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Nucleated Red Blood Cells

The presence of circulating NRBCs, outside of the neonatal period or occasionally during pregnancy, generally indicates either increased red blood cell production or bone marrow infiltration by malignant cells, fibrosis, granulomas, etc. When bone marrow infiltration occurs, granulocytic precursors are usually concurrently seen in circulation. The most common circulating NRBC is at the orthochromic stage of differentiation, although the term NRBC is used for all normoblasts regardless of the stage of maturation. It is critical to correctly identify these cells as they may not be recognized by automated hematology counters and will artificially raise the WBC count. This impacts patients receiving chemotherapy whose treatments are based on the absolute neutrophil count; they may be treated inappropriately if the circulating NRBCs erroneously elevate the WBC count. The newer hematology instruments automatically correct the WBC count when NRBCs are detected. However, even the most sophisticated instruments will, at times, misidentify these cells necessitating a manual correction of the WBC count. Therefore, review of the peripheral blood smear for confirmation of the number of circulating NRBCs is important when NRBCs are flagged as being detected. The formula for correcting WBC counts when NRBCs are present is as follows:

uncorrected WBC* x 100 = corrected WBC count/100 + # NRBCs

* If the instrument has already attempted to correct the WBC, it is necessary to retrieve the uncorrected WBC count from the instrument for the calculation.

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