Comparison of two methods for the assessment of intra-erythrocyte magnesium and its determinants 51 3 colorimetric assay, and triglycerides using a enzymatic colorimetric assay, not glycerol blanked, all measured routinely on a Cobas 8000 platform (Roche, Mannheim, Germany). Plasma sodium (indirect ISE) and potassium (indirect ISE)were measured routinely on a Cobas 8000 platform (Roche, Mannheim, Germany). Urine magnesium was measured on this platform as well, by means of the xylidyl blue colorimetric assay. Plasma and urine creatinine concentrations were determined using an Isotope Dilution Mass Spectrometry traceable enzymatic assay on a Roche Modular P analyzer 17. Creatinine clearance was calculated using the following formula 18: Creatinine clearance (ml/min) = ((U creatinine * V) / Pcreatinine)*(1000/1440) in which Ucreatinine represents urinary creatinine concentration in μmol/L, V represent the 24-hour urinary volume in liters and Pcreatinine represents plasma creatinine in μmol/L. Estimated glomerular filtration rate (eGFR) was estimated using the Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI), based on plasma creatinine 19. eGFR was categorized into 2 groups: ≥90 mL/min/1.73m2 and <90 mL/min/1.73m2. HbA1c concentrations were measured on a Tosoh G8 (HPLC, Sysmex Corporation, Norderstedt, Germany). Hemoglobin, hematocrit, erythrocyte count, leukocytes, and mean corpuscular volume (MCV) were determined using a Sysmex XN20 automated hematology analyzer (Sysmex Corporation). Data on alcohol consumption and smoking status was obtained from questionnaires. Non-drinkers were defined as not having consumed alcohol during the past month. Individuals were classified into four groups according to their daily alcohol intake: 0 drinks/day (non-drinker), ≤1 drink/day (light drinker), >1 to 2 drinks/day (moderate drinker) and >2 drinks/day (heavy drinker) 20. Smoking status was defined as selfreported non-smoker, former smoker or current smoker. Plasma magnesium assay The plasma magnesium concentration was assessed using a colorimetric assay (Cobas 8000, Roche, Mannheim, Germany) at the University Medical Center Groningen. The colorimetric assay is based on the complex formation between magnesium ions and xylidyl blue in an alkaline solution (tris(hydroxymethyl)-aminomethane / 6-aminocaproic acid buffer), forming a purple diazonium salt.The magnesium concentration is measured photometrically through the decrease in xylidyl blue absorbance. Ethylene glycol-bis(βaminoethyl ether)-N,N,N’,N’-tetraacetic acid (EGTA) is added to mask the calcium ions present in the sample. The intra-assay and inter-assay coefficients of variation are 1.1% and 1.3% at 0.59 mmol/L, 0.8% and 1.1% at 0.80 mmol/L and 0.7% and 0.9% at 1.35 mmol/L, respectively.
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