diagnostizieren
to diagnose
To identify an illness or medical problem in a person by analyzing symptoms and examination results.
Der Arzt diagnostizierte bei ihm eine schwere Grippe.
The doctor diagnosed him with a severe flu.

to diagnose
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to diagnose
To identify an illness or medical problem in a person by analyzing symptoms and examination results.
Der Arzt diagnostizierte bei ihm eine schwere Grippe.
The doctor diagnosed him with a severe flu.

The verb 'diagnostizieren' (to diagnose) belongs to the noun 'die Diagnose' (the diagnosis). The ending '-ieren' often signals a verb borrowed from a foreign language. These verbs are always regular and form their past participle with '-t' ('diagnostiziert').
Die Diagnose des Arztes war eindeutig.
The doctor's diagnosis was clear.
The context shows which meaning is intended. In medical texts with words like 'Arzt' (doctor), 'Patient' (patient), or names of illnesses, it is about the medical diagnosis. In contexts such as 'Unternehmen' (company), 'System' (system), or 'Gesellschaft' (society), the figurative meaning is intended.
Der Techniker diagnostizierte einen Fehler in der Software.
The technician diagnosed an error in the software.
In medical contexts, the person in whom something is identified is often indicated with the preposition 'bei' and the dative. The illness itself is the accusative object.
Bei dem Patienten wurde eine seltene Allergie diagnostiziert.
A rare allergy was diagnosed in the patient.
In the figurative meaning, the things that are usually 'diagnostiziert' (diagnosed) are abstract negative things, for example problems, errors, weaknesses, or a cause.
Die Analyse diagnostizierte grundlegende Schwächen im System.
The analysis diagnosed fundamental weaknesses in the system.
to put / to set
'eine Diagnose stellen' (to make a diagnosis) and 'diagnostizieren' (to diagnose) both belong to the medical field. Both describe that a doctor identifies and names which illness or problem someone has. 'eine Diagnose stellen' is a fixed expression with the noun 'Diagnose'. 'diagnostizieren' is the single verb and is often used directly with the illness. In terms of meaning, the words are very close, but the sentence structure is different: you say 'eine Diagnose stellen' (to make a diagnosis), or you 'diagnostizieren' (diagnose) an illness.

Der Arzt hat bei dem Patienten eine leichte Lungenentzündung diagnostiziert.
The doctor diagnosed the patient with mild pneumonia.
Nach der Untersuchung stellt die Ärztin eine klare Diagnose.
After the examination, the doctor makes a clear diagnosis.