Plasma oxalate: comparison of methodologies.

Area of Science:

• Clinical Chemistry
• Nephrology
• Analytical Chemistry

Background:

• Accurate blood oxalate measurement is vital for managing primary hyperoxaluria (PH) patients, especially those with kidney impairment or on dialysis.
• The absence of an external quality assurance (EQA) scheme for plasma oxalate analysis poses challenges for consistent patient monitoring.

Purpose of the Study:

• To assess the agreement of plasma oxalate measurements between laboratories involved in primary hyperoxaluria investigation.
• To identify factors contributing to discrepancies in plasma oxalate quantification.

Main Methods:

• A sample exchange scheme involving six laboratories was conducted.
• Participating laboratories utilized diverse analytical methods: gas chromatography/mass spectrometry (GCMS), ion chromatography with mass spectrometry (ICMS), and enzymatic methods.
• Reproducibility, linearity, and inter-laboratory agreement of results were evaluated.

Main Results:

• While individual laboratories demonstrated good reproducibility and linearity, significant poor agreement in absolute plasma oxalate values was observed between centers.
• Discrepancies were partly attributed to calibration differences but primarily to lower sample recovery rates, particularly with ultrafiltration.
• Longer-term patient data comparison confirmed inter-center variability in reported plasma oxalate concentrations.

Conclusions:

• Longitudinal monitoring of primary hyperoxaluria patients with declining kidney function should be conducted by a single, consistent laboratory using a defined methodology.
• Plasma oxalate concentrations reported in registry and risk-associated studies require careful interpretation considering the analytical methods employed.
• The development of a standardized reference method and an EQA scheme for plasma oxalate analysis is highly recommended to improve diagnostic accuracy and patient care.