For more than two decades, the diagnosis of narcolepsy type 1 has rested on a single laboratory measurement: the concentration of the neuropeptide hypocretin-1 (orexin-A) in cerebrospinal fluid, determined by radioimmunoassay. A value below 110 pg/mL is considered diagnostic. But the assay itself has long been a source of unease. Radioimmunoassay (RIA) does not measure orexin-A directly, it measures antibody binding, which can be influenced by cross-reactivity, peptide fragments, and batch-to-batch variability in kit reagents. In a review published online July 12 in the journal Sleep, Peijun Xue and Fang Han of Peking University People’s Hospital and the PKU-IDG/McGovern Institute for Brain Research make the case that the field is ready to move on.
The article, titled “From radioimmunoassay to mass spectrometry: redefining hypocretin/orexin measurement in narcolepsy,” arrives at a moment when the transition is already underway. Over the past decade, a series of studies have compared RIA results with those obtained by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and the discrepancies have been striking.
What the comparison studies found
In 2016, Hirtz and colleagues published the first quantitative mass spectrometry assay for orexin-A in Scientific Reports, demonstrating that LC-MS/MS could measure the peptide with high specificity and a wide dynamic range. Five years later, Lindström et al. reported in Clinical Biochemistry that orexin-A concentrations measured by a commercial RIA were on average five times higher than those obtained by LC-MS/MS, a difference large enough to shift clinical classifications at the boundary of the diagnostic cutoff.
The most recent and perhaps most revealing data came from Maus et al., published earlier this year in Sleep Medicine. Using LC-MS/MS to analyze immunopurified CSF samples from 80 patients referred for clinical orexin testing, the researchers discovered that full-length orexin-A (a 33-amino-acid peptide) was not detectable in CSF at all. Instead, two stable N-terminal fragments, orexin-A<sub>1–14</sub> and orexin-A<sub>1–16</sub>, were the dominant species. When they compared quantitative LC-MS measurements of these fragments against the clinical RIA results, the correlation was strong (Spearman’s ρ = 0.91 to 0.94), and the two methods agreed on the diagnostic classification (orexin-deficient vs. not) 88% of the time.
This finding resolves a long-standing puzzle: RIA had been detecting orexin-A immunoreactivity all along, but what it was actually binding to was a set of peptide fragments, not the full-length hormone. Mass spectrometry clarifies the picture by identifying the exact molecular species being measured.
Why it matters
The transition from RIA to mass spectrometry carries significance beyond analytical chemistry. For one, LC-MS/MS offers a path toward standardization. RIA kits vary between manufacturers and lot numbers, making it difficult to compare results across centers or over time. A mass spectrometry-based assay, calibrated against synthetic peptide standards, can be reproduced with far greater fidelity.
There is also the question of accessibility. The current RIA for orexin-A is performed in only a handful of specialized laboratories worldwide. Mass spectrometry platforms are increasingly common in clinical biochemistry departments, raising the possibility that orexin testing could become more widely available, reducing the diagnostic odyssey that many narcolepsy patients currently face.
Xue and Han, who lead one of China’s major sleep medicine centers, frame the shift as inevitable. The accumulated evidence, they argue, supports adopting LC-MS/MS as the new reference method for CSF orexin quantification, with the understanding that the diagnostic cutoff will need to be re-established for the new technique.
Limits
The review itself does not present new experimental data. The diagnostic cutoff for a mass spectrometry-based assay has not been formally validated in a prospective multicenter study, and the correlation between fragment concentrations and clinical outcomes, beyond the binary classification of orexin deficiency, remains to be established.
Bottom line
A methodological review from Peking University makes the case that mass spectrometry is ready to replace radioimmunoassay as the standard method for measuring CSF orexin-A in narcolepsy diagnosis. The shift, already supported by a growing body of comparative data, promises greater accuracy, reproducibility, and wider availability of testing.
Source: Xue P, Han F. From radioimmunoassay to mass spectrometry: redefining hypocretin/orexin measurement in narcolepsy. Sleep. 2026 Jul 12:zsag190. doi: 10.1093/sleep/zsag190. PMID: 42436637.