A patient comes to the hospital with a history of a sore throat, diarrhoea and sexual contact 2 weeks before. The best investigation to rule out HIV is -
The patient is giving symptoms of the acute viral syndrome.
The clinically acute viral syndrome is associated with nonspecific symptoms including a sore throat, myalgia, fever, rash, weight loss and fatigue
Patient with primary HIV infection develops the acute viral syndrome 3-6 weeks after infection, which resolves spontaneously in 2-4 weeks.
During this phase, the immune response to HIV infection has not yet developed. There are no HIV specific antibody ie., no seroconversion.
Since these phases are characterized by high level of viremia, the infection in these stages can be diagnosed by direct detection of virus or virus particles.
A variety of laboratory tests are available for the direct detection of HIV or its components.

p24 Ag Assay

The simplest of the direct detection test is p24 antigen capture assay.
During the first few weeks of infection before an immune response develops, there is a brisk rise in p 24 antigen. After the development of the p24 antibody, these levels decline. Thus p24 Ag assay has its greatest use as a screening test for HIV infection in patients suspected of having the acute HIV syndrome as high levels of p24 Ag are present prior to the development of antibody.

Detection of HIV RNA

Measurement of HIV RNA in the plasma of patients with HIV infection is also useful in conditions where measurement of anti-HIV antibodies is not possible e.g. in acute infections and in neonatal infections.

Currently, three assays are available for the diagnosis of HIV RNA.

HIV RNA by PCR
HIV RNA by bDNA
HIV RNA by NASBA

Note: In neonates, there is the passive transmission of antibodies from the mother. Thus the antibodies cannot be used for the diagnosis in this condition as it would be impossible to tell whether the antibodies detected are acquired from the mother or they have developed as a result of infection.
With the use of direct detection techniques, there is a decrease in the interval between infection and detection of (window period).
Window period for

Antibody testing → 22 days approximately
p24 Ag Assay → 16 days
HIV RNA → 12 days.
P24 antigen assay
ELISA
Western blot
Lymph node biopsy