS FOR HUMAN IMMUNODEFICIENCY VIRUS

There are multiple HIV testing methods available. Most are serologic methods based
upon detection of antibody to HIV in blood or body fluids, while the p24 assay detects HIV
antigen. The polymerase chain reaction and in situ hybridization techniques are used primarily
with fresh and fixed tissue samples, but can also be applied to blood samples. HIV viral culture
can be performed on both fluids and tissues. Immunologic alterations detected through
lymphocyte subset quantification in blood are used clinically to detect and follow the effect of
HIV infection on the immune system.
Though HIV can be present in a variety of body fluids, blood and blood components have
the greatest concentration of viral components. Oral secretions and urine have also been
employed for patient testing based upon ease of collection and processing. Laboratory personnel
must employ universal precautions. As an alternative to whole blood, serum, or plasma, dried
blood spots can be utilized because they are simpler to prepare, store, and transport. Such dried
blood spots have been employed for initial HIV testing, viral load quantification, and HIV drug
resistance testing.[326]
An acute HIV infection in the context of laboratory diagnosis may be defined as the time
from viral acquisition until a seroconversion is detected. However, detection depends upon the
host response and laboratory methodologies employed. An eclipse period from the time of
establishment of HIV infection at the site of exposure until HIV enters the systemic circulation
may last on average up to 10 days (range 7 to 21 days). When HIV-1 RNA is found at a
concentration of 1 to 5 copies per milliliter in plasma, then it is detectable by nucleic acid
amplification. At a concentrations of 50 copies per milliliter, HIV-1 is detectable using
quantitative assays used to monitor viral load, starting after the eclipse period.[327]
When HIV enters lymphoid tissues, replication increases rapidly along with viremia, and
HIV-1 RNA becomes detectable in blood, but antibodies may not yet be detectable. HIV core
antigen p24 may be detectable a week after viremia, and is useful primarily for detection of
perinatal infection because of passively acquired maternal HIV antibody. An acute retroviral
syndrome may be present at this point. HIV antibodies detectable by enzyme immunoassay
typically appear within 3 weeks of infection, and a positive Western blot is found a month after
infection. The HIV antibodies remain throughout the life of the infected person.[328]

ENZYME IMMUNOASSAY.—The most common initial laboratory method for
detection of HIV infection is the enzyme immunoassay (EIA) to detect HIV-1, HIV-2, and
potentially other HIV subtypes. These serologic tests for HIV antibody make use of the human
immunologic response to HIV infection in which antibodies, primarily directed against HIV
proteins and glycoproteins such as gp120 and gp160, appear after acute HIV infection. The EIA
is a simple test to perform for clinical laboratories with trained technicians and, therefore, is the
"gold standard" for testing used extensively in blood banking and patient screening in most
places. The sensitivity and specificity of EIA testing by standard methods using serum exceeds
99%.[329]
Use of rapid serum EIA methods, defined as any test that yields results in less than 30
minutes, provides accuracy nearly as good as routine EIAs. Rapid assays are simpler to perform