Lab Case 49

A 70 year old lady presents to your ED with a painful left knee. She has had three days of pain and swelling with intermittent fevers. On examination the knee is hot, red and swollen with a limited range of motion. She is apyrexial. She has severe difficulty weight bearing.

She has a history of recurrent UTI’s and is on prophylactic antibiotics.

You perform an aspirate of her knee which shows:


– volume 60 ml

– Description: turbid yellow


– Leucocytes  25000/uL

– Polymorphs  80%

– Mononuclear  20%

– RBC 1000/uL


1. Describe the abnormalities

2. Give 5 reasonable differential diagnosis

3. What 5 aspect of history are important?

4. What other investigations would you request?

5. What would you do next?