BACKGROUND OF THE INVENTION 
     The present invention relates to a method of verifying proper replacement of a used filter that is divided by a membrane into two chambers and that is arranged in the fluid system of a device for extracorporeal treatment of blood. In addition, the present invention relates to a device for extracorporeal treatment of blood with a fluid system containing at least one filter divided by a membrane into a first and a second chamber, so that proper filter replacement is verifiable. 
     DESCRIPTION OF RELATED ART 
     To remove substances usually eliminated with the urine and for removal of fluids, various methods of extracorporeal treatment or purification of blood are used to treat chronic renal failure. In hemodialysis, a patient&#39;s blood is purified outside the patient&#39;s body in an artificial kidney known as a dialyzer. The dialyzer has a blood chamber and a dialysis fluid chamber separated by a semipermeable membrane. During the treatment, the patient&#39;s blood flows through the blood chamber on one side of the membrane. To effectively purify blood of substances usually eliminated with the urine, fresh dialysis fluid flows continuously through the dialysis fluid chamber. 
     Diffuse mass transport is the predominant mechanism in hemodialysis (HD), while convective mass transport through the membrane dominates in hemofiltration (HF). Hemodiafiltration (HDF) is a combination of the two methods. In hemo(dia)filtration, a portion of the serum removed through the membrane is replaced by a sterile replacement fluid added to the extracorporeal blood circuit, either upstream from the dialyzer (predilution) or downstream from the dialyzer (postdilution). 
     In current devices for hemo(dia)filtration the dialysis fluid is prepared online from fresh water and an electrolyte concentrate, and the replacement fluid is prepared online from the dialysis fluid. To ensure that the dialysis fluid and replacement fluid prepared online are sterile and free of pyrogens, the fluids are passed through filters arranged in the fluid system of the hemo(dia)filtration machine. These filters are divided into two chambers by a microbe-retaining membrane. Such a device with two filters arranged in the dialysis fluid system is known from German Patent 34 44 671 C2. 
     German Patent 34 48 262 C2 describes a method of verifying that the filters of the hemo(dia)filtration machine known from German Patent 34 44 671 C2 are leakproof. The filter integrity test is performed using a pressure retaining test, with a partial vacuum being established in one of the two chambers of the filter. The pressure retaining test is based on the fact that when the membrane of the filter is wetted with a fluid, it is essentially impermeable to gases. An increase in pressure in the chamber can be detected during the pressure retaining test only when there is a defect. 
     German Patent 34 42 744 A1 describes a membrane integrity test for a used dialyzer, where the chambers of the dialyzer are filled with air, and equalization of pressure across the wetted membrane is observed. 
     Manufacturers of known sterile filters for hemo(dia)filtration machines suggest the replacement of filters at certain intervals, for reasons of safety. There are known hemo(dia)filtration machines which notify the user automatically when it is necessary to change the filter. To increase safety, the known hemo(dia)filtration machines also provide for manual confirmation of replacement of the filter by the user. However, this safety device cannot prevent the user from simply confirming that the filter has been replaced, without having actually replaced the filter. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a device and method for verifying proper replacement of a used filter in the fluid system of a machine for extracorporeal treatment of blood that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof, as well as the appended drawing. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention is a method for determining if a filter in the fluid system of a device for extracorporeal treatment of blood has been replaced with a new filter, that includes measuring whether a membrane of the filter that divides the filter into a first and a second chamber is permeable to gas and, in the event the membrane is permeable to gas, concluding that the filter has been replaced. 
     In another aspect, the invention is a device for extracorporeal treatment of blood, that includes at least one filter divided by a membrane into a first chamber and a second chamber, disposed along a fluid system of the device, and means for measuring a gas permeability of the membrane of the at least one filter. The device also includes means for determining whether the at least one filter has been replaced by a corresponding new filter, based on the gas permeability of the membrane. 
     One object of the present invention is to provide a method that will make it possible to ascertain with a high degree of certainty whether a used filter has been replaced by a new filter. In addition, another object of the present invention is to provide a device for extracorporeal treatment of blood that includes a device for verifying with a high degree of certainty whether proper replacement of the filter was carried out. 
     It is assumed in the present invention that the membrane of a new filter is dry, while the membrane of a used filter is wetted with fluid. A pressure retaining test is performed to verify the proper replacement of the filter. Verification is undertaken to determine whether the membrane of the filter is permeable to gas. If the membrane is impermeable to gas, it is concluded that the membrane is wetted with fluid, and that the filter installed is a used filter. 
     By using the pressure retaining test, proper replacement of all filters arranged in the fluid system of the machine for extracorporeal treatment of blood can be tested. This includes the filters for supplying sterile dialysis fluid and replacement fluid as well as the dialyzer filter itself. 
     The gas permeability of the membrane of the filter can be verified by a vacuum test or an excess pressure test. Preferably, a section of the fluid system including the first chamber of the filter and another section including the second chamber of the filter are isolated. Gas is then directed into one of the two sections to build up an excess pressure, which is monitored. The other section of the fluid system is then opened again, and if there is a pressure drop in the isolated section, it is deduced that the filter has been properly replaced. With this system it is also possible to verify whether an excess pressure can be built up and maintained in the isolated section of the fluid system that includes one of the two chambers. If that is the case, the filter is a used filter whose membrane is wetted with fluid. 
     The pressure drop per unit of time, or rate of pressure drop, is preferably compared to a predetermined limit value. If this limit value is exceeded, it can be deduced that the filter has been properly replaced. This ensures that it will be possible to differentiate between a pressure drop due to the fact that the membrane of the filter is not wetted with fluid, and the more minor pressure drop also detected with a membrane wetted with fluid over a long period of time. 
     To build up an excess pressure, it is preferable to direct atmospheric air into one of the two sections of the fluid system, where the excess pressure can be established with an air pump. To prevent microorganisms from entering the fluid system, the air is preferably passed through a hydrophobic filter. 
     When the used filter has not been replaced by a new filter, an optical and/or an acoustic alarm can be advantageously displayed or sounded. However, it is also possible in this case to prevent further operation of the extracorporeal blood treatment machine, so that treatment cannot be continued unless the used filter is replaced. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates an embodiment of the invention and, together with the description, serves to explain the objects, advantages, and principles of the invention. 
     The drawing shows a schematic diagram of a hemo(dia)filtration machine whose dialysis fluid system contains two filters 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The hemo(dia)filtration machine shown in the drawing has a dialyzer  1  divided by a membrane  2  into a first chamber  3  through which dialysis fluid flows and a second chamber  4  through which blood flows. The first chamber  3  is connected to a dialysis fluid path  5  of the dialysis fluid system of the extracorporeal blood treatment machine, which has an inlet line  6  and an outlet line  7 . The second chamber  4  is connected to a blood path  8 . 
     Inlet line  6  of dialysis fluid path  5  includes a first inlet line section  9 , a second inlet line section  10  and a third inlet line section  11 , and connects a dialysis fluid source  12  to the first chamber  3  of dialyzer  1 . The first inlet line section  9  leads from dialysis fluid source  12  to a first balancing chamber  13  of a balancing device  14  having two balancing chambers. First balancing chamber  13  of balancing device  14  is connected by second inlet line section  10  to the inlet of a first chamber  15  of a first sterile filter  16 , which is divided by a microorganism-retaining hydrophilic membrane  17  into a first chamber  15  and a second chamber  18 . To the third inlet line section  11  is connected the first chamber  19  of a second sterile filter  20 , which is also divided into a first chamber  19  and a second chamber  21  by a microorganism-retaining hydrophilic membrane  58 . 
     Outlet line  7  leads from the outlet of the first chamber  3  of dialyzer  1  to the second balancing chamber  22  of balancing device  14 . A dialysis fluid pump  23  is connected to the outlet line. An ultrafiltrate line  24  branches off from outlet line  7 , upstream from dialysis fluid pump  23 , and is connected to an ultrafiltration pump  25  for removing dialysis fluid. Ultrafiltrate line  24  leads to a drain  26  which is also connected to the outlet of the second balancing chamber  22  of balancing device  14 . 
     Blood path  8  has a blood inlet line  27  coming from the patient&#39;s blood vessels and connected to the inlet of the second chamber  4  of dialyzer  1 . A pump  28  is connected to blood inlet line  27 . The outlet of the second chamber  4  of dialyzer  1  leads through the first section of a blood outlet line  29  to a drip chamber  30  from which blood is carried to the patient over the second section of blood outlet line  29 . 
     The dialysis fluid system also has a replacement fluid line  31  that branches from the second chamber  21  of the second sterile filter  20  and may optionally be connected to blood inlet line  27  (predilution) or to drip chamber  30  (postdilution). The two connecting branches are indicated with dotted lines. A replacement fluid pump  32  is connected to replacement fluid line  31 . 
     The hemo(dia)filtration machine according to the invention has a device for verifying that two sterile filters  16 ,  20  have been properly replaced. The device for verifying proper filter replacement includes a first shut-off element  33  connected to the second inlet line section  10 , a second shut-off element  34  connected to the third inlet line section  11  upstream from the second sterile filter  20 , a third shut-off element  35  connected to the third inlet line section  11  downstream from the second sterile filter  20 , and a fourth shut-off element  36  connected to the replacement fluid line  31 . The shut-off elements are electromagnetically operated valves driven by a central control unit  37  over control lines  38 ,  39 ,  40 ,  41 . 
     Electric air pumps  42 ,  43  are connected by control lines  44 ,  45  to central control unit  37 , and are also connected to the third inlet line section  11  upstream and downstream from the second shut-off element  34 . Air pumps  42 ,  43  pump air from the atmosphere through a hydrophobic sterile filter  46 ,  47  to build up an excess pressure in the isolatable line sections  48 ,  49  of the dialysis fluid system. In addition, pressure gauges  50 ,  51  are connected to the third inlet line section  11  upstream and downstream from the second shut-off element  34  for monitoring the pressure in the isolatable line sections. 
     Pressure gauges  50 ,  51  are connected by control lines  52 ,  53  to an analyzing unit  54 , which is in turn connected by a data line  55  to central control unit  37 . An acoustic and/or optical alarm device  57  is connected to analyzing unit  54  by a signal line  56 . 
     The method according to the present invention for verifying whether the filter has been properly replaced will now be described in detail. As a first step, central control unit  37  closes shut-off elements  33  through  36 . Then control unit  37  starts operation of air pumps  42 ,  43 , so that an excess pressure is built up in isolated line sections  48 ,  49  of the fluid system of the hemo(dia)filtration machine. The pressure is monitored by pressure gauges  50 ,  51 . After a predetermined excess pressure has built up in the isolated line sections, the control unit stops the air pumps and opens shut-off elements  33  and  36 . Analyzing unit  54  compares the pressure measured by pressure gauges  50 ,  51  to a predetermined limit value. The appropriate predetermined limit value can be determined, for example, for each type of filter used. 
     If the pressure in the line sections of the fluid system including the second chamber  18  of the first sterile filter  16  and the first chamber  19  of the second sterile filter  20  does not drop below a predetermined limit value within a predetermined period of time, analyzing unit  54  triggers the alarm device  57  to deliver an acoustic and/or optical alarm. In a preferred embodiment according to the invention, the measurement time can be about ten seconds, and the predetermined limit can correspond to approximately half the pressure originally built up in the isolated line branches. 
     In another embodiment according to the invention, a common air pump and a common pressure gauge may also be provided for both the isolated line sections, without using two independent air pumps and pressure gauges. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the structure and methodology of the present invention, without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
A method and a device for verifying the proper replacement of a used filter that is divided into two chambers by a membrane in a device for extracorporeal treatment of blood. The verification of whether the filter has been properly replaced with a new filter is performed by a pressure retaining test, which determines whether the membrane of the filter is permeable to gas. In the event the membrane is permeable to gas, it is concluded that the used filter has been replaced by a new filter.