My mother’s left lung had collapsed but doctors don’t know why?

If your D-dimer is chronically high what does it mean mums had xrays scans no blood clots but she has had 3 episodes of not being able to get her breath she cant walk far without getting out of breath her left lung has collapsed at the bottom and they don’t know why she is 83 and had her left hip half replacement in june. They say they cant find anything wrong

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