Sure, here are some phrases that describe a medical condition that has a poor prognosis in 16 letters or less:
Terminale ziekte (Dutch for "terminal illness")
Onvervalbare aandoening (Dutch for "incurable condition")
Levensbedreigende ziekte (Dutch for "life-threatening illness")
Prognose is ongunstig (Dutch for "prognosis is poor")
Weinig hoop op herstel (Dutch for "little hope of recovery")
Ernstige, ongeneeslijke aandoening (Dutch for "serious, incurable condition")
Slechte prognose (Dutch for "poor prognosis")
Gezondheidstoestand verslechtert (Dutch for "health condition is deteriorating")
Korte levensverwachting (Dutch for "short life expectancy")
Uitgebreide ziekte (Dutch for "advanced disease")
These phrases all convey the idea that the medical condition is very serious and has a low chance of recovery. The specific phrase you choose will depend on the context and the nuance you want to convey.
Please note that I am not a medical professional and cannot provide any medical advice. If you are concerned about your health, please consult with a doctor.