4. Sample size calculation Our sample size calculation is based on the primary outcome, which is the area under the curve (AUC) of the platelet reactivity curve with P-selectin (CD62P) as platelet activation marker and ADP as platelet agonist. In a cohort of Dutch HIV-infected patients we found a mean AUC of 2500 with an SD of 660. Assuming that a decrease in AUC of 2500 to 2100 upon supplementation of vitamin D is clinically relevant, a sample size of 23 would be sufficient using a paired design with α 0.05 and a power of 80%.