How long until hepatitis a shows up in a blood test?

How long from what?: With an incubation period of averaging 28 days, HAV can be detected by antibody in the blood (igm anti-hav and igg anti-hav) shortly before the onset of symptoms if they occur. The period of viremia has already occurred prior to illness itself and by the time one is sick, generally the amount of virus in the stool is already decreasing.