Measurement of serum lanthanum in patients treated with lanthanum carbonate by inductively coupled plasma-mass spectrometry

BACKGROUND: Lanthanum carbonate is used as a phosphate binder in patients with stage V chronic kidney disease (CKD). While well tolerated in clinical trials, with no toxicity reported as regards bone and liver metabolism, and cognitive function, concerns remain over possible toxicity. Published methods for the measurement of lanthanum ion in biological samples include aggressive and complicated sample preparation steps that are unsuitable for routine use. A simple method has been developed and validated for the measurement of serum lanthanum. METHOD: A ThermoFisher Scientific XSERIES-II inductively coupled plasma-mass spectrometer was used to monitor ¹³⁹La. Validation was undertaken using internal quality control solutions containing lanthanum ion (0.20, 0.70 and 4.00 μg/L). Lanthanum was measured in patients (number = 20) with CKD prescribed lanthanum carbonate (500-1500 mg/d) and patients undergoing haemodialysis not prescribed lanthanum carbonate (number = 20). RESULTS: Accuracy and imprecision were >95% and <5%, respectively. Calibration was linear (range 0.1-5 μg/L, R² = 0.99). The lower limit of quantification (LLoQ) was 0.1 μg/L lanthanum ion. In patients with CKD not prescribed lanthanum carbonate, serum lanthanum was below the LLoQ. Out of 20 CKD patients prescribed lanthanum carbonate, serum lanthanum was measurable in only 12 (range 0.11-0.60 μg/L lanthanum ion). There was no apparent relationship between dose and serum lanthanum in these patients. CONCLUSIONS: A lack of relationship between the dose of lanthanum carbonate and the serum lanthanum concentration may have been due to poor adherence to the treatment regimen. However the concentrations measured were close to the LLoQ.