What is the Threshold Hct for Defining Polycythemia?

By William Aird

Most of us have memorized the WHO thresholds for anemia:1

  • Men Hb < 13 g/dL
  • Women Hb < 12 g/dL
  • Pregnant women Hb < 11 g/dL

What about thresholds for polycythemia?2

1. The first point to make is that when talking about polycythemia, we should refer to the Hct rather than the Hb since the effect of the Hct on blood viscosity is limiting on oxygen delivery.

2. Reported normal ranges for Hct are highly variable. They are of course lower in women than men. Tpically:

  • Women 36-47%
  • Men 38-54%

3. We can consider elevated Hct from the perspective of polycythemia vera (P vera) diagnostic criteria, but these values are a moving target:

  • WHO 2008:
    • Women >49.5%
    • Men >55.5%
  • WHO 2016 (lower thresholds to pick up masked polycythemia):
    • Women >48%
    • Men >49%

4. Other considerations for Hct thresholds: 5a. UpToDate uses the WHO 2016 criteria to define erythrocytosis whether from P vera or another cause (women >48%; men >49%) – this is a pretty low bar! Are we to work up all such patients?

  • Hct > 60% in men or > 56% in women always reflect increased RCM that is > 25% above normal predicted red cell mass.4
  • In men taking testosterone, erythrocytosis is defined as Hct > 54%, which is an indication to stop treatment (or phlebotomize).

CONCLUSION: an elevated Hct is in the eye of the beholder and context dependent!