It is hard to say exactly how long the window period lasts,
as there are variations between individuals and it is a difficult topic to
research (recently infected people would need to know exactly when they were
exposed to HIV and then give multiple blood samples over the following days and
weeks).
Nonetheless,
a study by Dr Kevin Delaney and colleagues calculated window periods for a
range of HIV testing assays. All these analyses were based on plasma
samples. Window periods are likely to be several days longer when testing
samples of fingerprick blood or of oral fluid, as will be normal when using
rapid, point-of-care tests and self-testing devices. Unfortunately, precise
figures for how much longer the window periods are have not yet been published.
The researchers’ analysis confirms that fourth-generation
laboratory tests (which detect both antibodies and p24 antigen) detect HIV
infections between one and three weeks earlier than older antibody-only tests.
Moreover, their data suggest that some countries’ guidelines which recommend
retesting 90 days after a possible exposure to HIV are more cautious than they
need to be.
A fourth-generation
laboratory test is recommended in UK and US guidelines. It uses
a sample of blood plasma or serum and can detect immunoglobulin G (IgG)
antibodies, immunoglobulin M (IgM) antibodies and p24 viral antigen (a protein
contained in HIV's viral core that can be detected sooner than antibodies). Commonly
used tests of this type include Abbott
Architect HIV Ag/Ab, GS Combo Ag/Ab EIA and
Siemens Combo HIV Ag-Ab.
- The median window period is 18 days (interquartile range 13
to 24 days). This indicates that half of all infections would be detected
between 13 and 24 days after exposure.
- 99% of HIV-infected individuals would be detectable within 44
days of exposure.
A fourth-generation rapid test is available (Determine HIV-1/2 Ag/Ab Combo). While results for this assay when
testing plasma were broadly similar to those of equivalent laboratory tests,
the window period is likely to be several days longer when testing fingerprick
blood, as the test is normally used.
A few third-generation rapid, point-of-care tests are available. They can
detect immunoglobulin G (IgG) antibodies and immunoglobulin M (IgM) antibodies.
Examples include the INSTI HIV-1/HIV-2
and Uni-Gold Recombigen HIV tests.
The estimated window period for INSTI when
testing plasma are as follows:
- The median window period
is 26 days (interquartile range 22 to 31 days). This indicates that half
of all infections would be detected between 22 and 31 days after exposure.
- 99% of HIV-infected
individuals would be detectable within 50 days of exposure.
However, those estimates were based on testing plasma. In
practice, tests are usually done on fingerprick blood and the window period is
likely to be several days longer.
Third-generation
laboratory tests are no longer recommended for use. They can detect
immunoglobulin G (IgG) antibodies and immunoglobulin M (IgM) antibodies, but
not p24 viral antigen. Their window periods are similar to those of the INSTI third generation rapid test
(plasma samples), but a little shorter (median 23 days).
Many rapid,
point-of-care tests are described as second generation. They can detect immunoglobulin G
(IgG) antibodies, but not immunoglobulin M (IgM) antibodies or p24 viral
antigen. As these two substances are detectable sooner after HIV infection than
IgG antibodies, second-generation tests have longer window periods. Examples
include OraQuick Advance Rapid HIV 1/2,
Clearview HIV 1/2 STAT-PACK and SURE CHECK HIV 1/2.
- The median window period
is 31 days (interquartile range 26 to 37 days). This indicates that half
of all infections would be detected between 26 and 37 days after exposure.
- 99% of HIV-infected
individuals would be detectable within 57 days of exposure.
However, those estimates were based on testing plasma. In
practice, tests are usually done on fingerprick blood or oral fluid and the
window period is likely to be several days longer.
No self-testing
devices were included in this study.
However, most self-tests are modified versions of rapid, point-of-care test
kits that were originally designed for healthcare professionals. Most are based
on second- generation tests, so are likely to have relatively long window
periods. A few, including the INSTI HIV
Self Test, are based on a third-generation test.
Similarly, self-sampling was not included. In the UK, this usually involve the user
sending a sample of fingerprick blood to be tested in a laboratory with a
fourth-generation antibody/antigen test. Plasma is extracted from the sample
using centrifugation. In theory, the test will be as accurate with plasma from a
self-collected sample of fingerprick blood as from venous blood, including in
relation to acute (recent) infection.