A serum assay for NELL1 membranous nephropathy that follows the disease

Design

Diagnostic assay development and validation using a luciferase immunoprecipitation systems immunoassay, with an independent evaluation cohort and longitudinal sampling

Population

Validation cohort of 20 biopsy-proven NELL1-positive and 20 PLA2R-positive NELL1-negative sera; independent cohort of 63 membranous nephropathy cases and 20 disease controls

Primary outcome

Sensitivity and specificity for NELL1-associated membranous nephropathy, and whether antibody levels track proteinuria

Effect

90% sensitivity, 100% specificity in validation, with better specificity than ELISA; six positives identified among 63 cases; antibody levels tracked proteinuria in three longitudinal cases

Membranous nephropathy is now a set of antigen-defined diseases rather than one entity, and NELL1 is one of the commonest non-PLA2R subtypes. Identifying it currently requires mass spectrometry or immunostaining of a biopsy, so it is diagnosed retrospectively, in centres with those facilities, and cannot be followed over time.

This group built a luciferase immunoprecipitation assay against an N-terminal fragment of NELL1 (amino acids 1-550), which outperformed the full-length protein. In a validation cohort of 20 biopsy-proven NELL1-positive and 20 PLA2R-positive, NELL1-negative sera, it gave 90% sensitivity and 100% specificity, agreeing with ELISA but with better specificity - the weakness that has kept ELISA out of primary diagnostic use. Applied to an independent cohort of 63 membranous nephropathy cases and 20 disease controls, it found six NELL1-positive cases. In three patients followed longitudinally, antibody levels tracked proteinuria.

That last point is what makes this a practice question rather than a laboratory one. If NELL1 antibody titres behave like PLA2R titres, then NELL1 membranous nephropathy becomes a disease you can monitor serologically - treat to immunological remission, detect relapse before proteinuria returns, and avoid repeat biopsies.

The restraint required is real: three patients is not a longitudinal validation, the test is not commercially available, and the cohorts are small and enriched. Nothing changes in the clinic this week. What changes is the expectation. When serological NELL1 testing does arrive, the right use is a baseline titre and serial monitoring, exactly as with PLA2R - and the reason to know now is that NELL1-associated disease carries different associations, including malignancy, so identifying the subtype is not an academic exercise.

• Ask whether antigen subtyping was done on any membranous nephropathy biopsy you receive
• NELL1-associated disease carries malignancy associations - subtype changes the workup
• Serological NELL1 testing is not yet commercially available; do not request it
• When it arrives, use it as PLA2R is used: baseline titre, then serial monitoring
• Ninety per cent sensitivity means a negative result will not exclude the subtype

The statistics, in plain English

Ninety per cent sensitivity and 100% specificity look excellent and come from 40 carefully chosen sera - 20 known positives and 20 known PLA2R-positive negatives - which is the easiest test a diagnostic assay ever faces. Real-world performance in unselected nephrotic patients, where other subtypes and non-membranous disease are in the mix, will be worse, and the wider cohort here found only six positives among 63 cases, too few to re-estimate accuracy. The longitudinal correlation with proteinuria rests on three patients, which is an observation rather than a validation.

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