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Fetal Red Blood Cell-to-Total Red Blood Cell Ratio, Blood

Published: August 9, 2026Updated: August 17, 2026

This blood test measures the proportion of red blood cells that are of fetal origin. It is most often used in maternal blood to help assess whether fetal blood has entered the mother's circulation.

Quick summary

["A higher-than-expected fetal RBC ratio may indicate fetomaternal bleeding, but the result must be interpreted with the clinical situation."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
55729-8
Short code
Fetal RBC/RBC Bld FC-Rto
Synonyms
Fetal RBC/RBC Bld FC-Rto
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Ref low
4.2
Ref high
5.9
Critical range
2 N/A – 7 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flow-cytometry
CPT
85041
SNOMED
900000000000003001
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

What the Test Measures

The test compares fetal red blood cells with the total red blood cell count in the sample. A flow-cytometry or other laboratory method may be used to identify and count fetal cells.

Why It May Be Ordered

It may be ordered after bleeding during pregnancy, delivery, trauma, or a procedure involving the placenta. It can also help evaluate unexplained maternal anemia when fetal blood loss is suspected.

How to Prepare

No special preparation is usually required. Follow the laboratory's instructions, and tell the healthcare team if you recently experienced bleeding, trauma, delivery, or a procedure involving the placenta.

Understanding the Result

Results are generally reported as a percentage or ratio together with any calculated estimate of fetal blood loss. The laboratory's reference interval and the specific testing method should be used when interpreting the value.

Reference range

Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A

Specimen: whole-blood

No special preparation required. Stay well hydrated.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Fetal RBC Ratio

Frequently asked questions

Does this test show how much fetal blood is in the mother's body?
It can help estimate the amount of fetal blood in the maternal circulation, especially when combined with the mother's blood count and other clinical findings.
Can this test diagnose fetomaternal hemorrhage?
No single result confirms the diagnosis. An elevated result may support it, while the overall assessment depends on symptoms, bleeding history, hemoglobin level, and other laboratory tests.
Why might the result be unexpected after delivery?
Blood from the fetus can enter the mother's circulation during delivery or through placental injury. Recent delivery, bleeding, trauma, or procedures can therefore increase the fetal RBC ratio without indicating a separate ongoing problem.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 17, 2026. · Read our editorial policy