Measurement of galactose-1-phosphate in dried blood spots by LC-MS/MS: method characteristics and therapeutic range.

Galactose-1-phosphate (Gal-1-P) is an important biomarker for diagnosis and therapeutic monitoring of classic galactosemia and related disorders. Measurement is traditionally performed in erythrocytes, requiring relatively large blood volumes and labor-intensive specimen processing. We developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for quantitation of Gal-1-P from dried blood spots (DBS). Analytical validation included evaluation of linearity, precision, stability, limit of detection (LOD), limit of quantitation (LOQ), and reference interval determination. DBS and erythrocyte specimens from patients undergoing routine galactosemia monitoring were analyzed in parallel to compare performance characteristics and establish a DBS-specific therapeutic range. The LOD and LOQ were 0.11 mg/dL and 0.17 mg/dL, respectively. The reference interval was established as ≤1.0 mg/dL. Comparison of paired DBS and erythrocyte samples demonstrated good correlation (R2 = 0.86) with an approximately 20% negative bias in DBS specimens. Based on 65 paired samples from treated individuals with galactosemia, a DBS therapeutic target of <4 mg/dL was established, which is lower than the therapeutic target of <5 mg/dL in erythrocytes. DBS-based measurement of Gal-1-P by LC-MS/MS provides a clinically reliable alternative to erythrocyte analysis with advantages including less blood volume required, simplified collection and shipping, and improved specimen stability. A DBS-specific therapeutic range is required because of systematic bias relative to erythrocyte measurements.