“Dies” refers back to “Therapieansprechen” (bzw. -erfolg), therapeutic success, which is sometimes difficult to measure in objective terms.

My suggestion for the sentence:
"A limited cognitive assessment may possibly also provide objective evidence of therapeutic success."

Don't trust anyone who comes up with howlers like "effektiviert" and "objektiviert".

German term (edited): vielleicht lässt dies objektiv festhalten

I see these as having like meanings: objektivieren, festlegen, objektiv festhalten.

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Note added at 1 hr (2018-05-09 06:45:42 GMT)
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Or instead of carried out, realized or actualized.

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Note added at 6 hrs (2018-05-09 12:00:54 GMT)
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Dies should refer to Zielkriterium, so festlegen and company would be to determine, ascertain, etc.

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Note added at 3 days 3 hrs (2018-05-12 08:38:29 GMT)
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Add "Sicherung" to my list. The text is rather repetitive, so I'm just following the pattern.
With Lancashireman's input in the discussion, I realized that "objectiv festhalten" is most likely another way of saying "festlegen" or "festschreiben." "Dies" is, then, best anchored to the text if it refers to "ein umschriebenes Zielkriterium."
https://dict.tu-chemnitz.de/
etw. festlegen; statuieren; festschreiben {vt} / to lay down ↔ sth.

Based on my experience reading and editing scientific articles written for an international audience of other science-minded people, I would use "clearly defined," as "objectively" could be taken the wrong way.

https://en.wikipedia.org/wiki/Encephalopathy
Encephalopathy is a difficult term because it can be used to denote either a disease or finding (i.e., an observable sign in a patient).

The ultimate Ziel is "Sicherung der Diagnose," which is plain difficult for an autoimmune disease. The author is saying that, for this patient, the reaction to a potential therapeutic treatment is the only means of getting a sure diagnosis. That's why this whole passage seems backwards.
Die Sicherung der Diagnose kann in so einer Situation letztendlich nur durch das potentielle Ansprechen auf eine Therapie erfolgen. Da es sich bei der Patientin vor allem um Symptome im Bereich der Kognition und des Verhaltens handelt, ist ein Therapieansprechen nicht ganz einfach zu objektivieren.

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Note added at 3 days 4 hrs (2018-05-12 09:25:48 GMT)
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Not part of your question, but this may also help:
Ein Therapieansprechen ist nicht ganz einfach zu objektivieren.
> Measuring/Gauging/Observing the response to a given therapy is not a simple task.

https://rd.springer.com/article/10.1007/s00337-007-0504-2
Triggerpunkte (TrP), kleine palpable Verhärtungen im Muskel mit Schmerzausstrahlung, spielen in der manuellen Medizin eine wichtige Rolle, da sie meist mit weiteren Funktionsstörungen im Bewegungssystem in Zusammenhang stehen. Zu ihrer Objektivierung wurde ein Messgerät zur Bestimmung der Gewebespannung, ein Tissue-Tensiometer (TTM), entwickelt... Das TTM erscheint geeignet, palpable Triggerpunkte für Studienzwecke zu „objektivieren“.

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Note added at 3 days 22 hrs (2018-05-13 04:08:53 GMT)
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To continue, the Zielkriterium may be a certain score on a small battery of cognitive tests (as opposed to full cognitive testing).
I think the rest falls into place. Apologize for the long string. This is indeed a difficult text. I might have tied it all together earlier with more help.

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Note added at 4 days (2018-05-14 01:59:06 GMT)
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To me (Ausländerin), the structure and diction appear nonstandard. This is what I think each sentence essentially means.
1. Clinical suspicion of encephalopathy linked to autoimmune disease or autoimmune encephalopathy. (Not sure about entire sentence, since Verdachtsdiagnose is repeated, but some type of Diagnose fits.)
2. Biochemical testing and MRI to confirm diagnosis did not find anything wrong. (Notice that "fehlende positive" is not the technical term "false positive" and should be completely reworded or omitted.)
3. This does not mean that the patient does not have an autoimmune disease.
4. Now, response to a potential therapeutic treatment is the only means of getting a sure diagnosis.
5. With this solution, there is a problem: picturing/gauging the response to therapy is not easy. (Better may be "quantifying the effect of therapy.")
http://care.diabetesjournals.org/content/35/2/446
6. I suggest that before trying trial immunoglobulin therapy, you and the treating psychologist and preceptors stipulate the criterium for success of the treatment.
7-8. This criterium could factor in the results from simple cognitive testing after the trial immunoglobulin therapy compared to the results before the two-week therapy.

In conclusion: If the therapy is considered successful, then the patient may be considered as having the disease.

By the way, I studied Philosophy, mainly Modern and Psychological, at a physical/concrete/material, if what they say is true, university for 4 years, so I'm particularly trained at recognizing when things aren't as they seem or need further clarification.