Page 61 - AIDSBK23C

Page 61

by persons without technical expertise, require no instrumentation, and can provide point-of-care
testing that is cost-effective. Diagnosis is typically accomplished through the sequential use of 2
separate rapid HIV-1/2 antibody tests, and in resource-limited settings, concordant serological
reactivity on 2 appropriately selected rapid tests is highly predictive of infection and is frequently
considered to be sufficient for presumptive diagnosis of HIV infection. In more-developed
countries, confirmation of antibody-positive rapid samples is accomplished using an independent
HIV test method, such as Western blot or line immunoassay.[330]
Rapid EIAs provide results to patients without a waiting period, which loses some
patients to follow up and counseling.[331] A rapid EIA can allow post-exposure prophylaxis to
begin within 2 hours for persons with occupational exposure to HIV. However, errors in
performance and interpretation are more likely to occur with rapid assays than with routine
laboratory EIAs.[332,333]
EIA tests have improved over time. The first generation tests that became widely used in
the 1990’s could detect only IgG antibodies, and the window period from infection to
seropositivity was 45 to 60 days. The fourth generation EIA tests combine both HIV antibody
and p24 antigen detection, which reduces the “window” period for detection following infection
th
to little more than 10 days, and reliably less than 3 weeks. However, these sensitive 4
generation assays complicate confirmatory testing, because EIA seropositivity may precede
Western blot positivity by 3 weeks.[334,335]
Use of EIA testing can also be applied to body fluid samples other than blood. Oral
mucosal transudate (OMT) is a fluid derived from serum that enters the saliva from the gingival
crevice and across oral mucosal surfaces. The OMT contains immunoglobulins that can be
concentrated via collection devices such as pads held next to gums and oral mucosa. Testing via
EIA of OMT yields results comparable to serum EIAs.[336] Saliva can also be utilized for rapid
EIA testing, and has the advantage of simplified collection and processing. However, the results
with rapid HIV tests using saliva are slightly less sensitive and specific than for serum
EIAs.[337] The sensitivity can be increased by use of an assay that employs an IgG antibody-
capture ELISA methodology (GACELISA).[338]
The EIA tests for HIV utilize either a whole virus lysate, recombinant proteins, or
synthetic peptides for the solid phase antigen, and tests based on the latter two antigens are more
sensitive and specific.[329] Kits are available that combine testing for both HIV-1 and HIV-2.
These assays are very reliable. When antibodies to one or more antigenic components of HIV
including reverse transcriptase (RT), p17, p24, p31, gp41, and gp120/160 are present, as in most
cases, sensitivity and specificity are over 99%. Highly sensitive immune complex transfer
enzyme immunoassay methods can also be employed.[339]
Many EIA assays detect HIV-2. The emergence of subtypes of HIV-1 complicates
testing, as evidenced by subtypes O and M, which are not detected by all routine methods for
HIV-1 testing. Group O and M infections can be diagnosed by EIA followed by viral RNA
quantification and genotypic resistance assays.[340] In addition, the EIA method can be used for
screening for HTLV. Tests employing synthetic peptide antigens can distinguish HTLV types I
and II.[329]
The standard protocol for EIA testing is initial determination of reactivity. Reactive tests
are repeated in duplicate. If both repeat tests are reactive, the sample is considered positive and a
confirmatory test is performed, If one of the repeat tests is negative, then there is a high
probability of error in testing and another blood specimen should be obtained for testing. The