If clinical judgement indicates a false negative result, the recommendation is to repeat testing on a fresh specimen in one to three months. False negative results may also be obtained in HIV-infected individuals who are receiving medication for treatment for HIV infection (ART – antiretroviral therapy) or individuals taking medication for prevention of HIV infection (PrEP – pre-exposure prophylaxis).
False positive results on fourth-generation assays are less likely to occur but may be obtained in situations where specimen integrity is questionable, or if a person has participated in a HIV vaccine study and has developed antibodies to the vaccine. In these situations, it may be hard to tell if a person is infected with HIV or not, so a comprehensive risk history and clinical judgement should be considered before coming to any conclusions.
For submissions to MTPHL for HIV testing, please submit 3 mL of serum and/or K2 EDTA plasma. A fourth-generation HIV assay will be performed first, and reflex confirmatory testing will be performed on all repeat reactive screens following the CDC HIV testing algorithm.
Confirmatory testing incudes performing the Geenius HIV 1/2 supplemental assay, which confirms the presence of HIV antibody and differentiates between HIV-1 and HIV-2. If results are discrepant between the screen and differentiation assay, HIV Nucleic-acid Amplification Testing (NAT) will be performed. The HIV NAT, also known as HIV RNA quantitative testing, confirms the presence of HIV p24 antigen, which indicates acute infection. HIV NAT can also be requested and performed for any newly detected cases. The specimen requirement for HIV NAT/HIV RNA quantitative testing is 3 mL K2 EDTA plasma.
Prior to submission, whole blood specimens should be centrifuged at 3000 g for 10 minutes. The serum and/or plasma should then be removed from the red cells, clot or gel separator and placed in a sterile secondary container prior to shipping.
Specimens may be kept at 2-8 degrees Celsius for up to seven days after sample collection for the screening and differentiation assays, but are only stable at 2-8 degrees Celsius for 5 days for the HIV RNA quantitative assay.