Jean-Jacques Kiladjian, MD, PhD, Saint-Louis Hospital & Paris Diderot University, Paris, France, discusses the relevance of red cell mass measurement in the diagnosis of polycythemia vera (PV). This test quantifies the number of circulating red cells and allows to confirm the presence of polycythemia. Although its use has significantly decreased as today, patients with PV are diagnosed with a JAK2 mutation test, this test may still have a role in patients who are JAK2-negative or in patients with splanchnic vein thrombosis (SVT). This interview took place virtually.