Blood Filter

What is a blood filter?

A blood filter primarily consists of a fiber membrane with pores diameter 170 to 260 microns. With these diameters, when blood passes through this membrane, blood clots, small clumps of platelets, and white blood cells formed during the collection and storage will be removed. Thus, impurities and debris with diameters of 170 microns and over will be filtered and removed from the blood, which will be infused for transfusion.

What are the types of blood filters?

1. Standard filters: These blood filters consist of a membrane with pores of 170 to 260 microns. They are used in transfusions wherein the macroaggregates, such as clots and fibrin, are removed before the blood is infused.

Blood transfusion set filter
Blood transfusion set

2. Leucocyte-depletion filters: There are blood transfusions wherein only the platelets are needed. If the available blood still contains leucocytes, this type of blood filter will be used to remove the leucocytes. These filters remove 80 to 95 percent of the white blood cells (leucocytes).

3. Microaggregate filters: These filters are used when the microaggregates, such as fibrin, platelets, and white blood cells, are meant to be removed from the whole blood before blood transfusions. The membranes of these filters have pores with a diameter of 40 microns. They are used in blood transfusions wherein only the red blood cells (erythrocytes) are needed. They are also used to remove cytomegalovirus that has infected the blood before being used in transfusions. 

What contraindications and cautions do you need to observe in using blood filters?

While we find many benefits from using blood filters, there are also several precautions and contraindications (when not to use) to be observed. These are:

1. A standard blood filter needs to be changed every 12 hours; however, if there is a noticeable slowing down in the blood flow rate, you can change the filter earlier than 12 hours.

2. If a filter for infusion of red blood cells, do not use it again for infusing platelets. The trapped red blood cells in the filter may block the effective filtration of platelets.

3. To prevent the blockage in the flow of blood in the filter, you can flush the standard blood filters with normal saline.

4. We can use each standard blood filter for up to 4 units of blood. Thus, changing the filter could be affected when 12-hour use has been reached or when it has been used for four units of blood, whichever comes first. 

5. Do not prime leukocyte-reduction filters with 0.9% normal saline because this will interfere with the filter’s normal functioning; instead, use normal saline. 

6. We can only use leukocyte reduction filters for 1 to 2 units of blood, which is the manufacturer’s recommendation. 

What needles and catheters can we use in the infusion of filtered blood?

We will have to discuss two essential gadgets before writing down the step-by-step procedure in effecting blood filtration and infusion. These are the needles and the catheters. They are so important because their sizes vary from one patient to another. No one size fits all; otherwise, the patient’s vein will be lacerated and damaged. For this reason, the needle and catheter sizes will be individualized, depending on the size and integrity of the patient’s vein. If the patient’s veins are so small in diameter and thin, there is no point in using large needles and catheters. 

Hypodermic Needle - 18G x 1.5
18G needle

An 18-gauge needle is standard, but patients can use a needle or catheter as small as 23-gauge for transfusion if that is the size that the patient’s vein can bear and tolerate. The only drawback in using a small-bore needle or catheter is that the blood flow rate may slow down, promoting blood clotting and stoppage in the flow. On some occasions, to restore the flow, pressure is applied; this is allowable to a certain extent, but we should avoid using too much pressure to prevent the destruction of the blood components known as hemolysis. To prevent the application of too much stress, we can use saline to dilute the red cells.

Hypodermic Needle - 23G x 1.5
23G needle

What are the steps in effecting the filtration and infusion of the blood? 

1. Get hold of a Y-type tubing. Hook one tail of the Y-type tubing with 0.9% standard saline solution and prime the tubing.

2. Fill the drip chamber, the filter area, and the tubing from the standard saline solution bag. Clamp the patient-line and the solution line.

3. Ensure that the filter is completely saturated with fluid to function correctly; this step is significant and critical because it will help prevent the breaking down, damaging, and lysis of the blood components. 

4. Close the patient-line’s roller clamp to prevent the saline solution’s entry to the patient’s vein. 

5. Hook the other tail of the Y-type tubing with the blood bag or blood component bag.

6. Ensure that the patient-line is still closed. Hold the blood bag and the filter upright, with the blood bag slightly over the normal saline solution level. Open the blood clamp slowly to allow the blood to enter the drip chamber. 

7. Open the patient-line clamp and then adjust the flow to the desired rate.   

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