Why patients don't use the nearest laboratory?

Hi,
I have built several big hospitals with a few separate  buildings. I have played this game over one thousand hours  and I am very annoyed about the fact that patients don't use the nearest lab but are walking to the farthest lab in the hospital which is located in different buildings. For example I made a department of infectious diseases and I build a lab there, because I find it sensible that those patients wouldn't be walking around the hospital.  Now my infectious patients use the farthest lab in hospital and other departments patients from different buildings use this lab.  I think that this problem could be solved somehow  and I hope it will be solved.  I wonder now on  which grounds patients choose the lab they use.

Best regards
Tuija