Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. There are few options that can cause a bloody aspiration while doing an arthrocentesis (medical term for aspiration of knee fluid). • You may have developed hemarthrosis (blood in the joint space) if the fluid surgeon has aspirated is bloody since the first appearance. People with osteoarthritis with degeneration of articular cartilage are more prone to develop hemarthrosis in future. To rule out the cause, you may need to do an MRI. • If you have any blood disorder like hemophilia, etc., then also you have bloody aspiration. For which your internist already has asked you to do PT/aPTT. If the values are in normal range, there is very rare chance it may have caused of any blood disorder. • There is little likely that during aspiration if you accidentally prick a vessel, then also you will get bloody aspiration. But, here the fluid you got in syringe will be clear initially, then comes bloody. There are few noted cases of recurrence of hemarthrosis post knee replacement surgery. This is usually evaluated by femoral arteriography to rule out any vascular causes. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Anuradha Das