Abstract:
An apheresis machine is provided, which can efficiently produce concentrated platelets products without excessively collecting platelets. 
     The apheresis machine  10  comprises a centrifuge  11  having an inlet port PT 1  and an outlet port PT 2  for separating whole blood into a lower density component, an intermediate density component and a higher density component, a first container  18  connected to receive the lower density component from the outlet port PT 2  and return it through the inlet port PT 1  to the centrifuge  11 , a first pump P 1  operated to collect whole blood from the inlet port PT 1  to the centrifuge  11 , a second pump P 2  operated to supply the lower density component from the first container  18 , and means for variable controlling the centrifuge  11  and/or the second pump P 2  for increasing or decreasing a process volume of whole blood in the centrifuge in response to at least one characteristic associated with the whole blood.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apheresis machine and a method for producing blood products using the same. More specifically, the present invention relates to customizing a collection process of whole blood during a cycle for improving cycle efficiency in an apheresis machine and production efficiency of blood products, and for lowering the level of contamination by white blood cells. 
     Apheresis is a procedure in which whole blood is separated into its various blood components, i.e., a higher density component such as red blood cells, at least one intermediate density component such as platelets and white blood cells, including lymphocytes and granulocytes, and a lower density component such as plasma, for collecting a desired blood component or components. Various methods are available for conducting apheresis, among which an intermittent blood flow method, according to which whole blood is intermittently processed with the use of centrifugation, is prevailing. 
     Among various blood component products obtainable through apheresis, the demand for concentrated platelet products is rapidly growing. This is particularly because, with the improvement in cancer therapy, there is a need to administrate more and more platelets to patients with lowered hemopoietic function. Platelets are fragments of a large cell located in the marrow called a megakaryocyte and primarily contribute to hemostasis by performing aggregation function, although they also have a role in tissue healing. Normal platelet counts are 150,000-400,000/mm 3  in the adult. Platelet counts under 20,000/mm 3  can cause various troubles such as spontaneous bleeding. 
     Platelets have a short half-life of 4-6 days and the number of donors is limited. Therefore, in producing concentrated platelet products, it is important to harvest platelets from the whole blood supplied by a donor at a maximum yield and in a required amount. Further, it is known that the contamination of concentrated platelet product by white blood cells can lead to serious medial complications, such as GVH reactions and, therefore, it is also very important to keep the level of contamination by white blood cells as low as possible, while efficiently collecting platelets. To this end, various excellent techniques have been developed. For example, according to the so-called “surge” technology developed by the assignee of the present application, after whole blood is collected and concentrically separated within a centrifuge into higher density, intermediate density and lower density components (so-called “draw” step) and plasma is harvested, the plasma is supplied through the centrifuge at a surge flow rate, that is, a flow rate that increases with time. By performing the surge, platelets can be preferentially displaced from the intermediate density components, which exist as a buffy coat mainly comprising a mixture of platelets and white blood cells, and concentrated platelet products can thereby be produced at an increased yield. Further, in Japanese Patent No. 2,776,988 (PCT/US94/01107) also owned by the assignee of the present application, a success in the improvement of separation between platelets and white blood cells was achieved by recirculating plasma at a constant rate through the centrifuge for a short period of time (so-called “dwell” step) so as to arrange platelets and white blood cells, which have close specific gravities, before displacing platelets from the centrifuge using the surge technology. In the common intermittent blood flow method, after harvesting a desired component or components, the residual blood components mostly comprising red blood cells are returned to the donor (so-called “return” step). 
     Usually, about 500 ml of whole blood is processed during one cycle which comprises the above-mentioned successive steps. This amount is based on 15% or less of the total amount of blood in humans and, if more than this amount is taken out of the body at once, the donor may suffer from blood pressure lowering or dizziness. This also means that there is a limit in the amount of a concentrated platelet product that can be harvested from one cycle and, in normal apheresis, a cycle which may require about 15 minutes is successively conducted for three to five times. The number of cycles is determined based on information previously obtained on the donor and his or her whole blood, for example, the donor&#39;s sex, height and weight, the number of cells in the whole blood, hematocrit value and the like. Typically, the amount of a concentrated platelet product that can be harvested per cycle is determined based on this information, and the number of cycles is selected so as to satisfy a target number of platelets. 
     As is well-known, concentrated platelet products are administered or traded on the basis of the number of platelets contained therein, i.e., number of units. For example, according to the Pharmaceutical Affairs Law in Japan, the presence of 1×10 11  platelets in a bag is prescribed as 5 units, and the products are used in a discrete number of units, 5, 10, 15 or 20. Accordingly, for example, 11 units and 14 units are both regarded as only 10 units. However, in actual apheresis, an attempt to produce a 10-unit concentrated platelet product does not always result in the desired product. Specifically, assume, for example, that it has been determined from previously obtained information that a 4-unit concentrated platelet product may be produced from one cycle. In this case, while 2.5 cycles should be sufficient for producing a 10-unit concentrated platelet product, it is actually necessary to perform 3 cycles because number of cycles must be an integer, resulting in 12 units of platelets contained in a bag. This presents a problem in terms of efficient production of concentrated platelet products. Further, even if 3.5 units is to be produced from one cycle, in which case 3 cycles theoretically result in 10.5 units, i.e., a 10-unit concentrated platelet product, the operator tends to select 4 cycles for the sake of certainty because the number of actually harvestable platelets may vary. This causes a problem that the number of platelets actually present in a bag tends to exceed the number of units indicated on the bag. In such a case, excessive platelets are collected from a donor, and the time needed to collect blood becomes unduly long. This also presents a problem in securing the safety of a donor. It is to solve these problems that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the volume of whole blood to be processed in a centrifuge during a “draw” step is variably controlled in response to at least one characteristic associated with the whole blood collected or to be collected from a donor. Specifically, in accordance with the present invention, a process volume of whole blood is increased or decreased so as to be customized with respect to each individual apheresis, so that the volume of an intermediate density blood component to be actuary harvested, in particular the number of platelets, becomes substantially equal to a desired number of units. The characteristics of whole blood are typically the number of platelets and hematocrit value, but the total amount of whole blood, which can be calculated based on the sex, height, weight and the like of a donor, as well as other characteristics, can also be considered. In addition, in case the separation of a desired number of white blood cells should be sought, the present invention is equally applicable. 
     In a draw step, using a first pump such as a peristaltic pump, whole blood is collected into a centrifuge from a donor directly, or after once pooled in a container such as a bag. The centrifuge may, for example, be a standard Latham bowl as described in US Pat. No. 3,145,713 (the contents of which is hereby incorporated by reference), has an inlet port and an outlet port and separates the collected whole blood into each component. In a draw step, it has been conventional to recirculate separated plasma by a second pump, which may also be a peristaltic pump, into the centrifuge at a constant flow rate such as 20 to 30 ml/min. for diluting the whole blood to facilitate its collection. Plasma is collected, for example, in a first container or bag which is connected to the centrifuge such that it can receive plasma from the outlet port and return it to the inlet port. In the second and the following cycles, the plasma collected in the previous cycle or cycles may be used. 
     According to one aspect of the present invention, a process volume of whole blood is decreased or increased by variably controlling the above second pump so as to increase or decrease the flow rate of plasma, i.e., the recirculation flow rate. The collection of whole blood in the draw step is continued until it is detected that the volume of a blood component separated in the centrifuge, typically the volume of a layer of red blood cells, has reached a predetermined amount. As mentioned above, the conventional amount of whole blood that may be collected from one cycle until this detection is, for example, about 500 ml. It is added that the detection can be made, for example, with an optical sensor which monitors the radius of the region occupied by the separated blood component in the centrifuge and detects when the radius has reached a particular value. 
     When, for example, the recirculation flow rate is increased here, the filling density of red blood cells within the layer of red blood cells is lowered at the time the radius has reached the above particular value, i.e., at the time the same volume has been occupied. This means that the number of platelets that can be harvested from one cycle decreases. Therefore, for example, when it has been previously determined from the hematocrit value, the number of platelets and the like of the whole blood of a donor that 3 units of platelets can be harvested from normal one cycle, the recirculation flow rate can be increased for decreasing the process volume of whole blood in the centrifuge so as to decrease the harvestable platelets units to about 2.7. While the same 4 total cycles are needed in either case to produce a 10-unit concentrated platelets product, when the recirculation flow rate is increased, the amount of excessively harvested platelets is decreased and the concentrated platelets product is produced more efficiently. On the other hand, in the above example, it is also possible to decrease the recirculation flow rate so that red blood cells are more densely filled in the layer of red blood cells and the number of platelets units to be harvested from one cycle becomes, for example, 3.5. According to this example, the desired 10-unit concentrated platelets product can be produced through three cycles, and this is preferred for efficient production and for not restricting the donor unduly longer. 
     In accordance with the present invention, it has been found that a decrease in the process volume of whole blood per cycle through increase in the recirculation flow rate also decreases, at the same time, the contamination by white blood cells in a resultant concentrated platelets product. Therefore, from this point of view, it is preferable to control the second pump so as to increase the recirculation flow rate for decreasing the process volume of whole blood per cycle, even if the total number of cycles remains the same, rather than decreasing the number of cycles by increasing the process volume of whole blood. In such a case, control means in accordance with the present invention can be adapted to perform an automatic operation in which a total number of cycles that exceeds a desired number of product units is determined based on the number of platelets that can be harvested per cycle and the process volume of whole blood per cycle is decreased so that the total amount of platelets to be harvested from the total cycles will not excessively go beyond the desired number of product units. Such decrease in the process volume of whole blood per cycle through an increase in the recirculation flow rate is also advantageous, especially if the donor is physically relatively petit and the total amount of available blood is limited, because the amount of extracorporeal blood circulation will be decreased and thus the danger of causing anemia or dizziness can be minimized. 
     On the other hand, however, it should be noted that decrease in the recirculation flow rate does not necessarily mean that contamination by white blood cells will increase. Such is a matter that may also be affected by the hematocrit value of donated whole blood and the like. Therefore, it is also possible to configure the above-mentioned automatic operation so that the process volume of whole blood per-cycle will be automatically decreased or increased according to hematocrit values or any other suitable threshold values. 
     Further, for example in Multi or CCS, which are the names of products marketed by Haemonetics Corporation, the assignee of the present application, the total amount of plasma flowing through the centrifuge, i.e., the so-called critical flow, is controlled to promote a separation between platelets and white blood cells. It is also possible to increase or decrease the recirculation flow rate by the second pump such that the critical flow is increased or decreased, and the process volume of whole blood is variably controlled thereby. As is known, the critical flow can be maintained constant by controlling the recirculation flow rate of plasma with the second pump, and the critical flow is advantageous in that whole blood is diluted, fluctuations in the flow rate of whole blood being collected from a donor is compensated for and the flow through the centrifuge, or bowl is stabilized to prevent the separation from being disturbed. In fact, depending on the condition of a donor, the volume of whole blood supplied sometimes drops by 20 to 30 ml during the draw step, even to zero under certain circumstances. According to the critical flow technology, however, the second pump operates to increase the recirculation flow rate so as to compensate for the drop, thereby maintaining the critical flow at a constant level. Then, according to the present invention, the second pump can be further controlled to increase or decrease the flow rate of the critical flow itself so as to increase or decrease the process volume of whole blood. 
     According to another aspect of the present invention, the same effect as that of variably controlling the second pump can also be obtained by variably controlling the number of rotations of the centrifuge. Namely, when the number of rotations of the centrifuge is decreased, red blood cells are filled more coarsely in the layer of red blood cells and the process volume of whole blood per cycle is decreased accordingly. On the other hand, when the number of rotations of the centrifuge is increased, red blood cells are filled more densely in the layer of red blood cells and the process volume of whole blood per cycle is increased. In this case, it is also preferable to perform control so that the number of rotations of the centrifuge is decreased for reducing the process volume of whole blood per cycle, when it is desired to decrease the contamination by white blood cells in concentrated platelets products. To decrease the number of cycles for reducing the burden of donors, it is preferable to control the centrifuge so that the number of rotations is increased. 
     The aforementioned variable control of the second pump and variable control of the centrifuge can be carried out independently. Alternatively, they can be coordinated with each other so as to optimize the process volume per cycle. Therefore, according to the present invention, in response to previously available at least one characteristic associated with whole blood, such as a hematocrit value, the number of platelets, total blood amount and the like, an appropriate number of cycles can be set by taking variations into account, thereby enabling efficient production of concentrated platelets products without excessive collection. Alternatively, it is also possible to select in advance a desired number of cycles, and to conveniently set the number of units of a concentrated platelets product to be produced on this basis. The means for implementing such control can be realized by the use of a micro computer, which may be appropriately pre-programmed or configured to be loaded with an appropriate program according to situations. The above-mentioned automatic operation can be most suitably implemented in such cases. 
     It is desirable to conduct the dwell step after the draw step. This step is described in detail in the aforementioned Japanese Patent No. 2,776,988 (PCT/US94/01107), the contents of which patent is hereby incorporated by reference. In the dwell step, after the line for collecting whole blood to the centrifuge is closed, the lower density component, i.e., plasma, is circulated into the centrifuge for a short period of time at a substantially constant flow rate which is greater than the flow rate for collecting the whole blood. The buffy coat is thereby diluted in the centrifuge by the plasma and the region it occupies in the centrifuge is widened, resulting in an improved separation of the intermediate density components in the buffy coat, i.e., platelets and white blood cells. Namely, the widened buffy coat allows the heavier white blood cells to sediment more completely than the lighter platelets to the outer side of the buffy coat, thereby improving the separation between the white blood cells and the platelets. The improved separation between white blood cells and platelets reduces the degree of contamination by white blood cells in eventually collected platelets. The operation of the second pump can begin in response to a detection, for example by the aforementioned optical sensor or the like, that the radius of the area occupied by the intermediate density components has reached a particular value. 
     The surge step is carried out following the dwell step. That is, the second pump can be further operated to supply plasma to the centrifuge at a surge flow rate so as to displace the intermediate density components from the centrifuge. The surge flow rate is a flow rate which increases over a predetermined period of time. Platelets first, and then white blood cells under further increased flow rate, are successively displaced from the centrifuge. Blood components displaced from the centrifuge are monitored by an optical line sensor to determine in accordance with the optical density of the components that a particular component has been displaced. An uninterrupted length of passage or tube is in fluid communication with the outlet port of the centrifuge and extends from the outlet port beyond the optical line sensor. The displaced blood components are collected in second and third containers or bags, respectively, which are in selective fluid communication with the outlet port of the centrifuge through the uninterrupted passage. The uninterrupted passage prevents foam associated with the component displaced from the centrifuge from mixing with the component more completely, thereby preventing the optical line sensor from making false optical readings. 
     When the processed whole blood is directly withdrawn from the donor, the return step follows the surge step, in which the remaining blood components in the centrifuge, for example, red blood cells and white blood cells, are returned to the donor. In this case, the efficiency of returning blood can be increased by diluting the blood components with the second pump as described in Japanese Patent No 2,575,769. The cycle can then be repeated a desired number of times. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following specific description of preferred embodiments shown in the drawings. In the drawings, like reference numerals refer to the same parts even if they are represented differently. The drawings are not necessarily to scale, and emphasis might have been placed to illustrate the principles of the present invention. 
     FIG. 1 is a schematic diagram of a preferred embodiment of the apheresis machine of the present invention. 
     FIG. 2 is a schematic diagram of a disposable system for use with the machine of FIG.  1 . 
     FIG. 3 is a side view of a centrifugal bowl with a section broken away and connected to an optical line sensor. 
     FIG. 4 is a graph showing recirculation pump speed in mi/min. at different steps of the blood separation process for the three pump machine of FIGS. 1 and 2 of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, an apheresis apparatus  10  uses a standard Latham type centrifuge bowl  11  for separating anticoagulated whole blood into its constituent components. However, of course, other types of centrifuge, for example an integral blow-molded centrifuge bowl may also be used. Examples of such bowl are described in U.S. Pat. Nos. 4,983,156 and 4,943,273. The centrifuge bowl  11  consists of a rotatable bowl  12  and stationary input and output ports PT 1  and PT 2  fluidly coupled to the bowl interior by a rotary seal  74  (see FIG.  3 ). The input port PT 1  of the centrifuge bowl  11  is in fluid communication with a phlebotomy needle  24  via a blood filter F 1 , a tube  28  and a Y-connector  30  when a valve V 1  is open. The phlebotomy needle may be replaced with a whole blood bag (not shown) in case the whole blood is to be first pooled and then supplied. The tube  28  has compatibility with blood, as is all the tubing in the apparatus  10 . The outlet port PT 2  of the centrifuge bowl  11  is selectively coupled by a tube  36 , a valve V 2  and a tube  37  with a first container  18  labeled plasma/air bag suspended from a weight scale  33 . A second container  20  labeled platelet bag is selectively coupled via the tube  36 , a valve V 3  and a tube  39  to the outlet port PT 2 . 
     A bag or container  16  for storing an anticoagulant is in fluid communication with the phlebotomy needle  24  via a bacteria filter F 2 , a tube  32  and the Y-connector  30 . The bacteria filter F 2  prevents any bacteria in the anticoagulant (ACD) container  16  from entering the system. Containers  16 ,  18  and  20  are preferably plastic bags made of a blood compatible material. Peristaltic pumps P 1 , P 2  and P 3 , together with the valves V 1 , V 2  and V 3 , control the direction and duration of flow through the apparatus  10  in response to signals generated by a line sensor  14 , a donor pressure monitor (DPM) M 1 , a system pressure monitor (SPM) M 2  and air detectors D 1 , D 2  and D 3 . The air detectors D 1 , D 2  and D 3  detect the absence or presence of fluid. The pressure monitors M 1  and M 2  monitor pressure levels within the apparatus  10 . The line sensor  14  is an optical sensor and detects the presence of blood components passing through the line sensor  14  from the output port PT 2 . 
     In initial operation, the pumps P 1  and P 3  are energized to prime the tube  28  of the apparatus  10  with the anticoagulant from the container  16 . The anticoagulant passes through the filter F 2  and Y-connector  30  before reaching the air detector D 1 . The air detector D 1  senses the presence of the anticoagulant at D 1  and terminates the anticoagulant priming operation. During the priming operation, the valve V 2  is open and sterile air displaced from bowl  12 ; by the anticoagulant enters the top port PT 3  of the air/plasma container  18 . 
     The phlebotomy needle  24  is then inserted into the donor and the draw step is commenced. The draw step is the first step in a sequence of  4  steps; draw, dwell, surge and return performed by the apparatus  10  for separating blood components. The pump speed of the pump P 2  during the draw, dwell and surge steps is exemplified in FIG.  4 . During the draw, whole blood is drawn from the donor at a rate of about 80 ml/min. and mixed with the anticoagulant using the pumps P 1  and P 3 . The pump P 3  mixes the anticoagulant from the container  16  with the whole blood drawn from the donor or a bag in which it is pooled. The valve V 1  is open, allowing the anticoagulated whole blood to pass through the tube  28  and blood filter F 1  before being pumped into the bowl  12  through the inlet port PT 1 . The whole blood is admitted to the bottom of the bowl  12  through a feed tube (not shown). The ratio of the anticoagulant to whole blood is typically about 1:10. The operation of each of the pumps and valves in the apheresis apparatus  10  can be performed in accordance with desired protocols under the control of a microcomputer, which is not shown. 
     Referring to FIG. 3, the centrifuge bowl  11  has the fixed inlet port PT 1  and the fixed outlet port PT 2 . The rotary seal  74  fluidly couples the stationary inlet port PT 1  to the lower interior portion of the bowl  12 , and the outlet port PT 2  to an upper portion of the bowl interior for collecting separated fractions. A core  72  occupies a volume coaxial with the interior of bowl  12  and provides a separation region between the wall of the core  72  and the outer bowl wall  70 . 
     As the bowl  12  is rotated, centrifugal forces separate the anticoagulated whole blood admitted into the bottom of the bowl into red blood cells (RBC), white blood cells (WBC), platelets and plasma. The number of rotations of the bowl  12  can be selected, for example, within a range of 4,000 to 6,000 rpm, and is typically 4,800 rpm. The blood is separated into different fractions in accordance with the component densities. The higher density component, i.e., RBC  60 , is forced to the outer wall  70  of the bowl  12  while the lower density plasma  66  lies nearer the core  72 . A buffy coat  61  is formed between the plasma  66  and the RBC  60 . The buffy coat  61  is made up of an inner layer of platelets  64 , a transitional layer  68  of platelets and WBC and an outer layer of WBC  62 . The plasma  66  is the component closest to the outlet port from the separation region and is the first fluid component displaced from the bowl  12  via the outlet port PT 2  as additional anticoagulated whole blood enters the bowl  12  through the inlet port PT 1 . 
     Returning to FIG. 1, the displaced plasma passes through the line sensor  14 , the tube  36 , a 3-way T-connector  26  and the valve V 2  (in the open position) and enters the first container  18 . The plasma entering the first container  18  is drawn from the container  18  by the pump P 2  via a tube  40  from the lower port PT 4  and is recirculated into the bowl  12  through the inlet port PT 1  via a Y-connector  91  and a line  41 . The recirculated plasma dilutes the anticoagulated whole blood entering the bowl  12  and allows the blood components to separate more readily. An optical sensor  21  is applied to a shoulder portion of the bowl  12  for monitoring each layer of the blood components as they gradually and coaxially advance toward the core  72  from the outer wall  70  of the bowl  12 . The optical sensor  21  may be mounted in a position at which it can detect the buffy coat reaching a particular radius, for example, the surge radius (≈3.81 cm) and the draw step may be terminated in response to the detection. 
     In accordance with the present invention, the amount of whole blood processed by the bowl  12  in the draw step is varied in response to at least one characteristic associated with the whole blood, such as the hematocrit value, the number of platelets, the total amount of blood or the like of the whole blood. This variable control may be implemented by the following steps. 
     (1) Calculation of the Total Amount of Blood 
     Generally, this can be calculated based on the sex, height and weight of the donor. 
     (2) Calculation of a Process Volume of Whole Blood per Cycle 
     Given the flow rate of plasma recirculated by the pump P 2  and/or the number of rotations of the centrifuge, the volume of whole blood that can be processed during one cycle is calculated, using the hematocrit value of the whole blood. The minimum recirculation flow rate, for example, can be selected as the recirculation flow rate to be given and the maximum number of rotations, for example, can be selected as the number of rotations of the centrifuge to be given. However, other values, for example, a standard recirculation flow rate within a range of 20 to 30 ml/min. and a standard umber of rotations on the order of 4800 rpm may also be used for the calculation. 
     (3) Determination of Number of Cycles 
     The number of cycles necessary for satisfying a target number of units, e.g., 5 units or 10 units, is then determined using the process volume of whole blood per cycle obtained in (2) and the number of platelets previously measured for the whole blood (pre-count). When the minimum recirculation flow rate or the maximum number of rotations of the centrifuge is used in (2) above, the number of cycles is determined by rounding up fractions to make an integer. In other cases, it is possible to make this determination by rounding down fractions to obtain the number of cycles as an integer. 
     (4) Variable Control of the Process Volume of Whole Blood 
     Using the number of cycles obtained in (3), the process volume of whole blood is controlled so that a concentrated platelets product is to be produced in an efficient manner. In case the minimum recirculation flow rate or the maximum number of rotations of the centrifuge is used in (2) above, the control can be made, for example, by gradually increasing the recirculation flow rate and/or gradually decreasing the number of rotations of the centrifuge for selecting an optimal recirculation flow rate and/or number of rotations of the centrifuge, such that the number of platelets to be obtained from the total cycles neighbors and also suffices the target number of units. 
     If the process volume of whole blood is to be controlled through the recirculation flow rate, use of the aforementioned critical flow enables to variably control the process volume of whole blood by the total amount of plasma entering the bowl. The critical flow is calculated by, for example, the following equation. 
     
       
           Qc=Qr +[1−( Hd /100)×(1−1 /ACD )]× Qd   
       
     
     wherein, 
     Qc=critical flow (ml/min.) 
     Qr=recirculation flow rate of plasma by pump P 2   
     Hd donor hematocrit (%) 
     ACD=anticoagulated whole blood/anticoagulant ratio 
     Qd=draw flow rate of wholeblood by pumpP 1  (ml/min.) 
     The critical flow can be variably controlled steplessly within a range of, for example, 50 to 120 ml/min. Alternatively, it can also be controlled to increase or decrease at a step of, for example, 5 ml/min. Further, in case the number of rotations of the centrifuge is to be varied, the process volume of whole blood can be increased or decreased by determining the number of rotations steplessly or stepwisely within a range of, for example, 4,000 to 6,000 rpm. These controls can be implemented under the control of a microcomputer, as aforementioned. Alternatively, each of them can be implemented manually. 
     After the draw step is completed, the valve V 1  is closed and the pump P 1  is stopped so that blood is no longer drawn from the donor, and the dwell step is commenced. During the dwell, the pump P 2  recirculates plasma  66  through the bowl  12  at a moderate rate (for example, about 100 ml/min. in FIG. 4) for about 20 to 30 seconds. At this flow rate, the buffy coat  61  is diluted by the plasma and widens but the platelets do not leave the bowl  12 . The dilution of the buffy coat allows the heavier white blood cells to sediment to the outer side of the buffy coat, resulting in a better separation between the lighter platelets layer  64  and the heavier white blood cells layer  62 . As a result, the transitional layer  68  is reduced. The dwell period also allows the flow patterns in the bowl  12  to stabilize and allows more time for microbubbles to leave the bowl  12  and be purged. 
     After the dwell, the surge step is commenced. In the surge, the speed of the pump P 2  (starting at 100 ml/min. in FIG. 4) is increased in 5-10 ml/min. increments to recirculate plasma until reaching a platelet surge velocity of about 200-250 ml/min. The platelet surge velocity is the velocity at which platelets can leave the bowl  12  but not red blood cells or white blood cells. The plasma exiting the bowl becomes cloudy with platelets and this cloudiness is detected by the line sensor  14 . The line sensor  14  consists of an LED which emits light through blood components leaving the bowl  12  and a photo detector which receives the light after it passes through the components. The amount of light received by the photo detector is correlated to the density of the fluid passing through the line. 
     When platelets first start leaving the bowl  12 , the line sensor output starts to decrease. The valve V 3  is opened and the valve V 2  is closed and the platelets are collected in the container  20 . Once the majority of the platelets are removed from the bowl  12 , the fluid exiting the bowl becomes less cloudy. This lessening of cloudiness is detected by the line sensor  14 . Then, the valve V 3  is closed and the collection is ended, or collection of white blood cells is initiated. 
     Optionally, the collection of white blood cells may be started, utilizing an additional valve V 4  and a third container, a WBC bag  22  (shown in dotted lines in FIG.  1 ). After the line sensor output has reached its minimum value, the fluid starts to clear out. Once the sensor output has risen a given percentage and the valve V 3  is closed, the valve V 4  is opened and the pump P 2  further increases its speed to the lymphocyte surge rate. This initiates the collection of white blood cells. The line sensor will soon reach a maximum, and the cloudiness of the fluid will start to increase again as larger particles start to leave the bowl. Thereafter, the valve V 4  is closed when red blood cells start leaving the bowl, and the collection is ended. 
     After the platelets and/or white blood cells have been collected, the apparatus begins the return step. During the return, the rotation of the bowl  12  is stopped and the remaining blood components in the bowl  12  are returned to the donor by reversal of rotation of the pump P 1  via the phlebotomy needle  24  with the valve V 1  open. The vale V 2  is also opened to allow air to enter the centrifuge bowl during the return. The plasma from the container  18  dilutes the remaining blood components in the bowl  12 . Namely, the pump P 2  mixes the plasma with the returning components in the bowl  12  with the valve V 2  open, diluting the returning red blood cells component with plasma to speed up the return time. When the remaining blood components in the bowl have been returned to the donor, the return step is terminated. 
     This process of draw, dwell, surge and return is performed the number of times determined as explained above. It is added that during the draw, the anticoagulated whole blood entering the bowl  12  may be diluted with a solution such as saline from a container  90  (shown in dotted lines) instead of plasma using valves V 6  and V 5  and the pump P 2 . 
     EXAMPLES 
     Working Example 1 
     A product sold under the product name Haemonetics Component Collection System (CCS), an apheresis machine of the assignee of the present application Haemonetics Corporation, was modified to enable individual setting of the number of rotations of the centrifuge, the number cycles and the amount of critical flow. A disposable circuit also sold by Haemonetics Corporation under the product name Haemonetics Multi-Component Set (Lot No. 995J) was fitted to the modified machine, and blood was withdrawn from a donor to produce a concentrated plaelets product. In this case, the total amount of blood was calculated from the sex, height and weight of the donor, and the process volume of whole blood per cycle was determined from the hematocrit value, with the speed of rotation of the centrifuge at 4800 rpm and the critical flow at 50 ml/min. Then, the number of platelets that can be harvested was calculated from the platelets pre-count, and the number of cycles was determined, with 10 units being a target. Next, the critical flow was increased at a step of 5 ml/min until it reached 120 ml/min. and a value, at which the number of platelets that can be harvested from the total cycles in closest to and still suffices 10 units, was selected as the value of the critical flow. In this step of selecting the critical flow, the process volume of whole blood and the number of harvestable platelets were also corrected, because they also change as the critical flow increases. Table 1 shows examples of the change of the process volume of whole blood with the change of critical flow of heratocrit values of 42% and 36%. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                   
                 Process volume 
                   
               
               
                   
                   
                 of whole blood 
                   
               
               
                   
                   
                 per cycle (ml) 
                   
               
             
          
           
               
                   
                 Hematocrit 
                   
                 42% 
                 36% 
               
               
                   
                   
               
             
          
           
               
                   
                 Critical 
                 50 
                 441 
                 514 
               
               
                   
                 flow 
                 55 
                 438 
                 511 
               
               
                   
                 (ml/min.) 
                 60 
                 434 
                 507 
               
               
                   
                   
                 65 
                 431 
                 503 
               
               
                   
                   
                 70 
                 428 
                 499 
               
               
                   
                   
                 75 
                 425 
                 496 
               
               
                   
                   
                 80 
                 422 
                 492 
               
               
                   
                   
                 85 
                 419 
                 488 
               
               
                   
                   
                 90 
                 415 
                 485 
               
               
                   
                   
                 95 
                 412 
                 481 
               
               
                   
                   
                 100 
                 409 
                 477 
               
               
                   
                   
                 105 
                 406 
                 473 
               
               
                   
                   
                 110 
                 403 
                 470 
               
               
                   
                   
                 115 
                 399 
                 466 
               
               
                   
                   
                 120 
                 396 
                 462 
               
               
                   
                   
               
             
          
         
       
     
     Whole blood was collected from 561 donors and platelets were harvested to produce blood products. The actual number of platelets, the degree of contamination by white blood cells and the cycle time were evaluated for each of the products. The speed of rotation of the centrifuge was fixed at 4,800 rpm, as stated above. 
     Comparative Example 1 
     Whole blood was collected from 253 donors and platelets were harvested as in the working example 1, with the exception that the critical flow was fixed at 61 ml/min. The actual number of platelets, the degree of contamination by white blood cells and the cycle time were evaluated for each of the products. It is added that the critical flow of 61 ml/min. is a value within a range of 60 to 80 ml/min., which is usually recommendable in a protocol for harvesting concentrated platelets. The results of the above are shown in Table 2. 
     
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Target unit of Platelets: 10 units (2 × 10 11  platelets) 
               
             
          
           
               
                   
                 Variable 
                   
               
               
                   
                 (50-120 
                 Fixed 
               
               
                 Critical Flow 
                 ml/min.) 
                 (61 ml/min.) 
               
               
                   
               
             
          
           
               
                 Number of Donors 
                 561 
                 253 
               
             
          
           
               
                 Number of 
                 Average (×10 11 ) 
                 2.48 
                 2.49 
               
               
                 platelets 
                 Maximum value (×10 11 ) 
                 3.22 
                 3.95 
               
               
                   
                 Minimum value (×10 11 ) 
                 1.65 
                 1.63 
               
               
                   
                 Standard deviation (×10 11 ) 
                 ±0.26 
                 ±0.34 
               
               
                   
                 Unacceptable ratio (%) 
                 1.4 
                 5.5 
               
               
                 Number of 
                 Average 
                 6.86 × 10 6   
                 1.18 × 10 7   
               
               
                 contami- 
                 Maximum value 
                 5.92 × 10 7   
                 7.45 × 10 7   
               
               
                 nant WBCs 
                 Minimum value 
                 1.83 × 10 6   
                 1.79 × 10 6   
               
               
                   
                 Standard deviation 
                 ±6.06 × 10 6    
                 ±1.19 × 10 7    
               
               
                 Cycle time 
                 Average (min.) 
                 52.6 
                 52.3 
               
               
                   
                 Maximum value (min.) 
                 99 
                 90 
               
               
                   
                 Minimum value (min.) 
                 27 
                 27 
               
               
                   
                 Standard deviation 
                 ±11.3 
                 ±10.9 
               
               
                   
               
             
          
         
       
     
     In the products obtained in the working example 1, dispersion among products is small and the ratio of products which fall short of the required units (thus are unacceptable as a 10-unit product) is low, meaning that concentrated platelets products were produced efficiently. Further, the number of contaminant WBCs is low. While no significant difference is observed in the cycle time, or process time, this is probably because the critical flow employed in the comparative example is in the proximity of the lower limit of the variable range of critical flow employed in the working example 1. 
     Working Example 2 
     Using the modified machine and the disposable as used in the working example 1, blood was withdrawn from a donor to produce a concentrated platelets product. In this case, the total amount of blood was calculated from the sex, height and weight of the donor, and a process volume of whole blood per cycle was determined from the hematocrit value, with the speed of rotation of the centrifuge at 5600 rpm and the critical flow at 65 ml/min. The number of platelets that can be harvested was calculated from the platelets pre-count, and the number of cycles was determined, with 10 units being a target. Next, the rotation speed of the centrifuge was decreased at a step of 100 rpm until it reached 4200 rpm and a value, at which the number of platelets that can be harvested from the total cycles is closest to and still suffices 10 units, was selected as the value of the rotation speed. As in the case of the working example 1, the process volume of whole blood and the number of harvestable platelets were also corrected. Table 3 shows examples of the change of the process volume of whole blood for hematocrit values of 42% and 36% with the change of the rotation speed of the centrifuge. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                   
                 Process 
                   
               
               
                   
                   
                 volume of whole 
                   
               
               
                   
                   
                 blood per cycle (ml.) 
                   
               
             
          
           
               
                   
                 Hematocrit 
                   
                 42% 
                 36% 
               
               
                   
                   
               
             
          
           
               
                   
                 Number of 
                 4,200 
                 415 
                 484 
               
               
                   
                 rotation of 
                 4,300 
                 418 
                 487 
               
               
                   
                 centrifuge 
                 4,400 
                 420 
                 490 
               
               
                   
                 (rpm) 
                 4,500 
                 423 
                 493 
               
               
                   
                   
                 4,600 
                 426 
                 497 
               
               
                   
                   
                 4,700 
                 428 
                 500 
               
               
                   
                   
                 4,800 
                 431 
                 503 
               
               
                   
                   
                 4,900 
                 433 
                 506 
               
               
                   
                   
                 5,000 
                 436 
                 509 
               
               
                   
                   
                 5,100 
                 439 
                 512 
               
               
                   
                   
                 5,200 
                 441 
                 515 
               
               
                   
                   
                 5,300 
                 444 
                 518 
               
               
                   
                   
                 5,400 
                 447 
                 521 
               
               
                   
                   
                 5,500 
                 449 
                 524 
               
               
                   
                   
                 5,600 
                 452 
                 527 
               
               
                   
                   
               
             
          
         
       
     
     Whole blood was collected from 124 donors and platelets were harvested to produce blood products. The actual number of platelets, the degree of contamination by white blood cells and the cycle time were evaluated for each of the products. The critical flow was fixed at 65 ml/min. 
     Comparative Example 2 
     Whole blood was collected from 114 donors and platelets were harvested as in the working example 2, with the exception:that the rotation speed of the centrifuge was fixed at 4800 rpm. The actual number of platelets, the degree of contamination by white blood cells and the cycle time were evaluated for each of the products. The results of the above are shown in Table 4. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Variable 
                   
               
               
                   
                 (5600-4200 
                 Fixed 
               
               
                 Number of Rotations of Centrifuge 
                 rpm) 
                 (4800 rpm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Number of donors 
                 124 
                 114 
               
             
          
           
               
                 Number of 
                 Average (×10 11 ) 
                 2.43 
                 2.59 
               
               
                 platelets 
                 Maximum value (×10 11 ) 
                 2.94 
                 3.80 
               
               
                   
                 Minimum value (×10 11 ) 
                 1087 
                 1.68 
               
               
                   
                 Standard deviation (×10 11 ) 
                 ±0.20 
                 ±0.37 
               
               
                   
                 Unacceptable ratio (%) 
                 2.4 
                 4.4 
               
               
                 Number of 
                 Average 
                 1.20 × 10 7   
                 1.08 × 10 7   
               
               
                 contami- 
                 Maximum value 
                 2.70 × 10 7   
                 4.39 × 10 7   
               
               
                 nant WBCs 
                 Minimum value 
                 5.56 × 10 6   
                 5.00 × 10 6   
               
               
                   
                 Standard deviation 
                 ±6.72 × 10 6    
                 ±7.20 × 10 6    
               
               
                 Cycle time 
                 Average (min.) 
                 51.3 
                 54.0 
               
               
                   
                 Maximum value (min.) 
                 81 
                 91 
               
               
                   
                 Minimum value (min.) 
                 28 
                 36 
               
               
                   
                 Standard deviation 
                 ±9.6 
                 ±10.7 
               
               
                   
               
             
          
         
       
     
     In the products obtained in the working example 2, dispersion among products is also small and the ratio of products which fall short of the required units is low, from which it is understood that concentrated platelets products were produced efficiently. It is also observed that the cycle time has a tendency to be shortened in general. 
     As has been described above, in accordance with the present invention, through the variable control of the second pump and/or the centrifuge for increasing or decreasing the process volume of whole blood per cycle, efficient production of concentrated platelets products, typically toward a target number of units, is enabled.