Help Please log in to determine if you are eligible to purchase PT Programs. SOLUBLE TRANSFERRIN RECEPTOR - STFR Analytes/procedures in bold type are regulated for proficiency testing by the Centers for Medicare & Medicaid Services (CMS). Analyte Challenges per Shipment Number of Shipments Soluble transferrin receptor (sTfR) 3 Two shipments per year Program Information Three 2.5‑mL liquid human serum specimens Shipping Schedule Shipment A: April 20 Shipment B: August 24 Guest Log in/Register