Even diabetic comes under immunosupressive catagory !!! What do you mean by old elisa test, You mean 3rd Generation test ?
No 168 days can be conclusive..... because old elisa test can't detect antibobies in 3 months..... so the test at 6 months can be conclusive.... if you are immuno compramised (i.e diabetic, t.b. vitamin deficiency. advanced cancer) take test at 6 months
this is stated in almost every post on this forum.