A 59-year-old male came with Hb 18.0 gm/dl on three occasions. The resident doctor wants to exclude Polycythemia Vera. Which of the following is the most relevant investigation :

Answer is C (Red cell mass): When confronted with an elevated Haemoglobin or Haematocrit the first step should be to determine whether this increase is due to absolute erythrocytosis or relative erythrocytosis. Absolute erythrocytosis Relative/ spurious erythrocytosis Apparent increase in red cell mass due to reduction in plasma volume True increase in red cell mass `Red cell mass' is the investigation of choice to determine such distinction. Once 'red cell mass' determination confirms absolute erythrocytosis, a diagnosis of polycythemia vera can be established by demonstrating absence of elevation in serum erythropoetin levels.