Is an elevated neurofilament light chain (NfL) level of 1.3 pg/mL (reference < 1.29 pg/mL) clinically significant in a 39‑year‑old man with memory complaints?

Minimal Clinical Significance in This Context

An NfL level of 1.3 pg/mL (just 0.01 pg/mL above the upper reference limit) in a 39-year-old man with memory complaints is not clinically significant and should not drive diagnostic or therapeutic decisions.

Why This Result Is Not Meaningful

The value is essentially at the reference threshold, falling within normal analytical and biological variation. The Alzheimer's Association guidelines explicitly excluded NfL from their 2025 blood biomarker recommendations for Alzheimer's disease diagnostic workup, prioritizing p-tau and Aβ markers instead 1. This exclusion reflects NfL's fundamental limitation: it is a non-specific marker of neuroaxonal injury that cannot distinguish between different neurological conditions 2.

Critical Age Context

At 39 years old, this patient is in a demographic where NfL interpretation is most reliable. The major challenge with NfL is its strong age relationship—levels increase substantially in people over 70 years due to multiple age-related changes 2. In younger patients like this 39-year-old, the diagnostic window is clearer, yet this value remains within normal limits.

What NfL Actually Tells You

NfL reflects neuroaxonal damage across multiple conditions:

• Highest elevations: ALS, frontotemporal dementia, atypical parkinsonian disorders, MS, HIV-associated neurocognitive dysfunction 2
• Modest elevations: Alzheimer's disease (but mainly at advanced dementia stages) 2
• Confounding factors: Renal function, BMI, peripheral neuropathy, cardiovascular disease 2

In sporadic AD, NfL shows large overlap between disease stages and is primarily useful at more advanced dementia stages, not for early memory complaints 2.

Practical Clinical Approach

For this 39-year-old with memory issues:

1. Do not rely on this NfL result for diagnostic decisions—it provides no actionable information at this borderline level

2. Pursue AD-specific biomarkers if Alzheimer's pathology is suspected:

   • Plasma p-tau217 or p-tau181
   • Plasma Aβ42/Aβ40 ratio
   • These have superior diagnostic accuracy for AD pathology 1

3. Consider alternative etiologies given the young age:

   • Psychiatric disorders (depression, anxiety)
   • Sleep disorders
   • Medication effects
   • Metabolic/endocrine causes
   • Substance use

4. Assess confounding factors that could affect NfL:

   • Kidney function (eGFR)
   • BMI
   • Recent head trauma
   • Peripheral neuropathy symptoms 2

When NfL Would Be Useful

NfL is perceived as clinically useful in memory clinics primarily for:

• Patients under 62 years (this patient qualifies by age) 3
• Confirming or excluding neurodegeneration when levels are clearly elevated (not borderline) 3
• Monitoring disease progression in established neurodegenerative conditions 4

However, a value barely above the reference range provides none of these benefits.

Common Pitfalls to Avoid

• Over-interpreting minimal elevations: Values within analytical variation (typically ±10-15%) are not clinically meaningful
• Ignoring age-adjustment: Reference ranges must be age-specific; this 39-year-old's threshold differs from older adults 5, 4
• Using NfL alone for AD diagnosis: It cannot differentiate AD from other neurodegenerative conditions 6, 7
• Neglecting biological variation: Day-to-day fluctuations and confounders affect interpretation 2

In summary: This borderline NfL elevation of 1.3 pg/mL should be disregarded. Focus diagnostic efforts on AD-specific biomarkers (p-tau, Aβ) if neurodegenerative disease is suspected, or pursue alternative causes of memory complaints appropriate for a 39-year-old patient.