Medium Fluorescence Reticulocytes (MFR%)

What are Medium Fluorescence Reticulocytes (MFR%)?

Reticulocytes are immature red blood cells freshly released from the bone marrow. Before maturing into fully functional red blood cells, they still contain residual RNA, which can be detected and quantified by fluorescent staining. The intensity of that fluorescence indicates how young a reticulocyte is.

MFR% specifically measures the fraction of reticulocytes at an intermediate stage of maturity. They are younger than the low-fluorescence (LFR%) group but more mature than the high-fluorescence (HFR%) group. Together, LFR%, MFR%, and HFR% make up 100% of all reticulocytes and give clinicians a nuanced picture of bone marrow activity.

Reference Range

2.9 – 12.9 %

Reference ranges can vary slightly between laboratories depending on the analyzer used. Always interpret your result in the context of the range printed on your own laboratory report.

Why Might It Be Elevated?

An elevated MFR% reflects increased bone marrow activity. Common reasons include:

• Recovery after blood loss.
• Start of anemia treatment (iron, vitamin B12, or folate supplementation).
• Hemolytic anemia — compensatory accelerated production.
• Erythropoietin therapy.

Why Might It Be Low?

A low MFR% suggests the bone marrow is not producing red cells at the expected rate:

• Nutritional deficiencies (iron, vitamin B12, or folate).
• Chronic inflammation or infection.
• Kidney disease (reduced erythropoietin).
• Bone marrow disorders.

What to Do Next

A single MFR% result is rarely interpreted in isolation. Your doctor will consider it alongside hemoglobin, total reticulocyte count, MCV, and other parameters. Do not adjust medications or supplements based on this value alone.

Monitoring Over Time

MFR% is a dynamic marker that can shift within days to weeks, making it particularly valuable for tracking treatment response. A rising MFR% in the 1–2 weeks following iron supplementation is an early indicator that the marrow is responding. Serial measurements give a more informative picture than any single data point.

Questions to Ask Your Doctor

1. Given my other blood count results, what does my MFR% level most likely indicate? 2. Are there additional tests (ferritin, vitamin B12, folate) that would help clarify the cause? 3. If I am starting treatment, how soon should I expect MFR% to normalize? 4. How frequently should I repeat this test to monitor progress?