HIV Screening (4th Gen) Rfx 1/2 AB

The Roche Elecsys HIV combi PT is an immunoassay for the in vitro qualitative determination of HIV‑1 p24 antigen and antibodies to HIV‑1 (HIV‑1 groups M and O) and HIV‑2 in human serum and plasma. Elecsys HIV combi PT is intended to be used as an aid in the diagnosis of HIV‑1 and/or HIV‑2 infection, including acute or primary HIV‑1 infection. The assay may also be used as an aid in the diagnosis of HIV‑1/HIV‑2 infection in subjects greater than 2 years of age and in pregnant women. The Elecsys HIV combi PT assay is not intended for the screening of blood or plasma donors.

A reactive result using the Elecsys HIV combi PT assay does not distinguish HIV‑1 p24 antigen, HIV‑1 antibody, HIV‑2 antibody, and HIV‑1 group O antibody. The human immunodeficiency virus (HIV), the causative agent of Acquired Immunodeficiency Syndrome (AIDS), belongs to the family of retroviruses. HIV can be transmitted through contaminated blood and blood products, through sexual contact or from a HIV infected mother to her child before, during and after birth. Two types of human deficiency viruses, called HIV‑1 and HIV‑2, have been identified to date. Various subtypes of the known human immunodeficiency viruses have been described, each of which has a different geographical distribution. HIV‑1 can be divided into 3 distantly related groups: group M (for main), group N (for non‑M, non‑O) and group O (for outlier). Based on their genetic relationship, at least 9 different subtypes (A to D, F to H, J, K) have been identified within HIV‑1 group M. Recombinant HIV‑1 viruses consisting of sequences of two or even more different subtypes exist and are spreading epidemically.

Antibodies to HIV proteins, indicating the presence of an HIV infection, can be found in the serum usually 6‑12 weeks after infection. Due to differences in the sequence of immunodominant epitopes, especially in the envelope proteins HIV‑1 group M, HIV‑1 group O and HIV‑2, specific antigens are necessary to avoid failure in the detection of an HIV infection by immunoassays.9,10 By detecting the HIV‑1 p24 antigen in blood specimens of recently infected patients with a high viral load, HIV infection can be detected about 6 days earlier than with traditional antibody assays.11,12 Anti‑HIV antibodies and the HIV‑1 p24 antigen can be detected simultaneously using a 4th generation HIV assay. This leads to improved sensitivity and therefore, a shorter diagnostic window as compared to anti‑HIV assays.

With the Elecsys HIV combi PT assay the HIV‑1 p24 antigen and antibodies to HIV‑1 and HIV‑2 can be detected simultaneously within one determination. The assay uses recombinant antigens derived from the env‑ and pol‑region of HIV‑1 (including group O) and HIV‑2 to determine HIV‑specific antibodies. For the detection of HIV‑1 p24, antigen specific monoclonal antibodies are used. Repeatedly reactive samples must be confirmed according to recommended CDC confirmatory algorithms.

Method Name

Roche Elecsys HIV combi PT (Elecsys 4th Generation)

Important Note

  • All reactive results are reported as “Unconfirmed Reactive” and  should be considered only as preliminary results. Results MUST be confirmed by an alternate testing method.  Specimens are sent for confirmation testing to a reference laboratory.
  • The Elecsys HIV combi PT assay is limited to the detection of p24 antigen and/or antibodies to HIV 1 (HIV 1 groups M and O) and/or HIV 2 in human serum and plasma.
  • The calculated values for anti HIV and/or p24 antigen in a given specimen as determined by assays from different manufacturers can vary due to differences in assay methods and reagent specificity. The results reported by the laboratory to the physician must include the identity of the assay used. Values obtained with different assay methods cannot be used interchangeably. The reported antibody level and/or p24 antigen cannot be correlated to an endpoint titer.
  • Heterophilic and Human Antibodies to Mouse antigens (HAMA) in human specimens can react with reagent antibodies, interfering with in vitro immunoassays.
  • The performance of the Elecsys HIV combi PT assay has not been established with cord blood, neonatal specimens, cadaver specimens, heat inactivated specimens, or body fluids other than serum and plasma such as saliva, urine, and amniotic or pleural fluids.
  • The Elecsys HIV combi PT assay may not detect all infected individuals. A negative test result does not exclude the possibility of exposure to or infection with HIV. HIV antibodies and/or p24 antigen may be undetectable in some stages of the infection and in some clinical conditions.


HIV, HIV 4th Gen, P24,, HIV Screening (4th Gen) Rfx 1/2 AB, HIV Screening 4th Gen, Exposure Protocol

Specimen Type


Specimen Required

One serum separator tube (SST).  Please allow 20 minutes after collection for SST specimen to clot before centrifugation.  All specimens must be centrifuged within four hours of collection.

Specimen Minimum Volume

4.0 mL of blood

Specimen Stability Information

7 days at room temperature, 4 weeks refrigerated, 3 months frozen (can thaw and refreeze up to 5 times)

Special Instructions

All repeatedly unconfirmed reactive results will be confirmed by HIV 1/2 antibody differentiation at a reference laboratory.  Conflicting results will reflex to HIV-1 Viral RNA qualitative testing.

Samples should not be taken from patients receiving therapy with high biotin doses (i.e. > 5 mg/day) until at least 8 hours following the last biotin administration.

Performing Laboratory

Glens Falls Hospital Chemistry and Special Chemistry Department

Reference Values


Day(s) and Time(s) Performed

As ordered.  Turn around time is 4 hours.

Specimen Retention Time

72 hours

Reflex Tests

HIV 1/2 Antibody determination. Turn around time 3-5 days.  If this assay is found to be indeterminant, the specimen will be tested for HIV-1 Qualitative NAT.