Mobilized peripheral blood stem cells (PBSCs) collection is used for 99% of autologous transplantations and 71% of procedures in an allogeneic setting.1 An efficient mobilization program is crucial for an adequate yield of CD34+ cells in the collection product and maximizes chances for an appropriate engraftment in the recipient. Depending on the policy of individual transplant centers, the type of transplant, and patient diagnosis, a minimum of 3–8×106 CD34+ cells/kg is necessary to perform the PBSC transplantation.2

Formulas for predicting CD34+ cell yield by leukapheresis have been proposed in the past few years. The predicted number of CD34+ cells can be calculated by the mathematical equation: Total number of CD34+ (cells/kg) yield=[(peripheral CD34+ cells/L)×(collection efficiency coefficient%)/recipient body weight (kg)]×total liters processed.3 The collection efficiency coefficient (CEC) varies from 30% to 50% depending on the automatic leukapheresis processor used, the skill of the collection staff and the venous access of the donor. In the original formula described above, the authors applied a CEC of 30% to guarantee the minimum number of CD34+ cells/kg that might be collected in a single day. The most commonly used system in Latin America is the COBE Spectra Apheresis System (Terumo BCT, Tokyo, Japan), which requires intermittent optical/manual input from the operator, making it labor-intensive and prone to user-dependent variability. The quality of cells collected with apheresis systems such as COBE has been reported to depend on the operator and a stable blood flow.4 The electronics-assisted Terumo Optia system has been largely used in Europe and the United States. Other systems, such as Amicus Fenwall and Com Tech Fenwall, are commonly used to collect PBSC, but are less popular.

The main predictor of a successful collection is the peripheral blood CD34+ cells count on the day of apheresis. Factors associated with a successful CD34+ cell mobilization in healthy donors have been reported. Bertani et al.5 in a retrospective multicenter study of 360 healthy donors identified a number of variables that correlated with PBSC mobilization yield. Male gender, younger donor age, elevated baseline white blood cell (WBC) count, higher doses of granulocyte-colony-stimulating factor (G-CSF) administration and the use of lenograstim instead of filgrastim were all associated with a good mobilization. Teipel et al.6 published the largest evaluation of predictors of good mobilization in healthy donors. In 7,216 unrelated allogeneic PBSC donors female sex, older age, smoking, elevation of lactate dehydrogenase, a higher relative lymphocyte count and an elevated large unstained cell count at baseline were negatively correlated with the CD34+ cells count on day +5 of mobilization. Conversely, a higher platelet, absolute lymphocyte and relative monocyte count, along with a higher body mass index at baseline was positively correlated with the CD34+ cells count on day +5. Billen et al.7 showed that female donors and donors who are of lower weight than their recipients were less likely to meet the CD34+ cell dose required for PBSCs transplantation. Ferritin levels,8 hematocrit (Htc) levels,9 race and ethnicity,10 and baseline peripheral blood CD34+ cells count11 have also been associated with mobilization outcome and collection of PBSC.

Studies evaluating predictors of successful PBSC collection in healthy donors are still scarce. Identification of relevant predictors and the ability to adequately estimate the CD34+ cell yield would improve collection strategies. The aim of the present study is to validate a formula to predict CD34+ cell yield and to describe predictors of successful collection among healthy donors at our institution.

We evaluated 338 collections from 307 allogeneic PBSC donors. In 279 (90.9%) donors we performed only one leukapheresis, in 26 (8.5%) donors two leukapheresis, in one donor (0.3%) three collections and in one (0.3%) four collections. Among them, 218 (71%) were men and the median inter-quartile range (IQR) values for age was 37 years-old (28; 46), for body weight was 73kg (64; 84), and for the total blood volume was 4,663mL (4103; 5204). Donors were mobilized with G-CSF with a median (IQR) dose of 10μg/kg/day (9.37; 11.69). The median (IQR) CBC values of the donors on the day of collection were: Hb level of 14.3g/dL (13.3; 15.2), WBC count of 50.04×109/L (38.11; 61.36), PLT count of 202×109/L (169; 246); the median (IQR) circulating CD34+ cells were 54.9/μL (32.5; 87). After collection, there were decreases in the median Hb level (11.2%), WBC count (28%) and PLT count (52%). Recipients’ median (IQR) for age and body weight were 37 years-old (27; 48) and 67kg (59; 76.5), respectively. Indications for PBSC transplantations were acute myelogenous leukemia (35.5%), acute lymphoblastic leukemia (17.6%), non-Hodgkin lymphoma (16.6%), chronic myelogenous leukemia (8.8%), myelodysplastic syndrome (8.4%), Hodgkin lymphoma (3.6%), myelofibrosis (3.6%) and others (12.3%).

Overall, 171 (55.7%) donors presented with at least 50 circulating CD34+ cells/μL on the first day of collection. Among the donors, median (IQR) values on the first collection day were 36 years-old (28; 44), 75.5kg of body weight (66; 85), 4,758mL of total blood volume (4215; 5313), Hb of 14.4g/dL (13.6; 15.3), WBC count of 54.8×109/L (45.4; 65.5), PLT count of 215.5×109/L (179; 259), and 82.05/μL circulating CD34+ cells (63.8; 109.1). Univariate analysis showed that a body weight <67kg (p=0.011) was associated with a circulating CD34+ cells count <50/μL, whereas age between 31 and 44 years (p=0.026), WBC ≥40×109/L (p<0.001) and PLT count ≥200×109/L (p<0.01) were each associated with a circulating CD34+ cells count ≥50 cells/μl. Multivariate analysis showed that donors with body weight <67kg were less likely to have a good mobilization (OR=0.44; 95% CI 0.24–0.81). Conversely, a WBC count >40×109/L (OR=3.69; 95% CI 2.11–6.46) and PLT count ≥200×109/L (OR=2.09; 95% CI 1.26–3.47) on the first day of collection were each associated with a good mobilization (Table 2).

A minimum of 4×106 CD34+ cells/kg of the recipients’ body weight was harvested with only a single leukapheresis in 75.3% of the procedures. The median (IQR) for CD34+ cells×106/kg collected in the products was 5.70 (3.3; 8.8) and the median CEC was 43%. Considering the median CEC achieved, the collection predicting formula to calculate the total number of CD34+ cells collected by leukapheresis showed that the median (IQR) CD34+ cells×106/kg estimated to be collected in each product was 5.75 (3.067; 9.022), was very close to the actual median (IQR) amount of 5.7 (3.3; 8.8) (Table 1). Predicted versus observed data showed that the number of CD34+ cells/kg collected had an r-value of 0.775 (0.726–0.816; p<0.0001), demonstrating a linear correlation (Figure 1). Male donors (p=0.013), younger age (≤31 years, p=0.009; between 31 and 44 years, p=0.003), body weight >67kg (p=0.003), a positive difference in weight between donor and recipient (p=0.004), Hb level ≥14g/dl (p<0.001), PLT count ≥200×109/L (p=0.004), WBC >40×109/L (p<0.001) and circulating CD34+ cells >40/μL on the day of collection (p<0.001) were all associated with a successful collection in univariate analysis. In the multivariate model, predictors of collecting at least 4×106 CD34+ cells with only one collection were circulating CD34+ cells/μL >40 (OR=16; 95% CI 6.94–36.93), Hb level ≥14g/dl (OR=3.40; 95% CI 1.53–7.57), WBC count >40×109/L (OR=4.61; 95% CI 2.10–10.10) on the collection day, and a positive difference in weight between donor and recipient (OR=3.10; 95% CI 1.36–7.06) (Table 2).

Figure 1.

Predicted versus actual yielded CD34+ cells in 307 leukapheresis donors.

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The number of CD34+ cells predicted by the formula and the actual number of CD34+ cells collected showed a strong correlation. Review of each PBSC collection and systematic calculation of the individual CEC for each procedure can be helpful in more precisely predicting the number of CD34+ cells that will be collected. A difference from other reports is that in the present study the predicting collection formula was validated exclusively from healthy donors. The number of circulating CD34+ cells on the day of leukapheresis, as broadly reported, was the strongest predictor of a successful collection and depended on a good mobilization. Additionally, a higher Hb level, higher WBC count and a positive delta weight between donor and recipient were also predictors of a successful PBSC collection. We found that higher WBC and PLT counts immediately before leukapheresis were associated with a good mobilization, while donors with low body weight were more likely to be poor mobilizers.

The first validated formula to predict collection of CD34+ cells by leukapheresis included only 9 healthy donors and the adjusted Terumo COBE Spectra CEC was 30% to guarantee at least the minimum number of CD34+ cells/kg that might be expected to be collected in a given day for any donor.12 Further, a similar predicting collection formula was validated only among autologous PBSC collections using a CEC of 40%.16 In 2014, Hosing at al.17 assessed the predictive value of the above-mentioned formula in 1,126 consecutive PBSC collections, 44 of which were from healthy donors. In our series, in contrast to other reports, all collections were performed on healthy allogeneic donors and we used the median CEC achieved in the collections to validate the efficacy of the formula. CEC depends on the collection processor, the skill of the collection team and the accumulated experience acquired in each service. We consider that using the historical CEC series in each procedure can improve the ability of PBSC collection services to more accurately predict the actual amount of CD34+ cells to be obtained by leukapheresis.

The identification of risk factors for poor PBSC mobilization and collection among healthy donors is essential to improve donor selection and strategies to ensure the most efficient recipient engraftment. A hematopoietic cell transplantation modality that is recently gaining importance is haploidentical transplantation. This modality of transplant requires higher amounts of stem cells to overcome the HLA barrier18 and, consequently, demands a selection of a “good mobilizer” donor and a specific leukapheresis schedule to optimize collection in large volume apheresis.

The G-CSF dose is calculated using the actual body weight and not the ideal body weight. Consequently, heavier donors receive a higher G-CSF dose and higher WBC counts can be found before leukapheresis. In line with this protocol, lower body weight was found to be negatively related with the CD34+ cells count on the collection day.6 Murine experiments showed that adipose tissue has several structural similarities with bone marrow and contains significant amounts of stem cells19 that can be mobilized into the bloodstream. Further investigation is needed to assess if these findings are similar and relevant for humans. Additionally, CBC on the day of collection is a simple and inexpensive test that is routinely performed as part of the collection process. WBC count and Hb levels are closely related with the overall hematopoiesis20 and can be used as a sensitive surrogate marker of successful collection.

Donor selection is primarily dependent on HLA compatibility and characteristics such as age, gender, and body weight. The preferential recruitment of male donors has been adopted by many services, considering that women and donors who are of lower weight than their recipients have a decreased likelihood of meeting the transplant physicians’ requested dose.6 A positive delta body weight between donor and recipient is an efficient predictor of successful collection and should be taken into consideration in selection of PBSC donors, especially when elevated amounts of CD34+ cells are needed.

There are some limitations to our study. Different from other studies4,5,7,21–23 we could not evaluate if there is an association between gender and poor mobilization or unsuccessful PBSC collection since most of the participants were men. We also did not collect data on the baseline CBC, before the first G-CSF dose, limiting the identification of other potential predictors of a good mobilization. However, we collected data from a consecutive group of PBSC healthy donors over a 10 year period. This group is representative of the current donors’ selection practices at our institution. Furthermore, our collection data cannot be extrapolated to other apheresis equipment. In the last year, new apheresis equipment, which achieves a higher CEC and a more efficient PBSC collection, has been launched. COBE Spectra is still the leading brand in Latin America and other parts of the world. This popularity reinforces the utility of our results to other collection services.

In conclusion, the described formula for predicting CD34+ cells yield is useful to determine the required length of the procedure, and whether a second day is likely to be necessary. An accurate prediction of the leukapheresis length also save donors’ time. Predictors identified have value to the refinement of PBSC leukapheresis procedures, optimizing human and material resources. Further studies to better identify predictors of poor mobilization will further help to select donors that may benefit from different regimens, for example in combination with plerixafor. Recent studies reported the use of plerixafor, off-label, in healthy family donors as a rescue strategy to achieve an appropriate amount of CD34+ cells and provide an adequate graft for PBSC transplantation.24–26 Finally, on-site computer applications to accurately achieve a successful PBSC yield can be assembled and prospectively validated in other populations using both the formula to predict PBSC yield and the predictors described herein.

None declared.

The authors declare no conflicts of interest.