That is also possible. I also read in internet that coinfection of HIV and hepatitis C delay in showing test results.
There is three ways an HIV test can be skewed.
1. A person taking chemo
2. A person taking antirejection drugs for transplants
3. A person that is a chronic IV drug abuser.
There is no other medications that will skew an HIV test.
If you want further information on testing contact your nearest HIV/STD clinic.
But how can it be justified? Suppose his immune system is weak and thereby reducing the chance of producing enough antibodies?
No, there is no way a person with diabetes will give a false negative after 3 months.