1. STDs
2. HIV
3. Testing

HIV Testing Singapore | Shim Clinic

Private & confidential human immunodeficiency virus (HIV) testing. Screening for HIV infection after unprotected sex, like a condom break, slip or if no condom was used. Private and confidential. Shim Clinic.

Check out the various types of HIV tests available at this clinic.

LABORATORY TESTS

The Centers for Disease Control and Prevention (CDC) recommends HIV screening for patients in all health-care settings, after the patient is notified that testing will be performed unless the patient declines (opt-out screening); the CDC recommends that persons at high risk for HIV infection be screened for HIV at least annually.

The diagnosis of HIV infection is made by the detection of circulating antibodies to HIV. Antibodies are identified by the use of a screening test, usually an enzyme-linked immunosorbent assay (ELISA), followed by definitive diagnosis using a Western Blot assay. HIV antibody is detectable in at least 95% of patients within 3 months after infection.

In some situations such as pre-seroconversion or neonatal infection, measurement of HIV antibodies may be unreliable. In these instances, diagnosis of infection may use direct detection of HIV itself such as quantification of plasma HIV RNA, HIV viral DNA, or HIV antigen or by detection and amplification of virus in a tissue culture.

Screening Antibody tests

The ELISA or EIA test is the standard screening test for HIV infection. Recombinant or native HIV antigens, fixed in a solid phase, are exposed to and bound by HIV antibodies in test serum. The presence of these antibodies is then detected by a second anti-human antibody, with a sensitivity of >99.5%. Most commercially available ELISA kits contain antigens from both HIV-1 and HIV-2 and are able to detect infection with either of these viruses. A positive ELISA test is usually observed within 3-6 weeks following infection. The weeks between infection and seropositivity are termed the “window period” and are associated with high levels of circulating HIV, and potentially more efficient transmission. Commercial fourth-generation screening assays, which combine antigen and antibody screening, may reduce this window period to 6 days. False-positive test results are rare and the specificity of the ELISA is >99.8%.

Confirmatory Antibody Tests

The Western Blot is the definitive diagnostic test for HIV infection. The Western Blot (WB) assay detects antibodies in patient sera that react with a number of different viral proteins. A positive WB is defined by the detection of antibodies to all of the 3 main groups of HIV proteins – envelope (gp160, gp120 or gp41), gag (p24) and polymerase (p66 or p51).

An indeterminate WB assay is most commonly caused by the presence of unrelated antibodies that are cross-reactive with HIV proteins. It is possible that an indeterminate result is due to early HIV infection and incomplete evolution of the anti-HIV immune response. An indeterminate test result should be repeated at 1, 2 and 3 months to exclude an evolving pattern.

Using both EIA and WB tests, the sensitivity and specificity exceed 99.9%. Antibody testing can be performed on individuals approximately 1 month after a high-risk sexual exposure. If negative, the test should be repeated again 3 months (window period) after the exposure.

Rapid Tests

Rapid tests are screening tests where results are available in 10-20 minutes. If performed correctly, they detect HIV antibodies with sensitivities similar to currently available EIAs. A negative rapid HIV test result requires no further confirmatory testing. A positive test requires confirmation by both EIA and WB testing.

Four rapid HIV tests have been approved by the US Food and Drug Administration (FDA):

• OraQuick® (and its newer version OraQuick® Advance) Rapid HIV-1/2 Antibody Test (OraSure Technologies, Inc., Bethlehem, PA);
• RevealTM (and its newer version RevealTM G2) Rapid HIV-1 Antibody Test (MedMira, Halifax, Nova Scotia);
• Uni-Gold Recombigen® HIV Test (Trinity BioTech, Bray, Ireland);
• Multispot HIV-1/HIV-2 Rapid Test (Bio-Rad Laboratories, Redmond, WA).

The Standard Diagnostics HIV-1/2 rapid test kit, and Alere™ Determine HIV-1/2 Ag/Ab Combo rapid test kit, are used at Shim Clinic (approved by HSA).

HIV p24 antigen detection

The first marker to appear following infection is free viral p24 antigen. This can be detected using an EIA test. Fourth generation HIV serology tests incorporate testing for both antibodies as well as for the p24 antigen, therefore reducing the window period further.

Polymerase chain reaction (PCR) test

PCR for HIV DNA is available in special circumstances e.g. for infants of mothers with HIV infection to distinguish active infection of the infant from passive transfer of maternal antibodies, and in cases where the WB test is indeterminate in a patient with high-risk behaviour. PCR technology is also employed for quantitative measurement of plasma HIV RNA, this is used to guide and monitor ARV treatment.

HIV TESTING

HIV infection and AIDS are notifiable conditions. HIV testing should be voluntary, persons should be informed orally or in writing that HIV testing will be performed unless they decline. Individuals must not be tested without their knowledge. Confidentiality of the result must be observed, failure to do so may result in prosecution. HIV screening after notifying the patient that an HIV test will be performed (unless the patient declines) is recommended in all health-care settings.

HIV testing is specifically recommended in the following situations:

• for all individuals who seek evaluation and treatment for STIs
• individuals with signs and symptoms suggestive of HIV-related illnesses
• individuals whose behaviour puts them at risk for HIV infection
• individuals who consider themselves at risk or request the test
• pregnant women
• individuals with active TB
• donors of blood, semen, and organs
• health care workers who perform exposure-prone invasive procedures

Post-test Counselling – Negative test

• Reinforce information on safer sex practices to reduce the risk of acquiring HIV
• The significance of “the window period” and the necessity and timing of a repeat test should be discussed with the patient

Post-test Counselling – Positive test

• Providers should expect individuals to be distressed when first informed of a positive HIV test result
• Individuals who test positive for HIV antibody should be counselled concerning the behavioral, psychosocial, and medical implications of HIV infection
• Prevention counseling must be given before leaving the testing site
• A referral letter should be written and an appointment made to see an infectious disease specialist

References

• HIV testing @ Wikipedia