All of the following methods are used for the diagnosis of HIV infection in a 2 month old child, except –
Diagnosis of HIV in Infant

In adults HIV can be easily diagnosed by detecting IgG antibody to HIV (Elisa and western blot test). But this method is not helpful in case of neonates because all neonates born to HIV infected mothers will have IgG antibody in their blood, regardless of infection status in them. These antibodies are passively transferred to the newborns from their mothers (IgG can cross placenta). These infants continue to test positive for IgG antibodies for upto 18 months. At the end of 18 month they will lose these antibodies so uptil 18 month of age this method of diagnosis cannot be used.
The presence of IgA or IgM anti HIV in the infant's blood can indicate HIV infection, because these classes of antibodies do not cross the placenta. However detectable quantities of IgA antibodies are detected only after 3 months of life and detection of IgM antibodies is very unreliable (both insensitive and nonspecific).
So direct viral detection assays are used for the diagnoses of HIV in newborn. These are


detection of HIV DNA or RNA by PCR.          
HIV culture
HIV p24 antigen
Immune complex dissociated p24 antigen        

These are very useful in young infants allowing a definitive diagnosis in most infected infants by 1— 6 months of age. Out of these method detection of viral DNA by PCR is the preferred method in developed countries.
DNA–PCR
Viral culture
HIV ELISA
p24 antigen assay