High light scatter reticulocytes (HLR%) are the fraction of the youngest, most immature red blood cells among the reticulocytes. Such cells contain the most RNA and scatter light most strongly in the analyzer, so HLR% reflects the "freshest" production of red blood.
HLR% is part of the immature reticulocyte fraction (IRF) and shows how actively the bone marrow is producing red blood cells — earlier than the hemoglobin level changes.
Reference ranges
The reference range depends on the analyzer; approximate values for adults:
| HLR% | ≈ 0.2 – 1.8% of red blood cells |
| Reticulocytes (context) | 0.5 – 2.5% |
| Meaning | the higher, the more active erythropoiesis |
What an elevated level means
A rise in HLR% means the bone marrow is vigorously releasing young red blood cells. This happens with blood loss, hemolysis and — importantly — as an early response to treatment with iron, vitamin B12 or folate. The rise in HLR% precedes the rise in hemoglobin by days.
What a low level means
A low HLR% (and low reticulocytes overall) points to weak erythropoiesis: the bone marrow is not responding to anemia. This happens with iron deficiency, B12 and folate deficiency, aplastic anemia, kidney disease and after chemotherapy.
How the test is done
HLR% is determined automatically in an extended blood count with a reticulocyte count (flow cytometry with RNA staining) — no separate preparation is needed. The parameter is interpreted together with the absolute reticulocyte count, hemoglobin and RBC.
What affects the value
- The start of treatment with iron, B12 or folate quickly raises HLR% — this is the expected response.
- Blood transfusion and erythropoietin change the value.
- The reference limits and labels (HLR, HFR, IRF) differ between analyzers.
Calculate using this marker
Calculators where High light scatter reticulocyte percentage (HLR%) is used directly:
Frequently asked questions
How does HLR% differ from the usual reticulocyte count?
The usual reticulocyte measure counts all immature red cells, while HLR% singles out the youngest of them — with the highest RNA content. So HLR% responds earlier and more sensitively to changes in bone-marrow work.
Why look at HLR% in anemia?
It shows whether the bone marrow is responding to the anemia. A high HLR% against a background of anemia is a sign of active recovery (blood loss, hemolysis or the start of treatment). A low HLR% means red-cell production is suppressed and one must look for a deficiency or marrow damage.