When a familial Alzheimer's genetic panel comes back with a borderline finding, doctors do not treat it as a final answer. They classify it as a variant of uncertain significance (VUS) and work through a structured process — gathering more family evidence, consulting additional databases, and sometimes seeking a second laboratory opinion — before deciding what, if anything, the result means for the patient and their relatives. The familial Alzheimer's panel tests three genes — PSEN1, PSEN2, and APP — together rather than one at a time, because ARUP Laboratories' test guideline notes that the three genes cause overlapping early-onset symptoms and a well-designed multigene panel limits the number of uncertain results returned in the first place.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- Why a "Borderline" Result Happens in the First Place
- How Doctors Resolve the Uncertainty
- What a Borderline Result Means for Patients Right Now
- Why a Negative or Borderline Result Doesn't Close the Case
- How Often the Panel Finds an Answer at All
- Frequently Asked Questions
Why a "Borderline" Result Happens in the First Place
A borderline result means the lab found a real genetic change but cannot yet say whether it causes disease. Geneticists call this a variant of uncertain significance, and it sits between "pathogenic" (known to cause Alzheimer's) and "benign" (harmless). This outcome is not rare. A 2021 systematic re-evaluation of 66 APP, 323 PSEN1, and 63 PSEN2 variants applied the current classification framework and found that 94.91% of previously reported pathogenic variants held up, while 5.09% were downgraded to uncertain significance, according to Xiao et al.
in Frontiers in Aging Neuroscience. That means roughly one in twenty variants once thought to clearly cause disease turned out to need a second look — a reminder that genetic certainty in this field keeps shifting as more families are studied. Reclassification also happens because different labs do not always agree. One analysis cited in the same Frontiers in Aging Neuroscience review found clinically significant discrepancies between independent laboratories in 29% of interpreted variants. That gap is why doctors facing an uncertain result commonly request a re-analysis or a second lab opinion rather than accepting one report as the final word.
How Doctors Resolve the Uncertainty
Doctors do not rely on the DNA sequence alone to resolve a borderline call. They apply the ACMG-AMP framework, a standardized scoring system that weighs several independent types of evidence at once: how common the variant is in the general population, computer-based predictions of whether it disrupts protein function, laboratory studies of the actual protein, and whether the variant tracks with disease through the family tree — a process called segregation analysis, as described in the Xiao et al. evaluation. A published case illustrates how this works in practice. A novel PSEN2 missense variant was initially classified as uncertain.
Doctors upgraded it to "likely pathogenic" only after testing additional affected siblings to see whether they carried the same variant, reviewing brain-imaging biomarker results, and running further bioinformatic analysis, according to a case study cited through the Alzforum network. No single test moved the needle — it took several pieces of evidence pointing the same direction. Segregation testing, however, does not always settle the question on its own. Alzforum's mutation database describes a PSEN1 variant, c.*9C>T, carried by two affected sisters and a cognitively impaired brother, while an unaffected 76-year-old sister did not carry it. But because the mother's genotype was never known, the Alzforum entry notes the case remained suggestive rather than definitive. Doctors weigh this kind of incomplete family data as one input among several, not as proof by itself.
What a Borderline Result Means for Patients Right Now
A current guideline from the UK's NHS Genomics Education Programme states plainly that a variant of uncertain significance is not considered clinically actionable. That means a VUS alone should not be used to change a patient's medical management or to guide whether relatives pursue predictive testing. In practice, this keeps doctors from overreacting to an uncertain result. A family member will not be told they are destined to develop Alzheimer's, and a patient will not be started on a different treatment path, based on a variant that has not yet been reclassified.
The result stays on file, flagged for review as more evidence becomes available, rather than driving an immediate decision. This caution extends even to variants that get upgraded toward pathogenic. PSEN2 variants in particular show reduced penetrance, meaning a person can carry a PSEN2 variant linked to disease and still not develop Alzheimer's with certainty, per ARUP's test guideline. Doctors counseling a family through a borderline PSEN2 result need to explain that even a stronger classification does not guarantee an outcome.
Why a Negative or Borderline Result Doesn't Close the Case
The standard panel only sequences coding regions and the boundaries between exons and introns — the protein-building parts of each gene and their immediate edges. According to ARUP's test guideline, this design does not reliably detect large deletions, duplications, insertions, or deep intronic and regulatory variants that sit outside those boundaries.
That technical limit matters for interpretation: a borderline or even a negative result does not rule out an inherited cause the assay was never built to detect. A family with a strong early-onset history and an uncertain or negative panel result may still carry a hereditary variant that this particular test cannot see. Doctors who know this limitation are more likely to recommend additional testing methods or continued surveillance rather than closing the case on one negative report.
How Often the Panel Finds an Answer at All
Context helps explain why borderline results carry real weight. ARUP's sample test report states that its familial Alzheimer's sequencing panel identifies a causative mutation in roughly 60% to 80% of familial early-onset cases, with the detection rate falling as the age of onset rises.
PSEN1 contributes the largest share of identified mutations, ahead of APP and PSEN2. That yield range means a sizable minority of families with a strong early-onset history will not get a clear mutation at all — borderline or otherwise. Doctors use this baseline to set expectations before testing: a clean negative does not mean there is no inherited cause, and an uncertain variant is still more informative than no finding, because it at least identifies a specific change to track as research advances.
Frequently Asked Questions
Does a variant of uncertain significance mean a family member will develop Alzheimer's?
No. The NHS Genomics Education Programme states a VUS is not considered clinically actionable and should not by itself change medical management or predictive testing decisions.
Can a negative panel result rule out a hereditary cause?
Not necessarily. ARUP's test guideline notes the panel does not reliably detect large deletions, duplications, or deep intronic variants, so an inherited cause can exist outside what the assay checks.
Why do different labs sometimes disagree on the same variant?
Independent labs apply judgment to overlapping but not identical evidence, and one analysis cited in the Frontiers in Aging Neuroscience review found clinically significant discrepancies in 29% of interpreted variants.
Does testing more family members always resolve a borderline result?
Not always. Alzforum's mutation database describes a PSEN1 case where segregation data remained suggestive rather than definitive because one key family member's genotype was unknown.
