Can reduced plasma volume cause relative erythrocytosis?

Reduced plasma volume can produce an apparent elevation in haemoglobin and haematocrit without increased erythrocyte production. This is termed relative erythrocytosis (or hemoconcentration), in contrast to absolute erythrocytosis, in which total red-cell mass is genuinely increased.

Clinical and diagnostic evidence

• Relative erythrocytosis is defined by normal total red-cell mass with reduced plasma volume. When the fluid compartment contracts, an unchanged quantity of circulating red cells occupies less volume, increasing measured haemoglobin and haematocrit concentrations.
• British Society for Haematology guidance distinguishes this from absolute erythrocytosis, which requires red-cell mass >25% above the predicted mean.
• Thus, a raised haemoglobin or haematocrit alone does not establish increased red-cell production or a true increase in total red-cell mass.

Mechanism and clinical context

• The mechanism is purely concentration-based: reduced plasma volume increases the red-cell fraction of blood (haematocrit) and haemoglobin concentration while erythrocyte mass remains normal.
• Dehydration, fluid losses, and diuretic-associated volume contraction are common clinical settings in which this pattern may occur.
• For a 77-year-old man, medication review and assessment of hydration or recent fluid loss are particularly relevant when an elevated haemoglobin/haematocrit is newly detected.

Practical implications

• A reasonable initial step when volume depletion is plausible is to correct it and repeat the complete blood count. Persistent elevation should lead to assessment for primary or secondary absolute erythrocytosis.
• Direct red-cell-mass measurement can distinguish hemoconcentration from true erythrocytosis, but is not widely available; clinicians often infer relative erythrocytosis from volume status and the clinical context.

Bottom line

• The claim is well supported: reduced plasma volume can cause relative erythrocytosis by concentrating red cells, raising haemoglobin and haematocrit despite a normal total red-cell mass.