No, a doctor would be considered an "expert witness" so long as his statements were within his area of expertise.
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A doctor is not an officer of the court. However, a doctor may sign an affirmation rather than having to do an affidavit (which must be notarized) unless the doctor is a party to the case.
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There is not enough information to answer this question.
A court appointed professional who testifies or provides "affidavits" to the court is not an "officer of the court", but rather is closked in immunity for his assertions.
So if a charlatan "doctor" rigs up his testimony to slant one way, he is immune though he is not an officer of the court. If you try tosue such an affiant, your case will be dismissed.