Can an attorney refuse to disclose the identity of a client in a civil proceeding?

"'[T]he authorities are clear that the privilege extends essentially only to the substance of matters communicated to an attorney in professional confidence. Thus the identity of a client, or the fact that a given individual has become a client are matters which an attorney normally may not refuse to disclose, even though the fact of having retained counsel may be used as evidence against the client.... To be sure, there may be circumstances under which the identification of a client may amount to the prejudicial disclosure of a confidential communication, as where the substance of a disclosure has already been revealed but not its source.'" (In re Osterhoudt, supra, 722 F.2d at p. 594, quoting Colton v. United States (2d Cir. 1962) 306 F.2d 633, 637, italics added.)