Determination of the Volume of HPC, Apheresis Products Based on Weight in Grams

Hematopoietic progenitor cells (HPC) collected by apheresis [HPC(A)] have become the most common source of HPC used for transplantation. HPC(A) products have a more variable content of CD34+ progenitor cells than marrow or umbilical cord blood, especially when collected from autologous donors. Most often the CD34+ cell content of previous collections is used to decide whether or not additional collections are needed to reach the target CD34+ cell infusion dose. Accurate assessment of CD34+ cell counts are imperitive in order to properly determine the need for additional apheresis procedures. CD34 content is typically measured by flow cytometry, either using a dual platform method that requires determination of the percentage of CD34+ cells within the total nucleated cells or a single platform method that uses counting beads to directly determine the number of CD34+ cells in a given volume of product. Both methods depend on accurate determination of the volume of the product in order to calculate the total number of CD34+ cells that have been collected. Volume estimates of HPC source products are most often performed using the product weight in grams (g) after subtracting the weight of the bag in which the product is collected. The weight is then divided by the product’s specific gravity to obtain the volume. The specific gravity of a substance is the ratio of the density of the substance to the density of a reference substance, such as water (1 g = 1 mL). The specific gravity of whole blood has been defined as approximately 1.058, whereas the specific gravity of platelet products is approximately 1.030. The difference between the two products is a function of the cellular content. Blood has a higher specific gravity since it contains more high-density cells (red blood cells (RBCs) and granulocytes) than platelet concentrates (largely platelets and lymphocytes). The composition of marrow is similar to blood, so a specific gravity of 1.058 is used. However, there are no published data that estimates the specific gravity of HPC(A) products and since these products are enriched for cells that are more dense than platelet products but less dense than whole blood products neither of the two established specific gravities is optimal to use. We sought to determine what the specific gravity of an apheresis product truly is so as to determine the optimal factor to estimate volume from weight for HPC(A) products. To this end our laboratory sampled 54 well-mixed HPC(A) products at receipt and used a reverse pipetting method and a sensitive analytical balance to weigh 1 mL aliquots of product. A hematology analyzer was used to measure the total number of white blood cells (WBC) and RBCs per mL of product. The 54 sampled products contained on average 2.7 ± 1.1 x 10E8 WBCs per mL and 5.5 ± 2.0E8 RBCs per mL, which is similar to the average seen for 823 products collected since 2010 of 2.81 ± 1.33 x 10E8 WBCs per mL. The weight in grams was significantly associated with the total number of RBCs + WBCs in a given product (p=0.026) but not with total WBC alone, indicating that RBC content was a contributing factor. The average weight (i.e. specific gravity) of 1.0 mL HPC(A) aliquots was 1.047 ± 0.009 g, a value between that of whole blood and platelet products. Based on these data we recommend that laboratories use a factor of 1.047 to convert the weight of HPC(A) products in grams to volume to more accurately determine the number of CD34+ cells contained in a given product.