CALCA and bacterial infectious disease: Biologic markers such as procalcitonin (PCT) and C-reactive protein (CRP) have been used to distinguish the bacterial infections from non-bacterial infections or disease flares in some autoimmune diseases, such as systemic lupus erythematosus, Behcet's disease, and Henoch-Schönlein purpura, while the results were controversial.5, 6, 7, 8, 9 PCT is a protein with 116 amino acids and a molecular weight of 13 kDa.