Question 1

What is the difference between RPI and absolute reticulocyte count?

Accepted Answer

The absolute reticulocyte count (ARC = Retic% × RBC count) corrects only for the haematocrit. RPI adds a second correction for the prolonged circulation time of stress reticulocytes. RPI is more accurate in moderate-to-severe anaemia.

Question 2

How does erythropoietin therapy affect RPI?

Accepted Answer

Exogenous erythropoietin (EPO) mimics the endogenous response, increasing RPI to 2–4 even in the absence of true marrow need. In these patients, the RPI reflects EPO effect rather than marrow compensation. The RPI must be interpreted in the context of EPO dose and iron availability.

Question 3

What is a normal RPI in a non-anaemic person?

Accepted Answer

A non-anaemic person should have an RPI of approximately 1.0 (the marrow replaces 1% of the red cell mass per day). An RPI >1 in a non-anaemic person suggests compensated haemolysis (e.g., early autoimmune haemolytic anaemia, hereditary spherocytosis).

Question 4

Can RPI differentiate between haemolysis and blood loss?

Accepted Answer

Both haemolysis and acute blood loss produce a high RPI. Haemolysis is distinguished by elevated LDH, low haptoglobin, elevated indirect bilirubin, and a positive direct antiglobulin (Coombs) test. Blood loss is usually evident by history.