A patient presents with redness and swelling. There had been a wound there 2 weeks before and the patient is worried about retained foreign body. Xrays are negative for foreign body. The student tells the patient “Good news! The xrays show there is no foreign body there!”
You wince a bit. Every mature clinician must know the limitations of the tools they use. I want to take a divining rod to the beach and see if it knows which direction is the ocean. If it doesn’t, that doesn’t mean the ocean is a mirage. It means the tool doesn’t work perfectly.
You clarify the student’s comments. The xrays do not show there is no foreign body. Instead they merely are negative. You manipulate the swelling and it reproduces pain. That is the hallmark of an infected foreign body (or any sharp foreign body, infected or not). Alas, anything infected hurts when you manipulate it, so this rule is not very specific.
You tell the patient there could be a foreign body there. You ask more about the history and it turns out there could be some plastic in there. Ultrasound is suggestive enough that you choose to dissect, and indeed uncover a broken piece of plastic.
-Always acknowledge the limitations of imaging studies (or any studies)
-Foreign bodies are suggested by pain with manipulation