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"What is the minimum amount of blood/fluid needed to determine infection?"
my knee was aspirated with a syringe after 3 to 4 minutes with the plunger pulled back it produced .5 cc of blood
The answer to your question depends on the requirements of the specific laboratory for which your fluid will be analyzed, so it is important to speak with your orthopedic surgeon. I can say that for many laboratories, 0.5 mL of fluid might be just enough to look for signs of infection. However, it might not and even if it is it might not be enough for other studies that your doctor might find to be more important. I think that the tests that your doctor will send off with this limited amount of fluid will depend somewhat on what the differential diagnosis is. If your doctor is very concerned about infection, then two things will probably need to be done. First, all the fluid that is currently aspirated will need to be sent off specifically for infectious studies. In addition, it is possible that you will need to have a second knee aspiration to look for additional fluid that might be sent off. My sense is that if you did not have very much fluid in the knee, then infection is less likely. Perhaps your doctor will look for other signs or causes of the pain such as crystals. For this problem, I suggest that you meet with your orthopedic surgeon or primary care physician, whoever aspirated your knee.
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