Maryland medical malpractice law places a limitation on who can testify in the cases to certify and/or testify to the standard of care to those medical providers who devote no more then 20% of their professional time to testifying in personal injury cases. Specifically, MD Code, Courts and Judicial Proceedings, § 3-2A-04 b(4)

A health care provider who attests in a certificate of a qualified expert or who testifies in relation to a proceeding before an arbitration panel or a court concerning compliance with or departure from standards of care may not devote annually more than 20 percent of the expert's professional activities to activities that directly involve testimony in personal injury claims.

The issue was addressed in  in Witte v. Azarian where the court addressed  what kind of activities “directly involve testimony in personal injury claims,” within the meaning of CJ section 3-2A-04(b)(4), so as to constitute the numerator in the 20 Percent Rule. The court reasoned that the operative statutory phrase at issue (“activities that directly involve testimony in personal injury claims”), was ambiguous, as its meaning could not be ascertained from its plain language, and that, in light of the legislative history of the Act as amended, including the amendments establishing the certificate of qualified expert requirement, that language had to be read narrowly, so as to avoid “creat[ing] an unreasonable impediment to the pursuit, or defense, of a common law right of action” for medical negligence. The Court determined a standard to determine the issue as follows:

A more reasonable approach, we think, is to regard the statute as including only (1) the time the doctor spends in, or traveling to or from, court or deposition for the purpose of testifying, waiting to testify, or observing events in preparation for testifying, (2) the time spent assisting an attorney or other member of a litigation team in developing or responding to interrogatories and other forms of discovery, (3) the time spent in reviewing notes and other materials, preparing reports, and conferring with attorneys, insurance adjusters, other members of a litigation team, the patient, or others after being informed that the doctor will likely be called upon to sign an affidavit or otherwise testify, and (4) the time spent on any similar activity that has a clear and direct relationship to testimony to be given by the doctor or the doctor's preparation to give testimony.
 

Furthermore the case of Waldt v. University of Maryland Medical System Corp. 181 Md.App. 217, 956 A.2d 223 Md.App.,2008. addressed the meaning of expert's professional activities.