A person has unprotected sex 3 weeks back, To rule out HIV infection the best test is
Laboratory diagnosis of HIV infection
Laboratory diagnosis is made by:
A. Antibody detection

It is the most common method used for diagnosis of HIV infection.
ELISA is the most sensitive test and therefore is the screening test of choice.
Western blot is the most specific (Confirmatory) test and therefore is the’gold standard’ for diagnosis.
Antibodies test usually nucleic acid or antigen.

B. Detection of HIV nucleic acid or antigen

P24 antigen detection by ELISA can be done 16 days after infection (earlier than antibody detection).
Nucleic acid detection tests are the earliest to become positive (12 days after infection), Most commonly used test is reverse transcriptase PCR, (RT-PCR), other methods used are DNA-PCR, RNA-PCR, RNA-bDNA (branched DNA), and RNA-NASBA (nuclei acid sequenced based assay).

C. Virus isolation

Co-cultivation of the patient’s lymphocyte with uninfected lymphocytes in the presence of IL-2.
Not suitable as a routine diagnostic procedure.

In infected individual HIV can be isolated from

Blood
Semen
Saliva
Urine
Lymphocytes
Cervical secretions
Tears
Breast milk
P 24 antigen assay
ELISA
Western blot
Lymphnode biopsy