Full Opinion

USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 1 of 41 PUBLISHED UNITED STATES COURT OF APPEALS FOR THE FOURTH CIRCUIT No. 24-4565 UNITED STATES OF AMERICA, Plaintiff – Appellee, v. COLBY EDWARD JOYNER, Defendant – Appellant. Appeal from the United States District Court for the Western District of North Carolina, at Charlotte. Robert J. Conrad, District Judge. (3:22-cr-00180-RJC-SCR-1) Argued: January 30, 2026 Decided: August 12, 2026 Before KING, THACKER, and RICHARDSON, Circuit Judges. Affirmed by published opinion. Judge Richardson wrote the opinion, in which Judges King and Thacker joined. ARGUED: Kristen Marie Santillo, GELBER & SANTILLO PLLC, New York, New York, for Appellant. Amy Elizabeth Ray, OFFICE OF THE UNITED STATES ATTORNEY, Asheville, North Carolina, for Appellee. ON BRIEF: Fern Mechlowitz, GELBER & SANTILLO PLLC, New York, New York, for Appellant. Russ Ferguson, United States Attorney, OFFICE OF THE UNITED STATES ATTORNEY, Charlotte, North Carolina, for Appellee. USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 2 of 41 RICHARDSON, Circuit Judge: Colby Joyner, a licensed physician assistant, took a part-time, remote job with MedCare Staffing, Inc., a telehealth company. His job was simple: review patient files that the company sent him and sign forms ordering genetic tests for Medicare beneficiaries. Over ten months, Joyner signed orders for some 607 beneficiaries—people he certified were his “patients,” despite having spoken with only about 20 of them. For each file he reviewed, he was paid $12 to $15, whether or not he approved the testing. Based on his orders, laboratories billed Medicare for more than 14,600 tests—over $10 million in claims. A jury convicted Joyner of healthcare fraud under 18 U.S.C. § 1347, and of making false statements related to healthcare matters under 18 U.S.C. § 1035(a). And the district court sentenced him to 72 months’ imprisonment. Joyner asks us to undo his conviction and sentence. His main contention is that the district court deprived him of a fair trial by excluding documents relating to MedCare’s internal compliance efforts, by quashing trial subpoenas for four witnesses who invoked their Fifth Amendment privilege, and by overruling his objection to the prosecution’s rebuttal summation. He also says the jury was improperly instructed, the evidence was insufficient, and the Guidelines calculation was wrong. We find no reversible error. The district court acted within its discretion in excluding the compliance documents under Federal Rule of Evidence 403, whatever their marginal relevance may have been. It permissibly excused the four witnesses after a proper and particularized inquiry. Assuming the prosecution’s rebuttal improperly invited an adverse inference from the absence of witnesses who had asserted the Fifth Amendment 2 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 3 of 41 privilege, the isolated comment did not prejudice Joyner’s substantial rights. Joyner’s remaining challenges to his convictions and sentence fare no better. We therefore affirm. I. BACKGROUND Joyner graduated from a physician-assistant program and held a North Carolina physician-assistant license. In August 2018, while working full-time at a separate clinic, he took a remote, part-time position with MedCare Staffing, a telehealth company also known as Provider Partners or Telehealth Solutions (collectively, “MCS”). Thomas Harbin and Rhonda Polhill owned MCS. Dr. Kevin Smith served as its medical director, and Sara Smola as its telehealth-operations manager. MCS employed doctors, nurses, and physician assistants across the country. These employees used patient information supplied by MCS’s clients to support approvals for medical equipment and genetic testing. Joyner was one of these employees. Christopher White was the owner of two of MCS’s corporate clients: Helix and Whitewater. At first, Joyner reviewed files to determine whether patients qualified for durable medical equipment. A few months in, White’s companies were expanding to include genetic testing. In October 2018, White and Dr. Smith trained Joyner and others on documenting medical necessity for two kinds of genetic tests and on completing laboratory requisition forms, letters of medical necessity, and cover letters transmitting results. For cancer genetic testing, Joyner was instructed that a personal or family history of cancer satisfied medical necessity. For pharmacogenetic testing, he was taught that medical necessity was met if a patient took multiple medications and risked adverse drug events, or reported side effects. MCS initially told Joyner to call patients to confirm they wanted 3 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 4 of 41 testing. But in December 2018, Smola advised him that he was responsible only for “chart review and signature” and did not need to call patients unless there was a discrepancy, because MCS had “a team of in-house Medical Assistants and Registered Nurses to call patients.” J.A. 1184. Joyner reviewed files under these criteria until he resigned in July 2019, citing concerns about MCS’s practices. He was paid per file regardless of whether he approved testing, earning roughly $17,628 in total. Joyner never personally billed Medicare. A grand jury charged Joyner with one count of healthcare fraud, 18 U.S.C. § 1347, six counts of false statements relating to healthcare matters, 18 U.S.C. § 1035(a), and with aiding and abetting those offenses, 18 U.S.C. § 2. The indictment alleged that the scheme arose out of Joyner’s work for MCS—specifically, his signing laboratory requisition forms that White’s companies had filled out in advance and that were “provided to him by [MCS] and its clients.” Those forms concerned beneficiaries whom others had already pre- selected for genetic and pharmacogenetic testing. J.A. 19–21. The government’s theory was that the forms Joyner signed contained at least three false statements: that the beneficiaries were Joyner’s “patients,” that he would use the test results to pursue care for them, and that the tests were medically necessary. Before trial, the government moved in limine to exclude a set of defense exhibits: MCS’s internal policies and procedures, emails among MCS management documenting compliance efforts, MCS’s correspondence with the American Telemedicine Association (“ATA”) about the definition of a “patient encounter,” materials reflecting that MCS retained a Medicare-compliance consultant, and a legal opinion letter. J.A. 1246. The 4 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 5 of 41 government argued that these documents were irrelevant to Joyner’s intent because he never received or reviewed them. Joyner responded that they reflected MCS’s due- diligence and good-faith compliance efforts, which bore on the existence (or not) of the charged scheme. The district court granted the motion, reasoning that the charged scheme was “very tailored to the conduct and intent of Mr. Joyner, not MCS.” It added that any probative value was substantially outweighed by “a high degree of risk of confusion” and “unfair prejudice, by distracting the jury from the conduct and the intent of Mr. Joyner to [that of] others.” J.A. 153. Earlier on, Joyner had sought a court order requiring MCS to produce documents. In that request, Joyner had explained that his theory of the case was that MCS, Smith, and White “knew the Company’s business model was illegal but withheld that information” from providers like Joyner. J.A. 30. That theory proved consequential when Joyner later served trial subpoenas on four MCS witnesses—Smith, Polhill, White, and Smola—each of whom naturally sought to invoke the Fifth Amendment. The day before trial, counsel for Smith and Polhill told the court that both would assert their Fifth Amendment privilege if called, citing what they described as false allegations against them in Joyner’s pretrial filings. The court deferred the issue, and the trial began on June 8, 2023. In his opening statement, Joyner’s counsel told the jury that it would “hear . . . testimony” that his employer told him not to call patients, thereby relieving him of any duty to do so. He also said that the jury would “hear” Smith, Polhill, and Smola say that they thought White’s companies “were on the up-and-up,” but that the evidence would 5 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 6 of 41 show White was a “fraudster” who lied, cheated, and stole, and that Smith and Polhill “took it hook, line, and sinker.” J.A. 179, 182. The government’s case featured Medicare expert Stephen Quindoza, who testified that Medicare covers genetic and pharmacogenetic testing only when ordered by a treating physician who uses the results in the patient’s further treatment, and that Medicare did not cover tests ordered through an asynchronous encounter (one in which provider and patient never interact in real time). The jury also heard that White had warned providers not to order too many tests at once to avoid being “flagged,” described the approval process as “plug and play,” and instructed staff to avoid “no-no words” like “free” and “Medicare.” J.A. 302, 800–01, 1197, 1200. Mid-trial, the district court held an evidentiary hearing on the subpoenaed witnesses, directing Joyner’s counsel to have the witnesses present on Monday, June 12. White appeared through counsel and represented that he would invoke the Fifth Amendment as to any questions related to the charges against Joyner. Although he had pleaded guilty in the Middle District of Florida to a kickback conspiracy, his plea agreement protected him only in that district. Smola appeared in person, said that Joyner’s counsel had made “false accusations” against her and MCS, and confirmed that she would invoke the privilege. J.A. 606–07. Smith and Polhill did not appear; their attorney reported that Joyner’s counsel had told them they need not appear until June 13, and Joyner’s counsel admitted telling Smith as much. Given that Joyner’s counsel “excused” witnesses whom the district court had directed to appear, the court found “fault with the way defense counsel handled these proceedings.” J.A. 628. The district court also found that all four witnesses had reasonable 6 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 7 of 41 cause to apprehend a danger of self-incrimination—particularly given the accusations leveled in Joyner’s pleadings and opening statement—and quashed the testimonial subpoenas. It ruled in the alternative that exclusion was warranted under Federal Rule of Evidence 611(a) given defense counsel’s “lack of transparency with the Court” and “side agreement[s]” with the witnesses. J.A. 82–83. Joyner then testified in his own defense: He was inexperienced with genetic testing, believed MCS was a reputable company run by seasoned professionals, raised questions with management and was led to believe the questions reached Harbin and Dr. Smith, and never believed his work violated the law or Medicare rules. He acknowledged speaking with only about 20 of the more than 600 beneficiaries and that he “did not use” the test results. J.A. 791. After the defense rested, the prosecution began its rebuttal summation: [Y]ou just heard from defense counsel about how they put on a case. But think back to what they said at the beginning of this case. They told you that they will put evidence in front of you that will show that Colby Joyner is innocent . . . because [MCS] was tricked by Dr. White. And . . . they told you you’re going to hear evidence, you’ll see legal documents, showing that all their policies were vetted by lawyers. He said you were going to hear from all those folks, he put up photos of all those employees. Did you see any of that? Did any of that actually come into evidence? . . . No, it didn’t. It fell flat. It went nowhere. J.A. 867. The court overruled Joyner’s objection and gave no curative instruction. The court gave three instructions that matter here. Those instructions stated that a representation is false “if it is known to be untrue” or “when it constitutes a half-truth or effectively omits or conceals a material fact, provided it is made with the intent to defraud,” J.A. 882; that intent to defraud could be found if Joyner “purposefully closed his eyes to 7 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 8 of 41 avoid knowing what was taking place around him,” J.A. 884; and that Joyner could be found guilty on an aiding-and-abetting theory if he “join[ed] another person and perform[ed] acts with the intent to commit a crime,” J.A. 884–85. During deliberations, the jury asked, “When is someone considered a patient under a [physician assistant]?” The court answered that the term carried its “ordinary, everyday meaning.” J.A. 898–99. The jury then convicted Joyner on all counts. The district court imposed a 72-month sentence, and Joyner timely appealed. II. DISCUSSION Joyner raises a number of challenges on appeal. We address each one in turn and find none convincing. A. The District Court Permissibly Excluded The Internal MCS Documents We review evidentiary rulings for abuse of discretion, and “we will only overturn a ruling that is arbitrary and irrational.” United States v. Nsahlai, 121 F.4th 1052, 1060 (4th Cir. 2024) (quoting United States v. Farrell, 921 F.3d 116, 143 (4th Cir. 2019)). Even then, evidentiary errors are subject to harmless-error review, and an error is harmless if we can say “with fair assurance, after pondering all that happened without stripping the erroneous action from the whole, that the judgment was not substantially swayed by the error.” Id. (quoting United States v. Johnson, 617 F.3d 286, 292 (4th Cir. 2010)). Joyner argues that the district court abused its discretion by excluding documents showing MCS’s compliance efforts. That evidence included the company’s internal policies and procedures; emails among MCS leadership; MCS’s correspondence with the ATA about how that industry association “officially defines a patient encounter,” J.A. 8 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 9 of 41 1237; materials reflecting that MCS retained a Medicare-compliance expert and scheduled monthly compliance meetings; assurances to providers that MCS was “in constant pursuit of compliance,” J.A. 1255; and the attorney opinion letter concluding that MCS was “quite compliant with applicable law,” J.A. 1246. On Joyner’s telling, the documents did three things. They tended to rebut the existence of a scheme to defraud, undermined the inference that fraud would have been obvious to him, and—through the ATA correspondence—bore on whether calling the beneficiaries his “patients” was false at all. He claims that these exclusions denied him his constitutional due process right to present a defense. We begin with framing. Joyner invokes the Constitution, but his complaint is, at bottom, that particular exhibits should have been admitted. That claim is “better framed as an evidentiary argument,” subject to ordinary abuse-of-discretion review and harmlessness principles. United States v. Malloy, 568 F.3d 166, 177 (4th Cir. 2009); see also Nsahlai, 121 F.4th at 1059 n.7. And the record undermines any suggestion that Joyner was prevented from defending himself: He testified at length about his training, his communications with MCS management, his understanding of the forms, and his belief that the operation was lawful, and the jury heard MCS’s compliance assurances through other admitted evidence. What remains is a routine ruling on the admissibility of evidence. On that question, we need not embrace the district court’s relevance rationale to affirm its ruling. The court excluded the inter-employee emails as irrelevant, explaining that Joyner had never seen these emails and that the charge was “very tailored to the conduct and intent of Mr. Joyner, not MCS.” J.A. 153. The court also excluded the 9 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 10 of 41 company policies and legal memoranda as irrelevant, and ruled in the alternative that— even if relevant—the documents’ probative value was substantially outweighed by “a high degree of risk of confusion, unfair prejudice, by distracting the jury from the conduct and the intent of Mr. Joyner to [that of] others.” Id. We may sustain an evidentiary ruling on any ground that the record supports, whether the district court invoked it or not. United States v. Ferguson, 140 F.4th 538, 546 (4th Cir. 2025) (“Evidence rulings may be affirmed if they are correct—whether the district court explains why or not.”). So even if the district court relied on the wrong reason—or gave no reason—we ask only whether the ruling was right. Id. Here, the district court excluded the company policies and legal memoranda under both Rule 401 and Rule 403, and it excluded the inter-employee emails under Rule 401 alone. We need not parse those rationales, because the same Rule 403 balancing that the district court conducted supports excluding all of the challenged documents. We rest our decision there and express no view on the district court’s Rule 401 analysis. 1 Rule 403 permits a district court to exclude relevant evidence “if its probative value is substantially outweighed by a danger of one or more of the following: unfair prejudice, 1 The government contends that Joyner abandoned any Rule 403 argument by failing to brief it. Gov’t Br. 39. Perhaps. Joyner’s opening brief does mention Rule 403 and notes exclusion under it was “erroneous and an abuse of discretion.” Opening Br. 22. But Joyner provided argument only as to relevance under Rule 401. See Grayson O Co. v. Agadir Int’l LLC, 856 F.3d 307, 316 (4th Cir. 2017) (“passing shot” insufficient). Even so, we exercise our discretion to reject the argument on the merits. We add that even if we ignored the Rule 403 basis for excluding the inter-employee emails, their exclusion was harmless. The identified emails documenting what MCS’s officers said to one another could not have substantially swayed the judgment. See Nsahlai, 121 F.4th at 1060. 10 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 11 of 41 confusing the issues, misleading the jury, undue delay, wasting time, or needlessly presenting cumulative evidence.” Fed. R. Evid. 403. Our review thus asks only how a reasonable jurist could have struck that balance, not whether we would have done so in the first instance. See Nsahlai, 121 F.4th at 1060. Start with the probative-value side of the ledger. Whether the documents were probative depends on what the government had to prove. The government’s theory of the case had two strands. First, Joyner himself knowingly executed a scheme to defraud Medicare through the false forms he signed. Second, he aided and abetted an existing scheme—run through MCS’s clients and, above all, White—by supplying the one thing that scheme could not function without: a licensed provider’s signature. On either strand, evidence that MCS’s officers tried to comply with the law does little work for Joyner. A “scheme to defraud” requires “a plot, plan, or arrangement” executed through fraudulent transactions, United States v. Bajoghli, 785 F.3d 957, 962–63 (4th Cir. 2015), and aiding- and-abetting liability requires that the underlying offense was committed by someone whom the defendant knowingly and intentionally assisted. See United States v. Moye, 454 F.3d 390, 398 (4th Cir. 2006); Rosemond v. United States, 572 U.S. 65, 71 (2014). Nothing about either theory required the government to prove that MCS’s officers possessed a fraudulent intent. For one, White’s underlying scheme—channeling pre-selected Medicare beneficiaries through pre-populated requisition forms to laboratories that billed Medicare millions—existed, or not, regardless of whether MCS’s principals were knowing 11 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 12 of 41 participants, useful dupes, or something in between. So evidence that MCS’s officers acted in good faith does not make the charged scheme less probable. What’s more, the documents are worth little as direct proof of Joyner’s own state of mind with respect to the underlying scheme. Joyner concedes that he never saw these documents, and a document that he never saw cannot speak to what he believed. Indeed, Joyner remained free to introduce—and did introduce—evidence of everything he actually saw and heard. The excluded materials, by contrast, concerned matters he did not and could not have known about. That leaves the indirect inference on which Joyner seeks to ground his relevance theory. The government argued at trial that the red flags surrounding this operation—the training video, the “plug and play” forms, the warnings about being “flagged,” the absence of any real patient encounter—would have been open and obvious to any professional in Joyner’s position. J.A. 835–36, 874. Joyner argues that if MCS’s own sophisticated officers, armed with lawyers and compliance consultants, purported to believe the model was lawful, then the fraud was not obvious after all—which in turn makes it marginally less likely that Joyner recognized it. Joyner has a plausible argument that this evidence might have had some relevance. If MCS’s officers—professionals with the advice of counsel, a compliance consultant, and an industry association—believed in good faith that the model was lawful, a jury might infer that its illegality was not so obvious. Joyner, who had less information and expertise than they did, might have been less likely to have recognized its illegality. 12 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 13 of 41 But probative value is only one side of the Rule 403 balance. And the inference that this evidence arguably supports is a step removed from the question that mattered. What MCS’s officers believed turned on what they told their lawyer, what they asked the association, and what their consultant reviewed—none of which bears directly on the red flags Joyner himself confronted when he reviewed files for hundreds of strangers, signed forms calling them his patients, and certified tests he would never use. Thus, on the question that actually mattered—what Joyner knew or should have known—the documents carried materially less weight than he suggests. The question, then, is whether the district court could reasonably conclude that even this diminished value was substantially outweighed by the dangers of admitting the evidence. It could. Three such dangers stood on the other side of the ledger. Each was evident on the face of this record. So exclusion was a reasonable resolution of the balance. First, confusion of the issues. This is not the unremarkable observation that admitting the documents would have introduced contested facts; contested facts are the stuff of every trial, and a factual dispute is not itself a reason to exclude relevant evidence. Our concern is more specific. In order to draw the inference that the fraud was not so obvious because MCS’s officers thought the model was lawful, the jury would first have had to decide what those officers actually knew and believed: that is, whether they were knowing participants in White’s scheme, negligent dupes, or completely innocent. That is a question about the culpability of people not on trial, and a different question from the one that the jury was empaneled to answer: What did Joyner know? Litigating this question would have required the jury to probe what facts MCS disclosed to its lawyer, whether its 13 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 14 of 41 compliance program was genuine or mere window dressing, and what its principals understood about White’s operation—in essence, inviting the jury to assess the guilt or innocence of absent non-parties as a predicate to judging Joyner. Rule 403’s worry about “confusing the issues” and “wasting time” concerns precisely this sort of excursion, which threatened to “distract[] the jury from the conduct and the intent of Joyner to [that of] others.” J.A. 153. Second—and relatedly—the risk of misleading the jury, particularly with regard to the legal opinion letter. Whether MCS’s model complied with Medicare’s requirements was a question of law on which the jury would be instructed by the court and informed by expert witnesses, not by a retained attorney’s letter to his client. A lawyer’s opinion cannot make lawful what the law forbids, and a jury shown a legal opinion stating that MCS’s business model was compliant may struggle to keep that opinion in its proper place. 2 More problematically, the letter did not even offer a legal opinion about whether MCS’s platform complied with Medicare’s coverage rules. Rather, it addressed only the platform’s compliance with Georgia law and with federal laws against kickbacks and self-referrals. Furthermore, its conclusion that the company was “quite compliant with applicable law” was conditioned on assumptions that the opining attorney did not test: that the physicians would adhere to each state’s telemedicine requirements, including “the use of electronic visual access where required” and the generation of genuine patient medical records. J.A. 2 True, the letter might have carried weight as advice-of-counsel evidence, but only for someone who saw and relied on it. Again, Joyner never saw the letter, so it could not support a reliance defense for him; its only realistic function was to suggest, obliquely, that the operation was lawful or that someone else thought so. 14 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 15 of 41 1246. So the district court could reasonably conclude that the letter could mislead the jury far more than it could illuminate how obvious the fraud would have been to Joyner. 3 Third, cumulativeness. The jury did not lack evidence that MCS held itself out— internally and to its providers—as a compliant operation. It heard assurances from Dr. Smith and Polhill that MCS had “dotted their Is and crossed their Ts,” J.A. 702, and was “conducting a professional and compliant operation,” J.A. 722. And MCS’s written communications to providers, including the December 2018 email restructuring Joyner’s duties, were admitted into evidence. Joyner himself testified to the assurances he received and the trust he placed in MCS’s leadership. The excluded documents would have only layered additional, secondhand iterations of the same basic point—MCS’s purported compliance—at the cost of the dangers just described. This is why Rule 403 expressly allows a district court to decline “needlessly presenting cumulative evidence.” Fed. R. Evid. 403. Weighing these dangers against whatever thin probative value the excluded documents may have provided, the district court’s exclusion was neither arbitrary nor 3 That is true of the ATA correspondence no less than the rest of the documents. Joyner casts that letter as proof that “patient encounter” can encompass asynchronous review, and thus that calling the beneficiaries his “patients” was not false. But the governing coverage requirements were established at trial through Medicare’s own rules and expert testimony, and a trade association’s definitional gloss—solicited by MCS, and never seen by Joyner—invited exactly the collateral dispute over third-party understandings that the district court was justified to avoid. Furthermore, the government’s falsity theory never rested on the word “patients” alone: The forms also represented that Joyner would use the test results to pursue care for the beneficiaries and that the tests were medically necessary, representations that Joyner’s own testimony undercut. 15 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 16 of 41 irrational. Nsahlai, 121 F.4th at 1060. Thus, the district court did not abuse its discretion in excluding these documents under Rule 403. 4 B. The District Court Permissibly Quashed The Testimonial Subpoenas A district court’s decision to quash the subpoena of a witness who will invoke his Fifth Amendment privilege is ultimately an evidentiary ruling reviewed under the same deferential standard, even when the defendant frames the exclusion as a denial of his right to present a complete defense. United States v. Oliver, 133 F.4th 329, 335 (4th Cir. 2025); see United States v. Branch, 537 F.3d 328, 342 (4th Cir. 2008). Joyner challenges the district court’s decision to excuse White, Smith, Polhill, and Smola from testifying after each invoked the Fifth Amendment privilege against self- incrimination. As Joyner sees it, the district court’s inquiry was insufficiently particularized in three ways. First, it did not proceed question-by-question with each witness in-person. Second, Smith, Polhill, and Smola faced no genuine incrimination risk once Joyner disclaimed any intent to accuse them of wrongdoing. Third, White, having already pleaded guilty and been sentenced in the Middle District of Florida, had nothing left to incriminate. Each fails. 4 Joyner separately contends that the district court abused its discretion by overruling his objection to the government’s cross-examination about his earnings as a physician assistant—questioning premised on salary figures never introduced into evidence, which the government later invoked in summation to portray Joyner as “one of the highest-paid physician assistants in the country, let alone North Carolina.” J.A. 873. We see no abuse of discretion. Joyner’s defense rested in substantial part on his professed inexperience and the modesty of his pay from MCS, so his compensation and professional standing were fair subjects of cross-examination. In any event, any error in permitting this brief exchange was harmless in light of the evidence of Joyner’s conduct. See Nsahlai, 121 F.4th at 1060. 16 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 17 of 41 Start with the governing principles. “A criminal defendant’s right to compel testimony is fundamental to sixth and fourteenth amendment due process rights.” Gaskins v. McKellar, 916 F.2d 941, 950 (4th Cir. 1990). But that right must yield to a witness’s valid privilege against self-incrimination. To police the boundary, “the trial judge must make a proper and particularized inquiry into the legitimacy and scope of the witness’ assertion of the privilege.” Id. A witness may be excused entirely only if he could legitimately refuse to answer every relevant question. Id. The privilege is generous in scope. It covers not only answers that would directly support a conviction but also those that “would furnish a link in the chain of evidence needed to prosecute” the witness. Hoffman v. United States, 341 U.S. 479, 486 (1951). At the same time, a witness’s bare “say-so does not of itself establish the hazard of incrimination.” Id. The hazard must be “substantial and real, and not merely trifling or imaginary.” Marchetti v. United States, 390 U.S. 39, 53 (1968) (cleaned up). And again: Whether a particular witness’s assertion clears that bar is committed in the first instance to the trial judge, whose ruling we review only for abuse of discretion. Oliver, 133 F.4th at 335. Here, the district court’s inquiry was sufficiently particularized. The court did not resolve the question on the papers the moment it arose. Rather, it cautiously deferred ruling until it could “talk to both sides.” J.A. 157. It then announced before the close of the government’s case that it would hold an evidentiary hearing. It also directed Joyner’s counsel to produce the subpoenaed witnesses on June 12. At that hearing, the court heard from Smola in person, who explained that a filing by Joyner’s counsel contained “false accusations” against her and MCS and confirmed her intent to invoke the privilege. J.A. 17 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 18 of 41 606–07. It received and considered the written submission of counsel for Smith and Polhill, which invoked the privilege in response to the same accusations. It heard from White’s counsel, who explained that his plea agreement foreclosed further charges only in the Middle District of Florida. And the court made witness-specific findings that each had reasonable cause to apprehend a danger of self-incrimination—findings it made with Joyner’s contrary representations in view. Joyner’s principal rejoinder is procedural: Because the privilege attaches question by question, he insists the district court was obliged to put each witness on the stand and rule question by question. See United States v. Miller, 434 F. App’x 287, 293 (4th Cir. 2011). That argument mistakes the privilege’s substance for a script. Our cases prescribe the substance of the inquiry, not its form: No case of ours requires live, seriatim questioning to satisfy that duty. See Oliver, 133 F.4th at 337–38; see also United States v. Ramos, 763 F.3d 45, 55 (1st Cir. 2014). What matters is whether the district court could reasonably conclude, on a witness-by-witness basis, that each could legitimately refuse to answer the full range of relevant questions Joyner proposed to ask. It could. Consider what Joyner wished to ask, and of whom. His own Rule 17 filing announced that his theory of the case was that MCS, Smith, and White “knew the Company’s business model was illegal but withheld that information” from Joyner and other providers, and that Smola had trained him to follow unlawful procedures. J.A. 30. His opening statement told the jury that White was a “fraudster” who lied, cheated, and stole. J.A. 182. And the topics he identified for examination—MCS’s structure, its relationship with White, its due diligence and compliance efforts, and the contents of its 18 USCA4 Appeal: 24-4565 Doc: 55 Filed: 08/12/2026 Pg: 19 of 41 training—called on these witnesses to give a sworn account of their own roles in designing and operating the very business model that the government was proving to be a vehicle for fraud. Answers about what they knew of White’s operation, what the laboratories were billing, what concerns were raised internally, and how the training and forms came to be could each “furnish a link in the chain” of a future prosecution. Hoffman, 341 U.S. at 486. Joyner responds by claiming that he had disavowed h