I posted a poll asking what the absolute nucleated red blood cell (nRBC) was based on the CBC below. See graphic for the correct answer and explanation.

Overview
- Nucleated red blood cells (nRBCs) (aka normoblasts, erythroblasts) are immature erythrocytes that are normally present in the peripheral blood of neonates but usually disappear in the first few days of life.12
- College of American Pathologists: “The term nucleated red blood cells is used to state the presence of normoblasts in the peripheral blood and includes all normoblasts regardless of the stage of maturation.”
- The presence of circulating nRBCs in healthy children and adults is considered an abnormal finding and possible harbinger of underlying disease.3
- When seen in peripheral blood, nRBCs should be enumerated and reported as number of nRBCs per 100 white cells or as an absolute count.
- Reports in hospitalized patients show that nRBCs are an indicator of poor prognosis and increased mortality, especially in critical care settings.4
History
- nRBCs were first described in peripheral blood by Paul Ehrlich in 1880, shortly after his introduction of staining methods in hematology. He wrote a paper, “Findings in Anemia-De- and Regeneration of Blood Cells,” in which he differentiated between the “normoblasts” present in the anemia of blood loss and the “megaloblasts” present in pernicious anemia.5
- The term erythroblastemia was used in the past to describe the presence of nucleated red cells in circulating blood.
- The first clinical application of nRBCs counts was published in 1969.6

Mechanisms:
- Erythrocyte differentiation involves a multistep pathway that begins with the hematopoietic stem cell and ends with the mature RBC.
- Adult mammalian erythrocytes are anucleate (they lack a nucleus); this has several advantages:
- The cell can assume a biconcave shape, which increases its surface-to-volume ratio:
- Allowing passage through narrow capillaries (increased deformability).
- Optimizing oxygen diffusion.
- Anucleate red cells lack mitochondria, so they do not consume the oxygen they are carrying to other tissues.
- These cells can pack in more hemoglobin (since they do not have space occupying nucleus), thus facilitating oxygen delivery.
- The cell can assume a biconcave shape, which increases its surface-to-volume ratio:
- Enucleation, the process by which the nucleus is removed from red blood cells, is the penultimate step in mammalian erythroid terminal differentiation (the final step involves differentiation of reticulocytes into mature RBCs).7
- Once blood cells are ready for release into the circulation they pass through fenestrae (holes) in the sinusoidal endothelium of the bone marrow. Normal mature bone marrow cells are deformable, so they can squeeze through small “portholes” in the endothelium to enter the peripheral circulation. Normoblasts and immature granulocytes, however, and enter the circulation in small numbers. The few normoblasts that escape into the circulation are removed by the spleen. Their presence in the peripheral blood indicates one of several possibilities:8
- The bone marrow barrier has been disrupted. The resulting release of excessive normoblasts can overwhelm the ability of a normal spleen to clear them from circulation.
- Hypoxic erythropoietin-induced compensatory erythropoiesis, as seen for example in hemolytic anemia, acute bleeding and severe hypoxic stress, results in intense marrow erythropoietic activity.
- Ineffective erythropoiesis may result in premature release of nRBCs into the peripheral blood.
- Extramedullary hematopoiesis has been activated, which lacks the normal barriers for release of nRBCs into the circulation.
- Hyposplenism or asplenia.9


Classification
- According to maturation stage:
- Most are at the polychromatophilic or orthochromatophilic stage
- According to pathological changes:
- Normal
- Megaloblastic:
- Nuclear cytoplasmic asynchrony
- Seen in megaloblastic anemia
- Dysplastic:
- Nuclear budding
- Nuclear fragmentation




Measurement
- Manual morphometry:
- The traditional method for identification and enumeration of nRBCs is by morphometric evaluation via light microscopy (morphological eye count).
- Methods:
- Nucleated RBCs are counted as part of the overall white blood cell (WBC) count.
- 100 nucleated cells are counted on a Wright-stained peripheral blood smear by highly trained laboratory staff.
- The number of nRBCs is reported as a percentage of the nucleated cells.
- Intepretation:
- Any number of nRBCs detected by this method is considered abnormal.
- Many laboratories still rely on reference intervals (RIs) established from manual methods.
- Disadvantages of manual counting:10
- Time consuming
- Subjective
- Requires a highly trained observer.
- Using this technique, it is difficult to detect nRBC concentration of less than 100 per μL or 1 nRBC per 100 WBCs.11
- Imprecision because only 100-200 WBCs are counted (significant propensity for interobserver and intraobserver variability).
- Because nRBCs have nuclei that are similar in size and shape to lymphocytes, they are erroneously recognized and counted as WBCs, resulting in an inaccurate high WBC count and automated differential (lymphocyte) count. It is necessary to correct the leukocyte count for the nRBC count.12
- Automated counters:
- The first generations of automated hematology analyzers did not have the ability to signal the presence of NRBCs.
- Further advances in technology resulted in the ability of generating a “suspect flag”.
- More recently, automated hematology analyzers are able to count nRBCs quickly (providing an absolute count) and with a high level of accuracy and precision.
- Advantages of automated counting:
- High throughput.
- Fast turnaround time.
- Reliable.
- Excellent correlation with manual method (see graphic below).
- > 20,000 cells are counted (vs. 100 for manual count).
- Total leukocyte and lymphocytes counts are corrected for number of nRBCs.

- Differential diagnosis:
- Physiologically seen at birth and disappear in 3-5 days.
- Outside of the neonatal period, the presence of nRBCs in the peripheral blood indicates a pathological process.
- nRBCs are seen in both hematological and non-hematological conditions.
- Their presence in the peripheral blood usually indicates increased erythroid activity, such as severe hypoxic states and hemolytic acute episodes, or damage to the bone marrow microenvironment from hematologic neoplasm, myelofibrosis, or metastatic malignant neoplasms. As mentioned above, hypo/asplenia can also lead to nRBCs.
- Recently, it has been observed that the duration and persistent presence of nRBCs in peripheral blood is linked to disease severity and more serious prognosis.
- Causes of increased release of nRBCs into peripheral blood:
- Compensatory erythropoiesis:
- Hemolytic anemia
- Hemorrhage
- Severe hypoxia
- Erythropoietin-induced erythropoiesis
- Bone marrow infiltration:
- Leukemia
- Lymphoma
- Myelofibrosis
- Multiple myeloma
- Myelodysplasia
- Solid tumor metastases
- Pernicious anemia
- Extramedullary hematopoiesis:
- Myelofibrosis
- Congenital hemolytic anemia
- Polycythemia vera
- Hypo/asplenia
- Other:
- Sepsis
- Liver disease
- Compensatory erythropoiesis:
Prognosis
- Elevated levels of nRBCs have been shown to predict mortality in: