Clinical Information

Human immunodeficiency virus (HIV) infection is caused by 2 known types: HIV-1, the most common worldwide, and HIV-2, which is endemic in West Africa but may occur elsewhere through travel or sexual contact. Both viruses share similar morphology, genomic structure, and clinical consequences, including the ability to cause acquired immunodeficiency syndrome (AIDS). Transmission occurs through sexual exposure, blood or blood products, or perinatal transmission from an infected mother to her infant. Seroconversion typically occurs 6–12 weeks after exposure, and HIV-1/2 antibodies are almost always detectable by 12 months. In late-stage AIDS, antibody levels may decline (seroreversion), but HIV p24 antigen is often detectable and will yield reactive results with combination antigen–antibody assays.  

Screening and Diagnostic Approach

Current CDC-recommended algorithms begin with an HIV-1/2 antigen–antibody combination assay, performed by FDA-approved rapid tests, enzyme immunoassays, or chemiluminescent immunoassays. Supplemental/confirmatory testing is performed only for repeatedly reactive specimens.
The Atellica® IM HIV Ag/Ab Combo (CHIV) assay is intended for the simultaneous qualitative detection of HIV-1 p24 antigen and antibodies to HIV-1 (including group O) and HIV-2 in serum. Results are reported as index values and interpreted as reactive or nonreactive.
• This assay is not validated for blood donor screening.
• A reactive result does not distinguish between HIV-1 p24 antigen, HIV-1 antibody, HIV-2 antibody, or group O antibody.  

Interpreting Negative and Reactive Results

Nonreactive HIV antigen/antibody results generally indicate the absence of HIV-1 or HIV-2 infection. However, acute HIV infection cannot be excluded. If acute infection is suspected, HIV-1 RNA testing is recommended.
Reactive screening results indicate possible HIV-1 or HIV-2 infection but are not diagnostic. All reactive results must be interpreted using a supplemental HIV-1/2 antibody differentiation test.
Confirmed positive results (serologic or molecular) are reportable to the Ohio Department of Health as required by state law.

Specimen Limitations

The performance of the Atellica IM CHIV assay has not been established for: cord blood or neonatal specimens, cadaveric samples, heat-inactivated specimens, body fluids other than serum/plasma (e.g., saliva, urine, amniotic or pleural fluid), pooled blood or plasma.
Assay Limitations and Interference
No current HIV assay detects all infected individuals. Antigen and antibodies may be undetectable in early infection or advanced disease. Individuals receiving HIV vaccines may develop reactive antibody results without true infection. Heterophilic antibodies and HAMA may cause assay interference and anomalous results, particularly in patients with frequent exposure to animal proteins.

Testing Algorithm

1.    HIV-1/2 Antigen/Antibody Screen — Initial Test
o    If nonreactive → HIV-1 RNA testing is recommended only if acute infection is clinically suspected.
o    If reactive → reflex to HIV-1/2 antibody differentiation test (additional charge).
2.    HIV-1/2 Antibody Differentiation Test
o    If negative for both HIV-1 and HIV-2 antibodies, or indeterminate, HIV-1 RNA detection is performed automatically (additional charge).
o    RNA testing is indicated for:
• Negative for both HIV-1 Ab and HIV-2 Ab
• Indeterminate HIV-1 Ab with negative HIV-2 Ab
• Negative HIV-1 Ab with indeterminate HIV-2 Ab
• Indeterminate for both HIV-1 Ab and HIV-2 Ab
3.    When HIV-1 RNA Testing Is Recommended (Provider-Ordered)
o    Positive for both HIV-1 and HIV-2 antibodies
o    First-time HIV-1 positive
o    First-time HIV-2 positive