Daubert Gatekeeping Must Evaluate a Physician’s Differential Diagnosis as a Whole (and Not Exclude Causation Opinions Solely Because a Supporting Quantitative Test Is Disputed)

Introduction

Walton v. Huron Regional Medical Center, Inc., 2026 S.D. 3, is a South Dakota Supreme Court decision arising from a medical malpractice action brought by Kevin Walton and Julie Walton against Huron Regional Medical Center (HRMC) and William J. Miner, M.D.. The Waltons alleged Kevin suffered a hypoxic brain injury after receiving high-dose opioid pain medications and being inadequately monitored during an April 2018 hospitalization for severe testicular pain.

The key evidentiary issue was whether the circuit court properly excluded the Waltons’ causation expert (Dr. Richard Adler) under SDCL 19-19-702 and Daubert. After excluding Dr. Adler, the circuit court granted summary judgment to HRMC and Dr. Miner on the ground that the Waltons lacked admissible expert evidence on causation. The Supreme Court reversed in part (allowing Dr. Adler’s causation testimony, including his reliance on qEEG as part of a differential diagnosis) and affirmed in part (excluding Dr. Adler’s testimony relying on Neurocloud-VOL and Neurocloud-PET).

Summary of the Opinion

• The Supreme Court held the circuit court abused its discretion by excluding all of Dr. Adler’s opinions while analyzing reliability only through the lens of certain quantitative tests (especially whether they “diagnose hypoxia”), instead of applying Daubert to the differential diagnosis methodology Dr. Adler used to rule out conversion disorder and rule in hypoxia.
• The Court held the circuit court did not abuse its discretion in excluding Dr. Adler’s testimony about Neurocloud-VOL and Neurocloud-PET, because the Waltons did not meet their burden to show those tools were reliable under Daubert (testability, peer review, error rate, etc.).
• Because Dr. Adler’s causation testimony (and Dr. Stein’s unchallenged opinions) should not have been excluded, the Court held there is a genuine issue of material fact on causation; thus summary judgment for HRMC and Dr. Miner was reversed and the case remanded.

Analysis

Precedents Cited

1) Standard of review framing

• State v. Fisher (2011 S.D. 74): Provided the abuse-of-discretion standard for admitting/excluding expert testimony. This mattered because the Court’s central holding is that the circuit court stepped outside permissible choices by misapplying Daubert to the wrong “method.”
• State v. Pretty Weasel (2023 S.D. 41): Supplied the definition of abuse of discretion as an arbitrary or unreasonable decision outside the permissible range—language the Court used to characterize the circuit court’s misapplication of the evidentiary rule.
• North Star Mut. Ins. v. Korzan (2015 S.D. 97): Anchored the de novo standard for summary judgment and the governing “no genuine issue of material fact” test, which became dispositive once Dr. Adler’s testimony was reinstated.

2) South Dakota’s Daubert framework and the gatekeeping role

• Tosh v. Schwab (2007 S.D. 132): Placed the burden on the proponent to prove admissibility (competent, relevant, reliable) by a preponderance of the evidence. The Court invoked this burden explicitly when affirming exclusion of Neurocloud-VOL and Neurocloud-PET due to a lack of foundational reliability evidence.
• Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993): The controlling admissibility standard. The Supreme Court’s critique was not that Daubert was ignored, but that it was applied to the wrong “target”—focusing narrowly on whether qEEG “diagnosed hypoxia,” instead of whether Dr. Adler’s overall reasoning process (differential diagnosis) reliably supported his causation conclusion.
• State v. Lemler (2009 S.D. 86): Reaffirmed Daubert in South Dakota and the “reliable foundation” and “relevant to the task at hand” formulation; also emphasized flexibility in Daubert factor application. This flexibility underpinned the Court’s conclusion that a differential diagnosis—while not reducible to a single lab test—can still be evaluated for reliability.
• State v. Guthrie (2001 S.D. 61): Quoted for reliability being grounded in science rather than subjective belief, and for the key remedial point: “When a trial court misapplies a rule of evidence … it abuses its discretion.” That misapplication rationale is the engine of the reversal.
• Kostel v. Schwartz (2008 S.D. 85): Cited for the gatekeeping duty to screen unreliable expert opinion, and later for the malpractice requirement of expert testimony to avoid verdicts based on speculation and conjecture. The decision uses Kostel both to justify robust evidentiary screening and to explain why exclusion of causation testimony typically drives summary judgment in malpractice cases.
• Glastetter v. Novartis Pharms. Corp., 252 F.3d 986 (8th Cir. 2001): Used to describe the gatekeeping function as separating “good grounds” from speculation masquerading as science. The Court used this to show the goal is not to “pick winners” among competing expert conclusions, but to ensure the reasoning is methodologically defensible.
• City of Pomona v. SQM N. Am. Corp., 750 F.3d 1036 (9th Cir. 2014): Quoted for a limiting principle on exclusion—courts should screen “unreliable nonsense,” but not exclude opinions merely because they are impeachable. This concept supported allowing Dr. Adler’s causation testimony even though defendants challenged parts of the data he considered.
• State v. Huber (2010 S.D. 63) and Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137 (1999): Provided the nonexclusive Daubert-factor list and emphasized those factors are flexible and not mandatory in all cases. This flexibility was significant because the Court concluded the circuit court’s analysis was too rigid and too narrow (treating “can this test diagnose hypoxia?” as the controlling question).

3) Differential diagnosis as a reliable methodology

• Westberry v. Gislaved Gummi AB, 178 F.3d 257 (8th Cir. 1999): Defined differential diagnosis/etiology and described its typical components (exam, history, clinical tests, and elimination of alternatives). The Court used Westberry to support the proposition that differential diagnosis is a recognized scientific technique, not merely subjective clinical intuition.
• Turner v. Iowa Fire Equip. Co., 229 F.3d 1202 (8th Cir. 2000): Cited for the general federal-circuit proposition that a reliable differential diagnosis typically satisfies Daubert and can be a valid foundation for expert causation testimony.
• Johnson v. Mead Johnson & Co., LLC, 754 F.3d 557 (8th Cir. 2014): Supported the idea that experts need not rule out all possible causes and that differential opinions can remain reliable even with less than full information—important here because defendants emphasized Dr. Adler’s incomplete review of all medical records.
• In re Paoli R.R. Yard PCB Litig., 35 F.3d 717 (3d Cir. 1994): Used for two complementary principles: (1) a doctor need not employ every diagnostic technique for a differential diagnosis to be reliable, but (2) where few standard techniques are used, the expert should provide a sound justification. This helped the Court frame Dr. Adler’s approach as one that could be evaluated as a whole, not invalidated by attacking a single component.

4) qEEG and the Frye/Daubert distinction

• Frye v. United States, 293 F. 1013 (D.C. Cir. 1923), and State v. Wimberly, 467 N.W.2d 499 (S.D. 1991): Mentioned to note the historical “general acceptance” standard that some jurisdictions used when excluding qEEG-related opinions.
• State v. Hofer, 512 N.W.2d 482 (S.D. 1994), and State v. Moeller, 1996 S.D. 60: Invoked to emphasize South Dakota’s adoption of Daubert and rejection of general acceptance as a strict requirement. This is crucial to the Court’s treatment of qEEG: the question is not whether qEEG is universally accepted as a standalone diagnostic tool for hypoxia, but whether it can be reliably used for a relevant purpose within the expert’s reasoning process.

5) Causation proof in malpractice and summary judgment

• Hanson v. Big Stone Therapies, Inc. (2018 S.D. 60): Provided the negligence elements and, most importantly, the causation burden in malpractice: proof must be “more probable” than alternative explanations, and causation is usually for the jury unless facts allow no difference of opinion. Once Dr. Adler and Dr. Stein were credited as admissible, Hanson’s jury-centric causation principle supported reversal of summary judgment.
• Kostel v. Schwartz (2008 S.D. 85): Reiterated that malpractice negligence and causation must generally be established by medical experts and not speculation—explaining why the evidentiary ruling on Dr. Adler effectively controlled the summary judgment outcome.

Legal Reasoning

1. The circuit court identified the correct overarching method (differential diagnosis) but assessed the wrong “unit” for reliability. The Supreme Court agreed the circuit court accurately described Dr. Adler’s clinical workflow (history, exams/tests, analysis, differential diagnosis, ruling out alternatives, final diagnosis). The error occurred when the circuit court effectively treated the quantitative tools (qEEG and Neurocloud) as the methodology itself and excluded the entire causation opinion based on that narrow focus.
2. The Court drew a key distinction between (a) using qEEG to diagnose hypoxia and (b) using qEEG to inform the differential diagnosis by identifying a biological basis for symptoms. The record showed Dr. Adler and Dr. Center did not present qEEG as a standalone diagnostic test for hypoxia; rather, Dr. Adler used it (and other data) to evaluate whether conversion disorder (a functional diagnosis often premised on the absence of an organic basis) should be ruled out and whether an organic injury should be ruled in. The Supreme Court held the circuit court’s “qEEG must diagnose hypoxia” framing misapprehended the proffered purpose and therefore misapplied Daubert.
3. Selective affirmance: Neurocloud evidence was properly excluded for failure of proof. The Supreme Court separated Dr. Adler’s causation opinion from the specific Neurocloud tools. Because Dr. Adler could not testify to foundational reliability markers (testing, peer review, error rates) and the Waltons’ showing was thin, the Court held the circuit court acted within discretion in excluding Neurocloud-VOL and Neurocloud-PET. This portion of the opinion reinforces that the “as-a-whole” approach is not a free pass: particular techniques still must be supported if they are offered as reliable scientific bases.
4. Summary judgment fell once the expert-causation exclusion fell. After reinstating admissible causation testimony (Dr. Adler, plus Dr. Stein’s standard-of-care and causation opinions), the Court held the Waltons created a genuine issue of material fact on causation. Accordingly, summary judgment could not stand.

Impact

• Clarifies how South Dakota courts must analyze medical causation opinions built on differential diagnosis. Walton makes explicit that Daubert reliability review cannot stop at evaluating a single disputed test when the expert’s actual methodology is differential diagnosis. Trial courts must evaluate whether the differential diagnosis process— including how alternatives are considered and excluded—is reliably applied to the facts.
• Enables more nuanced admissibility outcomes (partial exclusion rather than total exclusion). The split result (qEEG-related use permitted; Neurocloud excluded) signals that trial courts should consider surgical remedies: exclude unsupported components while allowing the remaining reliably grounded reasoning to reach the jury.
• Likely increases the role of “fit” and purpose-specific reliability in expert disputes. Future litigants will likely frame arguments around the purpose for which a tool is used (e.g., “to identify organic dysfunction” versus “to diagnose a specific etiology”), and courts will be pushed to articulate that “fit” analysis.
• Raises the bar for emerging or proprietary quantitative tools. The Court’s treatment of Neurocloud-VOL and Neurocloud-PET underscores that novelty or limited clinical uptake will require clear evidence on testing, peer review, and error rates—especially where results diverge from established, “known reliable” diagnostics.

Complex Concepts Simplified

Daubert / SDCL 19-19-702 gatekeeping

The judge must screen expert opinions to ensure they are based on reliable methods applied reliably to the case, rather than speculation. The judge is not supposed to decide which side is correct—only whether the expert’s approach is dependable enough for a jury to hear.

Differential diagnosis (differential etiology)

A standard medical reasoning process: list plausible causes of a patient’s symptoms, then use history, exams, and tests to rule causes in or out until the most likely cause remains. In court, it can be used to support causation if done rigorously and transparently.

qEEG (quantitative electroencephalography)

A computer-assisted analysis of EEG brainwave data, often comparing a patient’s electrical patterns to statistical “normative” databases. In this case, it was not used as a direct “hypoxia detector,” but as supporting evidence that brain functioning looked biologically abnormal—relevant to deciding whether symptoms were purely “functional.”

Conversion disorder / functional neurological disorder

A condition where neurological-like symptoms occur without a structural brain lesion explaining them. Because the diagnosis often relies on the absence of an organic explanation, evidence suggesting an organic abnormality can matter in ruling the diagnosis out.

Summary judgment

A pretrial ruling that ends the case if no reasonable jury could find for the nonmoving party on an essential element. In malpractice cases, if admissible expert testimony on causation is missing, summary judgment is often granted; if it exists, causation usually goes to the jury.

Conclusion

Walton v. Huron Regional Medical Center, Inc. establishes a practical evidentiary directive for South Dakota: when a medical expert bases causation on differential diagnosis, Daubert analysis must evaluate the reliability of that methodology as applied—not short-circuit the inquiry by disqualifying the entire opinion based solely on skepticism about a supporting quantitative tool’s ability to diagnose a particular condition.

At the same time, Walton confirms that proponents still bear a meaningful burden to demonstrate the reliability of particular scientific tools—illustrated by the affirmance excluding Neurocloud-VOL and Neurocloud-PET. The combined message is balanced: South Dakota courts must be careful gatekeepers, but they must also avoid excluding otherwise reliable medical causation reasoning merely because parts of it are contestable and therefore “impeachable” at trial.