Why is a second blood test necessary three months after a person thinks he or she has been exposed to hiv? it can take three months for the blood test to be read and a diagnosis made. it can take three months for antibodies to the virus to show up in an infected person’s blood. it can take three months for the virus to grow in the lab so a diagnosis can be made. it can take three months for immunity to the virus to show up in an infected person’s blood.