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The GRE (Graduate Record Exam) revised General Test, introduced in August 2011, features a new test-taker friendly design and new question types. It more closely reflects the kind of thinking you'll do in graduate or business school and demonstrates that you are ready for graduate-level work. Skills are assessed in the areas of verbal reasoning, quantitative reasoning, and analytical writing.

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About Riverbed Technology

Riverbed Technology, Inc. develops applications, websites, networks, data centers, the cloud, and remote offices. It offers visibility, intelligent optimization, and simplified control for all applications. The firm's solutions include application performance monitoring, network performance monitoring, end-to-end performance management, hyper-converge branch, wide area network optimization and hybrid wide area network. The company was founded by Jerry M. Kennelly and Steven McCanne in May 2002 and is headquartered in San Francisco, CA.

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What is the window period for HIV testing?

The window period refers to the time after infection and before seroconversion, during which markers of infection (p24 antigen and antibodies) are still absent or too scarce to be detectable. Tests cannot always detect HIV infection during the window period.

All tests have a window period, which varies from test to test. It also depends on the specimen that is being tested: window periods are usually reported based on a sample of blood plasma, but are longer when the specimen tested is fingerprick blood or oral fluid.

(Plasma is the colourless fluid part of blood, separated from whole blood using laboratory equipment. Fingerprick blood is produced by pricking the finger with a lancet, whereas oral fluid is obtained by swabbing the gums.)

There are two key questions to ask about a specific HIV test: 

The information given in UK testing guidelines about window periods is based on answers to the second question – specifically, how long after exposure to HIV 99% of infections will be detected by a specific type of test. At this stage, it is highly likely that a negative result is accurate.

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.

NAM aidsmap's Matthew Hodson explains about window periods for HIV testing.

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.

UK guidelines state that 45 days is the window period for fourth-generation laboratory tests.

A fourth-generation rapid test is available (Determine HIV Early Detect or Determine HIV-1/2). 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:

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.

UK guidelines state that 90 days is the window period for all rapid, point-of-care tests.

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).

UK guidelines state that 60 days is the window period for third-generation laboratory tests.

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.

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.

UK guidelines state that 90 days is the window period for all rapid, point-of-care tests.

In HIV testing, the period of time after infection and before seroconversion during which markers of infection are still absent or too scarce to be detectable. All tests have a window period, the length of which depends on the marker of infection (HIV RNA, p24 antigen or HIV antibodies) and the specific test used. During the window period, a person can have a negative result on an HIV test despite having HIV.

A protein substance (immunoglobulin) produced by the immune system in response to a foreign organism. Many diagnostic tests for HIV detect the presence of antibodies to HIV in blood.

The fluid portion of the blood.

Another name for antibodies. An antibody is a protein substance produced by the immune system in response to a foreign organism (such as bacteria, virus or parasite).

A test in which all stages, including reading the result, can be conducted in a doctor’s office or a community setting, without specialised laboratory equipment. Sometimes also described as a rapid test.

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.

In some situations, these figures should be interpreted with caution: