I've been poz for ten years. Not on ARVs for the past 6.5 years. My platelets have dropped from 211,000 to 81,000 in the past three month. We can't determine a medicine to be causing it, and my doctor doesn't believe the sudden drop is HIV related. What might it be?

Meds and then HIV would be at the top of the list. Although this is a low platelet count, it is just below the normal range. Is the lower result consistent on repeat? Are other blood cells affected as well? You don't indicate what your CD4 count or viral load are and whether other medical problems could be affecting the count. I consistently low or further decreasing, then other blood tests or a bone marrow biopsy may be needed to rule out other infections or immune system problems.