Determination of serum aldosterone by liquid chromatography and tandem mass spectrometry: a liquid-liquid extraction method for the ABSCIEX API-5000 mass spectrometry system

Aims Accurate serum aldosterone determination is critical to the screening and diagnosis of primary aldosteronism, the localisation of aldosterone producing tumours, and the investigation of other disorders of the renin-angiotensin system. Mass spectrometry offers a means to overcome problems with method-dependent bias between competitive immunoassays for aldosterone. The authors have developed a simple, sensitive and precise liquid-liquid extraction aldosterone method for the ABSCIEX API-5000 liquid chromatography and tandem mass spectrometry (LC-MS/MS) system. Methods Using d7-aldosterone internal standard, 500 mu l of sample is extracted with 2500 ml of methyl tertbutyl ether followed by dry-down, reconstitution and LC-MS/MS analysis in ESI negative mode. Method validation was undertaken using standard approaches and comparison made against a commercial radioimmunoassay. Accuracy was assessed using EQA material with assigned aldosterone concentrations. Results The assay was linear up to 3420 pmol/l (LOQ=50 pmol/l, LOD<22 pmol/l). Total CVs were <= 5% for concentrations >= 120 pmol/l and 10% at the LOQ. Mean accuracy was 98.5% against GCMS assigned material. Conclusion The authors present a precise, sensitive and simple aldosterone method suitable for routine clinical use that requires no solid phase extraction or specialised ion sources.