Age- and Sex-Specific Reference Intervals for Amino Acids and Acylcarnitines by Tandem Mass Spectrometry in a Kazakhstani Paediatric Population: An Indirect Data-Mining Study

Background/Objectives: Interpretation of amino-acid and acylcarnitine results in newborn screening depends on population-appropriate reference intervals (RIs), and no locally derived paediatric RIs have been published for the general Kazakhstan population. We aimed to establish the first age- and sex-specific RIs for a tandem mass spectrometry (MS/MS) panel measured from dried blood spots (DBSs) by flow injection analysis tandem mass spectrometry (FIA-MS/MS) in a Kazakhstani paediatric population using an indirect (data-mining) approach. Methods: All amino-acid and acylcarnitine results (1 January 2022 to the data freeze on 20 July 2026) were extracted from the consolidated laboratory information system of a 21-branch network; after cleaning, deduplication and selection of conditionally healthy subjects, RIs (2.5th, 50th, 97.5th percentiles with 90% bootstrap confidence intervals) were computed per CLSI EP28-A3c across CALIPER age strata, with the neonatal period split into 0–7/8–14/15–28-day strata, and cross-validated by the canonical refineR package run without flag-based selection, the CLIR/Mayo database and a comparable Turkish study. Results: From 687,726 results in 4198 patients, the conditionally healthy cohort comprised 3589 subjects; the large majority of analyte–age strata permitted full CLSI EP28 computation. Canonical refineR and flag-based estimates agreed within ±30% in 91% of key-marker limits (median relative difference 9.6%), indicating that flag-based selection did not materially bias the upper limits. Reference intervals are reported for all amino acids and all 36 acylcarnitines of the panel. The valine 97.5th-percentile upper limit exceeded its 250 µmol/L decision limit at 0–28 days (275.3 µmol/L). Conclusions: We provide locally derived and internally cross-checked paediatric MS/MS RIs, pending prospective verification, and flag two actionable findings: inter-site harmonisation of several key markers and the overlap of the upper-normal valine limit with its clinical decision limit.