COMPARISON OF THE DETERMINATION OF MAGNESIUM BY METHYLTHYMOL BLUE SPECTROPHOTOMETRY AND ATOMIC-ABSORPTION SPECTROPHOTOMETRY

Plasma samples (n = 155) of 30 patients on an intensive ward were anal ysed for magnesium simultaneously by atomic absorption spectrophotomet ry (AAS) and methylthymol blue spectrophotometry. Methylthymol blue sp ectrophotometry was performed at the bedside, using two different mult ianalysers, Easy ST 1 and Easy ST 2, Merck, D-Darmstadt. Precision was 12.2% (Easy ST 1) and 17. 1 % (Easy ST 2), and the average value was 0.89 mmol/l, which was above the expected range (0.72-0.88 mmol/1). Ac curacy was 16.25% (Easy ST 1) and 8.75% (Easy ST 2). Analyser 2 was mo re accurate (8.75% versus 6.25%) but less precise (17.1% versus 12.2%) than analyser 1. Precision of AAS was between the expected values of 0.69 and 0.84 mmol/l. Easy ST and AAS gave significantly different val ues (p < 0.0001) for 155 measurements. Comparison of AAS and methylthy mol blue spectrophotometry showed that methylthymol blue spectrophotom etry produced higher values than AAS (mean difference 0.186 mmol/l). F urthermore, analyses of 40 samples of a standardized plasma concentrat ion with methylthymol blue spectrophotometry showed a very low precisi on (15.3%). Easy ST cannot be assigned for urinary measurements of mag nesium. Experimentally measured samples gave unaccountable results.