Anaemia Is Often Dilution

01The evidence

• Dilutional anaemia associated with splenomegaly has been characterised as a plasma volume effect rather than a red cell deficit, which is the mechanism a dilution account requires.Annals of Surgery
• Dilution anaemia has also been described following repeated infusions, showing that the concentration can fall while red cell mass is unchanged.American Journal of Cardiology

02The tension

Anaemia is defined by concentration and treated by transfusion to a concentration threshold. The definition is convenient and universal, but it conflates two different states: fewer red cells, and the same cells in more plasma. The two have different physiology and different treatments.

03The theory

Haemoglobin concentration is red cell mass divided by plasma volume. In inflammation, pregnancy, splenomegaly and fluid resuscitation, plasma volume rises, so the concentration falls without any loss of oxygen-carrying capacity.

If dilution is common, then a substantial fraction of patients meeting a transfusion threshold are not actually anaemic in the sense that matters. Transfusing them adds red cells to an already expanded volume, which raises the concentration transiently and may worsen the underlying congestion.

The prediction is that measured red cell mass should be normal in a defined fraction of patients who meet concentration criteria, and that these patients should not benefit from transfusion. That is testable with existing red cell mass methods in a modest cohort.

The clinical consequence is that volume status should be assessed alongside concentration before transfusion, and that a red cell mass estimate, now feasible with cheap tracer methods, could refine the trigger.

04What this is not

05Predictions

• P1A measurable fraction of patients meeting transfusion thresholds should have normal red cell mass.
• P2Those patients should not benefit from transfusion.
• P3Volume assessment should improve prediction of benefit over concentration alone.

06What would refute it

Red cell mass tracking concentration across acute illness would leave the definition adequate.

07What it would change

It would add volume status to transfusion decisions and reduce unnecessary transfusion.