The traditional indicators of engraftment following PBSC transplantation (PBSCT) are the rising total WBC count and ANC. Reticulocytes may be an earlier indicator, since as reticulocytes mature, there is a gradual loss of cellular RNA, which can be measured using methylene blue and light scatter with an Abbott CD 3500 automated counter (Abbott Laboratories, Maidenhead, U.K.). Reticulocytes can be divided into three fluorescence ratios depending on the amount of light scatter generated, high, medium, and low. The most immature are the high fluorescence reticulocytes (HFR). Standard engraftment parameters together with HFR were measured in a homogeneous group of 25 patients with lymphoma after PBSCT using a standard conditioning protocol. An ANC of 0.5 x 109/L was achieved after a median of 10 days (mean 11.2 days, range 9-22). The recovery of the HFR to 2% of the total reticulocytes was significantly shorter, with a median of 8 days (mean 7.5 days, range 6-10) (p < 0.0001). The values of HFR to 2% preceded the ANC of 0.5 x 109/L in 24 of the 25 patients by a median of 3 days (mean 3.8 days, range 2-12 days). On this basis, it can be determined that in 96% of cases, engraftment was indicated earlier by HFR measurement. The HFR to 2% even preceded the ANC of 0.1 x 109/L in 23 of the 25 patients, showing that engraftment was indicated earlier in 92% of patients. Immature reticulocytes appearing in peripheral blood can be reliably measured by automated cytometers, and HFR can, therefore, be used as an earlier indicator of engraftment following PBSCT. This information provides the opportunity for earlier cessation of antibiotics and growth factors and could lead to earlier discharge from hospital, with cost savings.
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