Low Fluorescence Reticulocytes (LFR%) – mature reticulocytes in blood

What is LFR%

Low Fluorescence Reticulocytes (LFR%) is an indicator of the most mature fraction of reticulocytes. This value shows what proportion of all reticulocytes have the highest degree of maturity, containing the least amount of RNA.

Modern haematology analysers divide reticulocytes into three fractions based on fluorescence level, which reflects RNA content: LFR (low fluorescence), MFR (medium), and HFR (high). The total always equals 100%: LFR% + MFR% + HFR% = 100%. The immature reticulocyte fraction (IRF%) = MFR% + HFR%.

LFR% reticulocytes are the most mature and closely resemble fully mature erythrocytes — they have already lost most of their RNA. The more young reticulocytes circulate (e.g. during active production), the lower the LFR%.

Reference ranges

The normal range in adults is typically 83–97%. This means the vast majority of reticulocytes at any given time are already fairly mature cells. A higher LFR% indicates that the bone marrow is producing mostly mature reticulocytes, while a lower value indicates that more young cells are being released.

Why LFR% may be elevated

An elevated LFR% (close to or above 97%) may indicate:

• Reduced bone marrow production — when fewer new reticulocytes are released, only the already mature fractions remain in circulation.
• Stable state after recovery — as anaemia is treated and haemoglobin normalises, IRF% decreases and LFR% returns to normal.
• Aplastic anaemia — the total number of reticulocytes is markedly reduced, but those remaining are most often the mature ones (LFR%).

Why LFR% may be decreased

A decreased LFR% (low, e.g. 75–82%) means that younger fractions are increasing in the reticulocyte population:

• Recovery after anaemia treatment — iron, B12, or folic acid therapy triggers a 'reticulocyte crisis': the bone marrow releases the youngest cells en masse, IRF% rises, and LFR% temporarily decreases.
• Active bleeding or haemolysis — the body rapidly compensates by releasing more young cells.
• After bone marrow transplantation — the new system begins by producing the youngest reticulocytes first.
• EPO therapy — the effect of erythropoietin is seen as a rise in IRF% and a corresponding decrease in LFR%.

What to do next

LFR% is best interpreted together with IRF%, MFR%, HFR%, RET#, and RET%. A single LFR% change is usually just one small fragment of a broader picture.

If LFR% is very low, this indicates active release of the youngest reticulocytes — a good sign during treatment or recovery. If LFR% is very high alongside a low RET#, this may indicate reduced production that warrants further investigation.

Monitoring over time

Tracking LFR% changes is valuable for treatment monitoring:

• After starting iron therapy, LFR% temporarily decreases due to the reticulocyte crisis — this is a normal and positive phenomenon.
• When haemoglobin recovers and treatment is complete, LFR% normalises — typically returning above 85–90%.
• Long-term monitoring of LFR% decline helps assess whether the bone marrow continues to actively produce new erythrocytes or whether production has already stabilised.

Questions for your doctor

• What do my LFR% results mean together with IRF% and RET#?
• Do the changes in my LFR% reflect the course of anaemia treatment?
• Are additional tests needed to understand the distribution of reticulocyte fractions?
• When should LFR% normalise after starting treatment?