Study summary

This retrospective single-center study evaluated plasma neurofilament light chain (pNfL) measured in 253 people with multiple sclerosis using two automated assays:

• Roche Elecsys® NfL on cobas e801
• Fujirebio Lumipulse® G NfL Blood on LUMIPULSE G600

Main findings

• The assays were strongly correlated on the raw scale:
  • Pearson correlation: r = 0.879
• However, they showed poor absolute agreement:
  • Concordance correlation coefficient: CCC = 0.107
  • Mean concentrations differed substantially:
    • Lumipulse: 14.02 pg/mL
    • Elecsys: 1.50 pg/mL
• Therefore, the two assays should not be treated as interchangeable using raw pg/mL values.

Proposed conversion

The authors derived a directional Passing–Bablok conversion:

[
\text{Lumipulse-equivalent pNfL}

1.97 + 11.171 \times \text{Elecsys pNfL}
]

This equation is intended for conversion from Elecsys to Lumipulse values, within the analytical range studied. It should not automatically be assumed to work in the reverse direction or outside that range.

Simoa-referenced harmonization

Published equations were used to convert both platforms to Simoa-equivalent values:

• Elecsys-derived Simoa value:

[
6.31 \times \text{Elecsys} + 2.33
]

• Lumipulse-derived Simoa value:

[
0.94 \times \text{Lumipulse} + 1.30
]

Alignment improved after conversion, with a regression coefficient of approximately 1.03, suggesting better comparability on the harmonized scale. However, these conversions are literature-based and do not establish that either assay is clinically identical to Simoa.

Clinical validation of Elecsys

Higher Elecsys pNfL was associated with greater disability:

• EDSS coefficient: 0.302
• BMI-adjusted model: 0.335

Elecsys pNfL was also lower among treated than untreated patients, supporting expected biological and clinical validity in routine MS care.

Clinical interpretation

The practical message is:

1. Do not compare raw Elecsys and Lumipulse pNfL concentrations directly.
2. A patient switching platforms may appear to have a large change in pNfL solely because of assay bias.
3. During platform transitions, clinicians should:
   • Continue interpreting results using the assay-specific reference range;
   • Prefer longitudinal monitoring on the same platform;
   • Consider conversion equations cautiously when necessary;
   • Avoid interpreting converted values as fully interchangeable measurements.

Important limitations

• Retrospective, single-center design.
• All samples were measured in singlicate.
• Precision estimates such as cumulative intra- and inter-session CVs were not calculated.
• Only one reagent lot per platform was used.
• The Simoa conversions were based on published formulas rather than a direct three-platform calibration in the same samples.
• The Elecsys-to-Lumipulse equation is directional and may not generalize beyond the observed concentration range.
• Treatment status and disease characteristics may confound clinical associations.
• Improved regression alignment after conversion does not necessarily demonstrate complete clinical agreement or equivalent decision thresholds.

Overall, the study supports clinical validity of Elecsys pNfL in MS, but emphasizes that Elecsys and Lumipulse values are assay-specific and cannot be directly substituted on the raw pg/mL scale.